NCERT Solutions Class 7 Science Chapter 3 Electricity Circuits and Their Components

F R E E S T U D Y M AT E R I A L F O R E V E R Y S T U D E N T

CLASS 7 · SCIENCE

NCERT Solutions

Chapter 3: Electricity: Circuits
and their Components

NCERT Textbook — Curiosity

BOOK PAGES SECTIONS QUESTIONS MEDIUM

23 – 40 21 51 English

Solutions, notes, sample papers & more at 60 pages

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

CLASS 7 · SCIENCE · CURIOSITY

NCERT Solutions — Chapter 3: Electricity: Circuits and
their Components
Nihal's trip to the Bhakra Nangal Dam opens a chapter that takes an ordinary torch apart. You meet the
electric cell and the battery, the incandescent lamp and the LED, the switch, the symbols used to draw a
circuit diagram, and finally you sort everyday materials into conductors and insulators.

TEXTBOOK BOOK PAGES

Curiosity (Class 7) 23 – 40

SECTIONS QUESTIONS

21 51

MEDIUM

English

In-text Questions — Page 24
Chapter opener and Section 3.1 A Torchlight

Q1 Can you help Nihal by adding some more uses to his lists? Also, suggest some other
ways of grouping the uses of electricity.

Yes. Every heading in Nihal's chart can be made longer, and the whole chart can be re-drawn in
other ways.
More uses under his headings:

Page 1 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

HEADING MORE USES YOU CAN ADD

Lighting Tube lights, LED bulbs, table lamp, emergency lamp, traffic signals, hospital
operation-theatre lamps, decorative lights at Diwali

Cooking Induction cooktop, electric rice cooker, roti maker, water purifier, electric tandoor

Heating and Cooler, hair dryer, iron, cold storage for vegetables, air cooler in a bus
Cooling

Transportation Metro train, e-rickshaw, electric bus, electric scooter, ropeway

Communication Telephone, television broadcast, radio station, Wi-Fi router, mobile tower

Entertainment Music system, video game, cinema projector, loudspeaker at a mela

Others Tube-well for irrigation, washing machine, sewing machine, X-ray machine, printing
press, ATM

Other ways of grouping the uses:

By place — home, school, hospital, farm, factory, shop, road.
By what electricity is changed into — light (bulb), heat (iron), sound (radio), motion (fan), or a
mixture (hair dryer gives heat + motion).
By the source used — devices that run on cells/batteries (torch, remote) and devices that run
from a wall socket (refrigerator, geyser).
By who uses it — household use, farming use, industrial use, medical use, transport use.

Try This: Walk through your own house with a notebook and count the devices that
would stop working if the electricity went off. Most students are surprised — the
count usually crosses 20.

Q2 Why does the torch lamp glow in one position of its switch?

Because in that position the switch completes the circuit.
Inside a torch there is a cell (or battery), a lamp, and a switch, all joined by metal strips and
wires. The switch is simply a movable metal piece that either bridges a gap or leaves it open.

ON position — the switch closes the gap, so there is an unbroken path from the + terminal
of the cell, through the lamp, and back to the – terminal. Current flows through the filament,
it gets hot, and the lamp glows.

Page 2 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

OFF position — the switch leaves an air gap. Air is an insulator, so the path is broken, no
current flows and the lamp stays dark.

Why it happens: A lamp does not glow just because a cell is nearby. Current must
actually pass through the filament, and current can only flow round a closed loop.
Break the loop anywhere — at the switch, at a loose wire, or at a broken filament —
and the lamp goes out.

Activity 3.1: Let us explore — Page 24
Section 3.1 A Torchlight

ACTIVITY

Q1 Take a torchlight similar to the one shown in Fig. 3.1. Observe it carefully. Do you
notice a lamp? And a switch?

Switch
Lamp

Wrist strap Body (cells inside)

Fig. 3.1, page 24 — a torchlight; redrawn here as a labelled sketch.

Yes — both are easy to find.

The lamp sits behind the glass or plastic window at the front of the torch, inside a shiny
curved cup (the reflector) that throws the light forward. It may be a small incandescent bulb
or one or more LEDs.
The switch is the small sliding button (or push button) on the side of the barrel, placed where
your thumb naturally rests.

Page 3 of 60

as e
a g l
Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

co m
m.
Check it yourself: Hold the torch and slide the switch without looking at the lamp.

as e
com l
You can usually feel a small click — that click is the moment the metal strip inside
. a g
m
touches and the circuit closes.

l a se
ag

co m
ag
Slide its switch and observe. Does the torch lamp glow? Now slide the switch back
.
Q2

e m
as
to its original position and observe the torch lamp.

a g l

Yes, in one position of the switch the lamp glows; when the switch is slid back, it goes off.
co m
se m.
o m l a
g (ON)
Switch.cslid one way → circuit closed → current flows → lamp glows
m a
l a se slid back → circuit open → no current → lamp does not glow (OFF)
a g Switch

m a s
.co agl
The position in which the lamp glows is called the ON position of the switch; the other position
is the OFF position.
se m
g l a
a
Now, open the torchlight. What do you find inside?
co m
.
Q3

e m
m l as
.co a g
em the barrel you find two or more electric cells, placed one behind the other, along with the

a s
gl
Inside
a parts that connect them.

se m
com a
A careful look shows all of these:

. a g l
e m
Electric cells — placed in a line, the + cap of one cell touching the – disc of the next.

g l as presses the cells firmly against the contacts.
A spring at the back end, which
a along the inside of the barrel to carry the current to the lamp.
Metal strips or wires running

m
The lamp and its reflector at the front, and the switch in between.
. co
se m
o m l a
ag (the device) and a switch
.ccells (the source), wires and metal strips (the path), a lamp
Why it happens: A torch is nothing more than a neat, packed-up electrical circuit —

se m
g l a
a
(the control). Everything you will study in this chapter is already inside your hand.

.c
s e m
m a
m . co agl
se — Page 25
Activity 3.2: Let us observe
a
agl

co m
m .
m ase
.co


a g l Page 4 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Section 3.2.1 Electric cell

ACTIVITY

Q1 Take an electric cell, turn it around and look at it carefully (Fig. 3.2). Do you notice a
positive (+) sign and a negative (–) sign marked on the electric cell?

Positive terminal Negative terminal

+ –

Fig. 3.2, page 25 — an electric cell, with its two terminals marked.

Yes. Printed on the outer cover of every cell there is a + sign at one end and a – sign at the other
end.
These two marks name the two terminals of the cell:

MARK ON THE CELL NAME OF THE TERMINAL SHORT FORM

+ Positive terminal + ve

– Negative terminal – ve

Tip: The signs matter a great deal for an LED and for putting cells into a battery in
the right order, so never ignore them while making a circuit.

Q2 Do you also notice that it has a small protruding metal cap on one side and a flat
metal disc on the other side?

Yes — and the two are not decoration; they are the terminals themselves.

Page 5 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

PART OF THE CELL WHAT IT LOOKS LIKE WHICH TERMINAL IT IS

Metal cap A small raised knob sticking out at one end Positive (+) terminal

Metal disc A flat metal plate covering the other end Negative (–) terminal

So even if the printing on a cell has worn away, you can still tell the terminals apart by feeling
for the raised cap.

Why it happens: The cap and the disc are the only two metal parts a wire can be
joined to. Everything in between is the chemical material of the cell, which turns
chemical energy into electrical energy. That is why an electric cell is called a portable
source of electrical energy.

In-text Questions — Page 25
Section 3.2.2 Battery

Q1 In a torch, we generally use more than one cell. Are those placed in any particular
order?

Yes. They must be placed in a definite order — the positive terminal of one cell touching the
negative terminal of the next.

Correct: [– cell +] [– cell +] → the + cap of the first meets the – disc of the second

Wrong: [– cell +] [+ cell –] → two like terminals face each other, and the cells oppose each

other

When cells are placed in the correct order the arrangement is called a battery, and the lamp
glows. If one cell is turned round, the two cells push current in opposite directions, the effects
cancel and the lamp does not glow.

Tip: Look inside the cell compartment of any remote or toy — a small '+' and '–'
picture is printed there to show you the correct order.

Activity 3.3: Let us experiment — Page 25

Page 6 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Section 3.2.2 Battery

ACTIVITY

Q1 Put the cells back in a different order. Also, try reversing the direction of one cell.
Then, slide the switch and check whether the lamp glows in each case.

The lamp glows in only one of the arrangements — the one in which the + terminal of one cell
touches the – terminal of the other.

HOW THE TWO CELLS ARE DO THE CELLS HELP OR OPPOSE? LAMP
PLACED

–+ –+ + of first meets – of second — they help each Glows
other

–+ +– + meets + — they oppose each other Does not
glow

+– –+ – meets – — they oppose each other Does not
glow

+– +– – of first meets + of second — they help each Glows
other

Why it happens: Two cells joined + to – add their pushes, so the current through the
filament is strong enough to make it glow. Two cells joined + to + (or – to –) push in
opposite directions; their effects cancel and almost no current flows.

Q2 Check the order in which the cells were placed in the torch when the lamp glows.

When the lamp glows, the cells are in the order shown in Fig. 3.3 — the positive terminal of
one cell is connected to the negative terminal of the next cell.

[ – cell 1 + ] → [ – cell 2 + ]

The free – of cell 1 and the free + of cell 2 become the two terminals of the battery.

Such a combination of two or more cells is called a battery. Fig. 3.3a shows a battery of two
cells and Fig. 3.3b a battery of four cells — in every case the same + to – rule is followed.

Page 7 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Did you know? The word battery is also used for a single cell. The single cell that
powers a mobile phone is called its battery.

Activity 3.4: Let us observe — Page 26
Section 3.2.3 Electric lamp — Incandescent Lamp

ACTIVITY

Q1 Take the torch and examine its lamp. What do you see? Do you notice a thin wire
fixed in the middle of the glass bulb?

Yes. Inside the small glass bulb there is a very thin, coiled wire held in the middle, supported at
its two ends by two much thicker wires.
What you see, from outside inwards:

A clear glass bulb.
Two thick wires rising from the base — they only hold and feed the thin wire.
The thin wire between them, usually wound like a tiny spring.
A ring of insulator at the base that keeps the two terminals from touching.

Q2 Now, switch on the torch. Which part of the lamp glows?

Only the thin wire inside the glass bulb glows. This glowing thin wire is called the filament of
the lamp.
The thick supporting wires, the glass bulb and the metal base stay dark and comparatively cool.

Why it happens: The same current passes through the thick wires and the thin
filament, but the thin filament resists the current far more. That extra opposition
makes the filament heat up until it is white-hot and gives out light. A lamp that
works this way is called an incandescent lamp — 'incandescent' means 'glowing
because it is hot'.

Page 8 of 60

as e
a g l
Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

co m
m.
Take out the lamp with the help of your teacher and inspect it from all sides. How is
e
Q3

m l as
.co
the filament fixed?

a g
se m
g l a
a

The filament is fixed between two thicker wires that both support it and carry current to it (Fig.

m
3.4a). Those two thick wires lead down to the two terminals of the lamp:
. co ag
e m
THICK WIRE WHERE IT ENDS
g l as NAME OF THAT TERMINAL

One
a
The metal case at the base of the lamp One terminal

co m
m.
The other The metal tip at the centre of the base The other terminal

o m l a se
c
A ring of .insulating a g so the two terminals
m
se touch each other (Fig. 3.4b).
material separates the metal tip from the metal case,

l a
ag
never

Why it happens: If the tip and the case touched, the current would take that easy
m a s
.co
short path and skip the filament altogether — the lamp would not glow. The
m agl
l a se
insulator forces the current to go up one thick wire, through the filament, and down
the other. a g

co m
m .
m Let us observe — Page 27 as e
Activityo3.5:
. c a g l
s e m
a
agl
se m
com g l a
m . a
ase
agl

co m
m .
m as e
.co a g l
se m
g l a
a c
m .
m a s e
e m . co agl
g l as
a

co m
m .
m ase
.co


a g l Page 9 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Section 3.2.3 Electric lamp — LED Lamp

ACTIVITY

Q1 Take an LED of any colour (Fig. 3.6) and observe. Do you see any filament inside it?

+ – + – + – + – + –

Fig. 3.6, page 27 — LEDs of different colours. The longer wire of each LED is its +
terminal.

No. Unlike an incandescent lamp, an LED has no filament (Fig. 3.6).
Inside the small coloured dome you can only see a tiny cup holding a speck of material, joined
to the two wires. That speck gives out light directly when current passes through it — nothing
has to get red-hot first.

Did you know? LED stands for Light Emitting Diode. Because it does not waste
energy in heating a filament, an LED gives the same brightness while using far less
electricity, and it lasts much longer. That is why most new torches, street lights and
home bulbs are LEDs.

Q2 Notice the length of two wires attached to the LED. Do you find one of those longer
than the other?

Yes. Of the two wires coming out of an LED, one is clearly longer than the other, and this
difference in length is a code for the terminals.

Page 10 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

WIRE OF THE LED TERMINAL IT IS MUST BE JOINED TO

Longer wire Positive (+) terminal + terminal of the battery

Shorter wire Negative (–) terminal – terminal of the battery

Why it happens: Current can pass through an LED in one direction only. The makers
therefore leave one wire longer so that you can identify the direction even without
any printing on the LED. Join it the wrong way round and the LED simply stays dark
— it does not glow dimly, it does not glow at all.

Activity 3.6: Let us construct — Page 28

Page 11 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Section 3.2.4 Making an electric lamp glow using an electric cell or battery

ACTIVITY

Q1 Predict, for each arrangement, if the lamp will glow or not and write your
prediction in Table 3.1.

Table 3.1: Trying to make the lamp glow
Note: The lamps are not shown glowing in any circuit

Page 12 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

S.NO. ARRANGEMENT OF CELL AND LAMP PREDICTION OBSERVATION

1.

+

–

2.

+

–

3.

+

–

4.

+

–

5.

+

–

Page 13 of 60

as e
a g l
Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

co m
m.
6.

m as e
.co
+
a g l
se m
g l a
a –

com
e m . ag
g l as
a
Table 3.1, page 29 — the six arrangements of the cell and the lamp, redrawn from the
photographs.

co m
em.
m l as
.co a g

a s em
l
Use one simple rule to predict: the lamp glows only if one of its terminals is joined to one

a gterminal of the cell and its other terminal to the other terminal of the cell — so that current has
an unbroken loop to travel round.

om a s
HOW THE CELL AND LAMP ARE.cJOINED IN TABLE 3.1
em agl
s
S.NO. PREDICTION
a
1. agtol one lamp wire and the wire from – is joined to
The wire from + is joined Will glow
the other lamp wire — both joints made

. comWill not glow
Only one joint is made; the wire from – and the second lamp wire are left
e m
2.

m l as
.co
lying apart

a g
se m
l a
3. Only the – side is joined; the wire from + is left free Will not glow

ag
m
4. Both lamp wires are joined together to the + wire at the same point; the – Will not glow

a se
com l
wire is free

. a g
m
ase
5. Both lamp wires are joined together to the – wire; the + wire is free Will not glow

6. agl
Again one lamp wire to +, the other lamp wire to – — both joints made Will glow

c o m
Tip: Before predicting, trace the path with your finger startingm .
s e at the + terminal. If
a the lamp, the lamp will
om finger can reach the – terminal only by passing through
.cyour agl
se m glow.
g l a
a c
m .
m a s e
e m . co agl
g l as
a

co m
m .
m ase
.co


a g l Page 14 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Q2 Now, connect the lamp and the cell, and observe if the lamp glows or not. Note
down your observation in Table 3.1. Also, for the lamps which glow, colour their
glass bulbs yellow.

Table 3.1: Trying to make the lamp glow
Note: The lamps are not shown glowing in any circuit

Page 15 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

S.NO. ARRANGEMENT OF CELL AND LAMP PREDICTION OBSERVATION

1.

+

–

2.

+

–

3.

+

–

4.

+

–

5.

+

–

Page 16 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

6.

+

–

Table 3.1, page 29 — the six arrangements of the cell and the lamp, redrawn from the
photographs.

On actually making the connections, the lamp glows in the arrangements at S.No. 1 and S.No. 6
and does not glow in the rest. So only those two bulbs are to be coloured yellow.

S.NO. OBSERVATION REASON

1. Lamp glows Complete circuit — current goes from + through the lamp and back to –

2. Lamp does not glow The loop is open at the second joint, so no current flows

3. Lamp does not glow The + wire is free — the loop is open

4. Lamp does not glow Both lamp terminals touch the same wire, and the – wire is free

5. Lamp does not glow Both lamp terminals touch the same wire, and the + wire is free

6. Lamp glows Complete circuit, made the same way as in S.No. 1

Why it happens: In 1 and 6 the two terminals of the lamp are joined to two different
terminals of the cell. In 4 and 5 both lamp terminals are joined to the same terminal,
so there is no push from one side to the other. In 2 and 3 the loop is simply left open.

In-text Questions — Page 30

Page 17 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Section 3.2.5 An electrical circuit

Q1 Now, carefully look at the arrangements in which the lamp glows. Compare these
with those in which the lamp does not glow. Can you find the reason for the
difference?

Yes. In the arrangements where the lamp glows, the connections form a complete, unbroken
loop; in the others the loop is broken somewhere.

Lamp

current

Cell

+ –
A closed loop: current leaves the + terminal, passes through the lamp, and returns to the – terminal.
Break the loop anywhere and the lamp goes out.

The difference, stated as a rule:

Lamp glows — one terminal of the lamp is joined to one terminal of the cell, and the other
terminal of the lamp to the other terminal of the cell.
Lamp does not glow — either a joint is left open (S.No. 2, 3), or both lamp terminals are
joined to the same cell terminal (S.No. 4, 5).

An arrangement of the first kind is called an electrical circuit. It provides a complete path for
electric current to flow through the lamp, and the lamp glows only when current passes through
the circuit. By agreement, the direction of the current is taken to be from the positive terminal
to the negative terminal of the cell.

Page 18 of 60

as e
a g l
Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

co m
m.
Did you know? Sometimes a lamp does not glow even in a correct circuit. We then

m as e
l
say the lamp has 'fused' — its filament has broken, so the path inside the bulb itself
is open. .co a g
a s em
agl
om31
Activity 3.7: Let us experiment — Page
. c ag
s e m
a
Section 3.2.5 An electrical circuit — making an LED glow

ACTIVITY agl

co m
se m.
Q1 Insert two cells in the holder, taking care that for each cell, its negative terminal is

o m l a
towards the spring side of the holder (Fig. 3.10b) and the battery is ready to use.
g
m .c will you decide which is the positive terminal of this abattery?
se
How
a
agl
m a s
.co agl
+ –

se m
g l a + –
a

co m
m .
m as
–
e +

.co a g l
se m
l a
(a) (b)

a g
m
Fig. 3.10 (a) and (b), page 31 — the empty two-cell holder, and the holder with two cells

a se
com l
in it, each cell with its negative terminal towards the spring. Redrawn.

. a g
m
ase
ANSWER agl
om
Look at which end of each cell the two ends of the holder touch.
. c
s e
The end of the holder that is connected to the positive terminal of mone cell is the positive
. om of the battery.
cterminal a gla
a s em The end of the holder that is connected to the negative terminal of the other cell is the
agl negative terminal of the battery.
c
m .
m a s e
. co
Holder end A — touches the + cap of cell 1 → + terminal of the battery
e m agl
g l as
Holder end B — touches the – disc of cell 2 → – terminal of the battery
a

co m
m .
m ase
.co


a g l Page 19 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Tip: In a cell holder the spring always presses against the flat disc, so the spring side
of each cell is its negative side. Follow the springs and you can never get the polarity
wrong.

Q2 Now, connect the free end of the battery positive terminal wire to the longer wire
of LED, and the free end of the second wire to the shorter wire of LED (Fig. 3.10c).
Does the LED glow?

+ –

+ –

+ –

– +

Fig. 3.10c, page 31 — the + terminal of the battery joined to the longer (+) wire of the
LED, and the second wire to the shorter (–) wire. Redrawn.

Yes, the LED glows.

Page 20 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

+ of battery → longer wire (+ terminal) of LED

– of battery → shorter wire (– terminal) of LED

Current can pass in this direction → LED lights up

Why it happens: An LED allows current in one direction only. Here that direction
agrees with the direction in which the battery pushes the current, so current flows
and the LED emits light. Two cells are used because a single cell usually does not
give enough push to light an LED.

Page 21 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Q3 Repeat the above step but interchange the wires connected to the LED (Fig. 3.10d).
Does the LED glow again?

– +

+ –

+ –

– +

Fig. 3.10d, page 31 — the same circuit with the two wires on the LED interchanged.
Redrawn.

No, it does not glow this time.

+ of battery → shorter wire (– terminal) of LED

– of battery → longer wire (+ terminal) of LED

Current cannot pass in this direction → LED stays dark

Page 22 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Why it happens: Electric current can pass through an LED in one direction only.
When the connections are interchanged, the battery tries to push current the wrong
way through the LED and the LED blocks it. So always take care to connect an LED
correctly in a circuit.

Compare: An incandescent lamp behaves differently — it glows whichever way
round it is joined, because the current simply heats its filament and heating does not
depend on direction.

Dive Deeper — Page 31
Section 3.2.5 An electrical circuit

DIVE DEEPER

Q1 Sometimes you may come across a device in which the cells are placed side by side.
Then, how are the terminals of the cells connected?

They are still connected + of one cell to – of the next — but the joining is done by a hidden metal
piece instead of the cells touching each other.
If you look carefully inside the battery compartment you will usually see:

A thick wire or metal strip running across, which connects the positive terminal of one cell
to the negative terminal of the next.
'+' and '–' symbols printed inside the compartment, one pair for each cell, to help you
place the cells the right way round.

Why it happens: The rule for making a battery never changes — the + terminal of
one cell must reach the – terminal of the next. When the cells lie side by side and
cannot touch end to end, a metal strip does the joining for them. This is why a
remote or a toy stops working if even one cell is put in the wrong way round.

Check it yourself: Open the battery compartment of a TV remote (with permission)
and find the metal strip. Notice that the printed '+' and '–' marks for the two cells
point in opposite directions — that is the + to – rule at work.

In-text Questions — Page 32

Page 23 of 60

as e
a g l
Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Section 3.2.6 Electric switch
co m
e m.
m l as
Q1
.co
How does a switch turn on or off the torchlight?
m a g
l a se
a g

m
A switch works by closing or opening a small gap in the circuit — nothing more.

. co ag
e m
POSITION WHAT HAPPENS INSIDE
g l as STATE OF THE LAMP

a CIRCUIT

ON A movable metal piece bridges the gap Closed
co m
Current flows from + to

m.
securrent flows, so the
and touches both contacts –, so the lamp glows

m l a
OFF
m .co The metal piece is moved away, Open
ag No

l a se leaving an air gap between the lamp does not glow

a g contacts

m a s
.co agl
A switch can be placed anywhere in a circuit — before the lamp or after it — because breaking

se m
the loop at any one point stops the current everywhere in that loop. The switches used for lights

g l a
and other devices at home work in exactly the same way, though they are designed differently
and are covered with insulating material. a

. com
Why it happens: Air is an insulator. So an air gap of even a millimetre is enough to
stop the current completely. A switch is simply a convenient, safeem
m a s way of making and
. co that gap. a gl
m
removing

ase
agl
se m
Activity 3.9: Let us test — Page 32m
co g l a
m . a
ase
agl

com
m .
m ase
.co a g l
se m
g l a
a c
m .
m a s e
e m . co agl
g l as
a

co m
m .
m as e
.co


a g l Page 24 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Section 3.2.6 Electric switch

ACTIVITY

Q1 Connect the electric cell, lamp, and switch as shown in Fig. 3.12a. Does the lamp
glow?

+ Safety pin

–

Fig. 3.12a, page 32 — the circuit with the safety-pin switch in the ‘OFF’ position.
Redrawn.

No, the lamp does not glow.
In Fig. 3.12a the free end of the safety pin is turned away, so it does not touch the second
drawing pin. There is a gap between the two drawing pins, and air fills that gap.

Gap between the drawing pins → circuit open → no current → lamp dark

This is the OFF position of the switch, and we say the circuit is open.

Page 25 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Q2 Rotate the free end of the safety pin till it touches the other drawing pin as shown
in Fig. 3.12b. Does the lamp glow now?

+ Safety pin

–

Fig. 3.12b, page 32 — the safety pin turned till it touches the second drawing pin (‘ON’
position). Redrawn.

Yes, now the lamp glows.
The safety pin is made of metal, a good conductor. When its free end touches the second
drawing pin, it closes the gap and completes the path.

Safety pin touches both drawing pins → circuit closed

Current flows from the cell's + terminal → through the lamp → back to the – terminal

Lamp glows (ON position)

Why it happens: The cardboard the pins are fixed to is an insulator, so the current
cannot cross from one pin to the other through the board. Only the metal safety pin
can carry it across. That is the whole idea of a switch — a movable conductor over a
fixed gap.

In-text Questions — Page 33

Page 26 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Section 3.3 Circuit Diagrams

Q1 Can we represent the circuits in a simpler manner?

Yes. Instead of drawing the cell, the bulb and the wires as pictures, we replace each component
by an agreed symbol. A drawing of a circuit made with these symbols is called its circuit
diagram.

S.NO. ELECTRICAL HOW ITS SYMBOL IS DRAWN
COMPONENT

1. Electric cell One long line (+ terminal) and one short thick line (– terminal),
side by side

2. Battery Two or more cell symbols drawn in a row, long–short, long–short

3. Electric lamp A circle with a cross inside it

4. Light Emitting Diode A triangle touching a bar, with two small arrows pointing away to
(LED) show that light is emitted

5. Switch in 'ON' position Two small circles joined by a straight line

6. Switch in 'OFF' position Two small circles with a line lifted away from one of them

7. Wire A plain straight line

Two things to remember while reading these symbols:

In the cell symbol, the long line is the positive terminal and the short line is the negative
terminal (Fig. 3.13a).
In the LED symbol, the triangle points in the direction in which the current can flow, and
the two arrows show that the LED emits light (Fig. 3.13b).

Why it happens: Symbols are quicker to draw, take less space, and mean the same
thing to everyone. Bodies such as the IEC, ANSI and IEEE fix these standard symbols
so that an engineer in Chennai and one in Canada read the same diagram in the
same way.

Activity 3.10: Let us draw — Page 34

Page 27 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Section 3.3 Circuit Diagrams

ACTIVITY

Q1 Using symbols shown in Table 3.2, draw the circuit diagram of an electrical circuit
given in Fig. 3.12a and Fig. 3.10c. Are your circuit diagrams similar to Fig. 3.14a and
Fig. 3.14b respectively?

Table 3.2: Electrical components and their symbols

Page 28 of 60

as e
a g l
Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

co m
m.
S.NO. ELECTRICAL SYMBOL

as e
com
COMPONENT

. a g l
s e m1. Electric cell
a
agl

co m
e m . ag
g l as
a
m
Battery

co
2.

e m.
m l as
m .co a g
l a se
a g
m a s
Electric lamp
m .co agl
se
3.

g l a
a

co m
m .
m as e
.co a g l
se m
g l a 4. Light Emitting Diode (LED)

a
se m
com g l a
m . a
ase
agl

co m
m .
m ase
.co a g l
m
ase
5. Switch in ‘ON’ position

agl c
m .
m a s e
e m . co agl
g l as
a

co m
m .
m as e
.co


a g l Page 29 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

6. Switch in ‘OFF’ position

7. Wire

Table 3.2, page 33 — electrical components and their symbols.

+ Safety pin

–

Fig. 3.12a, page 32 — the circuit with the safety-pin switch in the ‘OFF’ position.
Redrawn.

Page 30 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

+ –

+ –

+ –

– +

Fig. 3.10c, page 31 — the + terminal of the battery joined to the longer (+) wire of the
LED, and the second wire to the shorter (–) wire. Redrawn.

Yes — drawn correctly, they come out exactly like Fig. 3.14a and Fig. 3.14b.

lamp

+ – + –

switch (OFF)
(a) circuit of Fig. 3.12a (b) circuit of Fig. 3.10c

Page 31 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

(a) A cell, an electric lamp and a switch shown in the OFF position — the circuit diagram of Fig. 3.12a.
(b) A battery of two cells and an LED — the circuit diagram of Fig. 3.10c. The LED's triangle points
away from the + terminal of the battery, so current can pass and the LED glows.

How to draw them, step by step:

1. Draw a rectangle of thin straight lines — these are the connecting wires.
2. Break the top side and put in the lamp symbol (circle with a cross) for Fig. 3.12a, or the LED
symbol (triangle and bar) for Fig. 3.10c.
3. Break the bottom side and put in the cell symbol (Fig. 3.12a) or the battery symbol of two
cells (Fig. 3.10c). Keep the long line on the + side.
4. For Fig. 3.12a also break the bottom side once more for the switch, drawn in the 'OFF'
position because that is how Fig. 3.12a shows it.
5. For the LED, check that the triangle points away from the + terminal of the battery —
otherwise the LED would not glow.

In-text Questions — Page 34
Section 3.4 Electrical Conductors and Insulators

Q1 Why did we use metal wires for making electric circuits? Can we not use some other
materials for wires?

Metals are used because electric current flows through them easily — metals are good
conductors of electricity. Most other materials do not let current pass at all, so a circuit made
with them would never work.

MATERIAL DOES CURRENT PASS? CAN IT BE USED FOR WIRES?

Copper, silver, gold, aluminium Yes, easily Yes — these are conductors

Plastic, rubber, wood, glass, wax No No — these are insulators

Silver, copper and gold are the best conductors of electricity. Even so, copper is used for almost
all wiring because it is much cheaper than silver or gold and is available in plenty. Different
kinds of wires are made for different uses — thin ones for a torch, thick ones for a house.

Page 32 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Q2 Also, why are electric wires covered with plastic or rubber?

Because plastic and rubber are insulators — current cannot pass through them. The covering
keeps the current inside the metal wire where it belongs.
The covering does three jobs:

Protects people. Our body is a conductor. If we touched a bare wire carrying current, the
current would pass through us and could cause severe injury or even death.
Stops short circuits. If two bare wires touched each other, current would take the short path
instead of going through the device.
Protects the wire itself from moisture, rust and damage.

Why it happens: Conductors and insulators are both needed in every electrical
device. Wires, switch contacts, plug pins and socket parts are made of conductors so
that current can flow; their coverings, plug tops and switch bodies are made of
rubber, plastic or ceramic so that current cannot reach our hands.

Caution: Never touch a switch or plug with wet hands, never use electrical devices in
wet places, and never use equipment with damaged insulation or a broken plug.

In-text Questions — Page 35
Section 3.4 Electrical Conductors and Insulators

Q1 Suppose, we make wires of materials other than metal and use them for making
the electrical circuit. Do you think the electric current will pass through those
materials in such a circuit?

Not through most of them. Current will pass only if the material happens to be a conductor.

If the 'wire' is made of a metal such as copper, aluminium or iron — yes, current passes and
the lamp glows.
If it is made of plastic, rubber, wood, glass, wax, paper or cloth — no, current does not pass
and the lamp stays dark.
A few non-metals are exceptions. Pencil lead (graphite) is not a metal but it does conduct, so
the lamp glows dimly with it.

Page 33 of 60

as e
a g l
Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

The only honest way to find out for any given material is to test it — which is exactly what the
co m
tester in Activity 3.11 is for.
e m.
m l as
m .co a g
l a se
g
a 3.11: Let us identify — Page 35
Activity

m
Section 3.4 Electrical Conductors and Insulators

. co ag
ACTIVITY
e m
g l as
Q1
a
Touch the two free ends of the wires momentarily. Does the lamp glow? If yes, our
tester is ready.
co m
em.
m l as
.co a g

a s em
Yes, the lamp glows for that moment.

a l the two bare ends are apart, the circuit has a gap in it and no current flows. The instant
gWhile
you make them touch, the metal of one wire meets the metal of the other, the loop is completed
m a s
.co agl
and the lamp lights up.

a s em
gl off
Ends apart → circuit open →alamp

m
Ends touching → circuit closed → lamp on
. co
se m
o m l a
g the place of that touch.
.c — whatever you place between the two free ends willatake
This test proves that the cell, the lamp and all the joints are working. Your conduction tester is

se m
now ready

g l a
a
m
Tip: If the lamp does not glow on touching the ends, check the cell, the bulb, and

a se
com l
whether about 1 cm of plastic covering has really been removed from each end of
. a g
m
ase
every wire.

agl

co m
m .
m as e
.co a g l
se m
g l a
a c
m .
m a s e
e m . co agl
g l as
a

com
m .
m ase
.co


a g l Page 34 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Q2 One by one, touch the free ends of the tester's wires to both ends of each object
you have collected (Fig. 3.15b). Make sure the wires don't touch each other. Does
the lamp glow every time?

+

–

Free ends of the wires
(a) Conduction tester

+

–

Metal spoon

(b) Testing a material

Page 35 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Fig. 3.15, page 35 — (a) the conduction tester, (b) the tester used on a metal spoon.
Redrawn.

No. The lamp glows for some objects and stays dark for others.

It glows for the metal spoon, coin, key, pin, sewing needle, aluminium foil and pencil lead.
It does not glow for cork, rubber, glass, the plastic scale, the wooden block, the candle,
cardboard and paper.

This shows that electric current can pass easily through some materials but not through others
— which is exactly the point of the activity.

Check it yourself: The two wires must never touch each other while testing. If they
do, current takes the short cut and the lamp glows even for a piece of wood, giving a
wrong result.

Page 36 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Q3 Record your observations in Table 3.3.

Table 3.3: Identifying Conductors and Insulators

S.NO. OBJECT MATERIAL LAMP CONCLUSION
IT IS MADE GLOWS (CONDUCTOR/INSULATOR)
UP OF (YES/NO)

1. Stick Wood No

2. Scale Plastic

3. Bangle Glass

4. Paper Paper
strip

5. Candle Wax

6. Key Metal

7. Eraser Rubber

8.

9.

Table 3.3, page 35 — the recording table for conductors and insulators.

A completed Table 3.3 looks like this. Rows 8 and 9 are left for objects of your own choice — two
useful ones are given here.

Page 37 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

S.NO. OBJECT MATERIAL IT IS LAMP CONCLUSION
MADE UP OF GLOWS (CONDUCTOR/INSULATOR)
(YES/NO)

1. Stick Wood No Insulator

2. Scale Plastic No Insulator

3. Bangle Glass No Insulator

4. Paper strip Paper No Insulator

5. Candle Wax No Insulator

6. Key Metal Yes Conductor

7. Eraser Rubber No Insulator

8. Coin Metal Yes Conductor

9. Aluminium Aluminium Yes Conductor
foil (metal)

Try This: Test a pencil sharpened at both ends. The lamp glows through the lead
(graphite) but not through the wood — one object, two different answers.

In-text Questions — Page 36
Section 3.4 Electrical Conductors and Insulators

Q1 Analyse your observations. Did the lamp glow for all materials?

No. The lamp glowed for some materials only.
That single observation carries the whole idea of the section:

Materials through which electric current can flow easily are called good conductors, or
conductors of electricity.
Materials through which current cannot pass are called insulators, or poor conductors of
electricity.

Page 38 of 60

as e
a g l
Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

co m
m.
Why it happens: In a metal, some of the tiny charged particles are free to move

m l a se
through the whole piece, so they can carry current from end to end. In wood, plastic,
o
m .c the charged particles are held tightly in place andagcannot travel, so
glass or rubber
se flows.
no acurrent
l
ag
o m
e
. c
mrecorded in Table 3.3, conclude which materials ag
s
Based on the observations you have
a which are insulators. Note it in Table 3.3.
Q2

agl
are conductors of electricity and

co m
m.
Table 3.3: Identifying Conductors and Insulators

m as e
.coS.NO. OBJECT MATERIAL LAMP
a g
CONCLUSIONl
a s em IT IS MADE GLOWS (CONDUCTOR/INSULATOR)

ag l UP OF (YES/NO)

a s
com
1. Stick Wood No

m . agl
ase
2. Scale Plastic

3. Bangle gl
aGlass

co m
.
4. Paper Paper
strip
e m
m l as
.co 5. a g
sem
Candle Wax

a
agl 6. Key Metal

se m
Eraser Rubber
com l a
7.

. a g
m
ase
agl
8.

9.

co m
m
Table 3.3, page 35 — the recording table for conductors and insulators.
.
m as e
.co a g l
a s em
agl ANSWER
.c
Sorting the objects of Table 3.3 by material gives a clean division:
s e m
m a
e m . co agl
g l as
a

co m
m .
m ase
.co


a g l Page 39 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

CONDUCTORS OF ELECTRICITY INSULATORS (POOR CONDUCTORS)

Metals — key, coin, spoon, pin, sewing needle, aluminium foil Wood — stick, wooden block

Pencil lead (graphite) Plastic — scale, wire covering

Glass — bangle

Rubber — eraser, cork

Paper and cardboard — paper strip

Wax — candle

The main conclusion is that metals are conductors of electricity, and that is why they are used
for making wires. Silver, copper and gold are the best conductors, but wires are mainly made of
copper because it costs much less and is available in plenty.

Page 40 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Q3 From Table 3.3, you would have also realised that plastic, rubber, and ceramics are
electrical insulators. Have you now understood why wires are covered with those
materials?

Table 3.3: Identifying Conductors and Insulators

S.NO. OBJECT MATERIAL LAMP CONCLUSION
IT IS MADE GLOWS (CONDUCTOR/INSULATOR)
UP OF (YES/NO)

1. Stick Wood No

2. Scale Plastic

3. Bangle Glass

4. Paper Paper
strip

5. Candle Wax

6. Key Metal

7. Eraser Rubber

8.

9.

Table 3.3, page 35 — the recording table for conductors and insulators.

Yes. The metal inside carries the current; the plastic or rubber outside makes sure the current
cannot reach our hands.

PART OF A DEVICE MADE OF BECAUSE

Wire core, switch contacts, plug pins, socket Conductors (copper, brass) Current must flow through
connectors them

Wire covering, plug top, switch body, holder Insulators (plastic, rubber, Current must not flow out of
ceramic) them

Page 41 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Why it happens: Our body is itself a conductor of electricity. Current passing
through the body may cause severe injury or even death. The insulating cover is the
barrier that stands between the current and the person holding the wire — which is
why a wire with cracked or peeled insulation must never be used.

Let Us Enhance Our Learning — Pages 37–39
End-of-chapter questions

Q1 Choose the incorrect statement. (i) A switch is the source of electric current in a
circuit. (ii) A switch helps to complete or break the circuit. (iii) A switch helps us to
use electricity as per our requirement. (iv) When the switch is in ‘OFF’ position,
there is an air gap between its terminals.

The incorrect statement is (i) — 'A switch is the source of electric current in a circuit.'
A switch is not a source of current at all. The source in these circuits is the electric cell or
battery; the switch only controls whether the current is allowed to flow.

STATEMENT CORRECT? REASON

(i) A switch is the source of electric current Incorrect The cell or battery is the source; the switch
supplies no energy

(ii) A switch helps to complete or break the Correct This is exactly what a switch does
circuit

(iii) A switch helps us use electricity as per Correct We can switch a fan or lamp on only when it
our requirement is needed

(iv) In 'OFF' position there is an air gap Correct Air is an insulator, so the gap stops the
between its terminals current

Page 42 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Q2 Observe Fig. 3.16. With which material connected between the ends A and B, the
lamp will not glow?

A

B

+

–

Fig. 3.16, page 37 — a tester circuit with a gap between the ends A and B. Redrawn.

The lamp will not glow if the material connected between A and B is an electrical insulator.
Fig. 3.16 is a conduction tester — a cell and a lamp joined by wires whose two ends, A and B, are
left free. Whatever is placed between A and B becomes part of the circuit.

MATERIAL JOINED BETWEEN A AND B TYPE LAMP

Rubber, plastic, wood, glass, paper, wax, dry cloth, thermocol Insulator Does not glow

Copper wire, iron nail, key, coin, aluminium foil, pencil lead Conductor Glows

Why it happens: Current cannot pass through an insulator, so the loop stays broken
at the gap between A and B even though the cell and lamp are perfectly good. Only
a conductor bridges the gap and completes the circuit.

Page 43 of 60

as e
a g l
Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

co m
m.
In Fig. 3.17, if the filament of one of the lamps is broken, will the other glow? Justify
e
Q3

m l as
.co
your answer.

a g
se m
g l a
a

co m
e m . ag
g l as
a

co m
em.
m l as
m .co a g
l a se
a g Fig. 3.17, page 38 — two lamps and a battery in one loop.

m a s
m .co agl
l a se
a g

No, the other lamp will not glow either.
In Fig. 3.17 the two lamps and the battery are joined one after another in a single loop — the
co m
m .
e
current has only one path to follow, and that path passes through both filaments.
m l as
m .co a g
l a se
ag lamp 1 lamp 2 — filament broken

se m
com g l a
m . a
ase
agl
no current anywhere

co m
m .
m as e
.co a g l
se m
g l a battery
a c
m .
e
The two lamps lie in the same single loop. A break in one filament opens the whole loop, so no

m a s
co agl
current flows and neither lamp glows.

m .
as e
a g l

co m
m .
m ase
.co


a g l Page 44 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Justification: A broken filament leaves a gap inside that bulb. The circuit is therefore open, so
no current flows through any part of the loop — including the second lamp. The second lamp is
perfectly good, but there is no current for it to use.

Did you know? Old strings of decorative lights were wired like this, which is why one
fused bulb used to switch off the whole string.

Q4 A student forgot to remove the insulator covering from the connecting wires while
making a circuit. If the lamp and the cell are working properly, will the lamp glow?

No, the lamp will not glow.
The plastic covering on a connecting wire is an insulator. If it is not stripped off at the ends, the
copper inside never actually touches the terminals of the cell or of the lamp.

Cell terminal → plastic → copper → plastic → lamp terminal

Current cannot cross the plastic → circuit is open → lamp stays dark

Why it happens: A circuit needs an unbroken path of conductors from one terminal
of the cell to the other. Insulating plastic at the two joints breaks that path just as
surely as a missing wire would. This is why the activities tell you to remove about 1
cm of the covering from both ends of every wire.

Tip: After stripping, the exposed copper should be shiny. If it looks dull or blackened,
rub it gently with sandpaper — a coating of dirt or oxide can also stop the current.

Q5 Draw a circuit diagram for a simple torch using symbols for electric components.

A simple torch has just three components in a single loop — a battery of two cells, an electric
lamp, and a switch, joined by wires.

Page 45 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

electric lamp

current

+ –

battery (two cells) switch (ON)

Circuit diagram of a simple torch: a battery of two cells, a switch shown in the ON position, and an
electric lamp, all in one loop. Current is taken to flow from the + terminal, through the lamp, back to
the – terminal.

Points to check in your own drawing:

The long line of each cell is its + terminal, the short line its – terminal, and the cells are
drawn long–short, long–short.
The lamp is a circle with a cross inside it.
The switch is two small circles; join them with a straight line for 'ON', lift the line for 'OFF'.
All the wires are plain straight lines and the loop must be closed for the lamp to glow.

Tip: If your torch uses an LED instead of a bulb, replace the lamp symbol with the
LED symbol and make sure the triangle points away from the + terminal of the
battery.

Page 46 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Q6 In Fig. 3.18: (i) If S2 is in ‘ON’ position, S1 is in ‘OFF’ position, which lamp(s) will glow?
(ii) If S2 is in ‘OFF’ position, S1 is in ‘ON’ position, which lamp(s) will glow? (iii) If S1
and S2 both are in ‘ON’ position, which lamp(s) will glow? (iv) If both S1 and S2 are in
‘OFF’ position, which lamp(s) will glow?

L1 S2 L2

S1

Fig. 3.18, page 38 — lamps L₁ and L₂ with switches S₁ and S₂, both switches drawn OFF.

Look at Fig. 3.18 carefully: the battery, S1, L1, S2 and L2 all lie on one single loop. There is no
branch anywhere, so current must pass through every component or through none.

CASE S1 S2 IS THE LOOP COMPLETE? LAMP(S) THAT GLOW

(i) OFF ON No — broken at S1 None

(ii) ON OFF No — broken at S2 None

(iii) ON ON Yes — the loop is closed Both L1 and L2

(iv) OFF OFF No — broken at two places None

Answers in one line each:

1. No lamp glows — S1 is open, so the circuit is broken.
2. No lamp glows — S2 is open, so the circuit is broken.
3. Both L1 and L2 glow — the circuit is complete and the same current passes through both
lamps.
4. No lamp glows — the circuit is broken at both switches.

Page 47 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Why it happens: In a single loop every component shares one path. Opening a
switch anywhere in that path stops the current everywhere in it. That is also why a
switch can be placed anywhere in a circuit and still control the whole circuit.

Q7 Vidyut has made the circuit as shown in Fig. 3.19. Even after closing the circuit, the
lamp does not glow. What can be the possible reasons? List as many possible
reasons as you can for this faulty operation. What will you do to find out why the
lamp did not glow?

+ Safety pin

–

Fig. 3.19, page 38 — the circuit Vidyut made, with a safety-pin switch on a piece of
cardboard. Redrawn.

The circuit in Fig. 3.19 has only four kinds of parts — the cell, the lamp, the wires and the switch
— so the fault must lie in one of them.
Possible reasons:

Page 48 of 60

as e
a g l
Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

co m
m.
PART WHAT COULD BE WRONG

m as e
Cell
. c o a g l
The cell is used up (dead), or has been placed in the holder the wrong way round, or is not

sem
pressed firmly against the spring and the contact

gla
aLamp The lamp is fused — its filament is broken; or it is not screwed tightly into its holder, so a
terminal is not touching

co m
Wires
e m .
The plastic covering has not been removed from a wire end; a wire is broken inside its ag
g l as
covering; a wire has come loose from a screw or a drawing pin; the exposed copper is dirty or
rusted a
Switch
co
The safety pin is not really touching the second drawing pin, so the gap is still open; or the
m
e m.
drawing pins have become loose in the cardboard

o m l a sare
Holders
.c
m contact
The screws of the lamp holder or the cell holder are loose, so
ag
the joints not making proper

l a se
g
a How to find out — test one part at a time:
m a s
.co agl
1. Check the switch first. Bypass it — touch the two wires that go to the drawing pins directly

a s em
to each other. If the lamp now glows, the fault was in the switch.

a gla cell that is known to be working. If the lamp glows, the old
2. Check the cell. Replace it with
cell was dead.
3. Check the lamp. Put the doubtful lamp into a torch that works, or replace it with a lamp
co m
m .
as e
known to be good. Also hold the bulb up to the light and look for a broken filament.
m l
.co a g
4. Check the wires and joints. Look at every end for unstripped plastic; tighten every screw

a s em and drawing pin; gently tug each wire to see whether it is broken inside.

agl 5. Make a tester. Use a cell and a lamp that surely work, leave two ends free, and touch them
m
across each wire in turn — the lamp glows only through a wire that is whole.

a se
. com a g l
e m
Tip: Change only one thing at a time and test after each change. If you change two

g l
things together and the lampas lights up, you will never know which one was the fault.
a

co m
m .
m as e
.co a g l
se m
g l a
a c
m .
m a s e
e m . co agl
g l as
a

co m
m .
m ase
.co


a g l Page 49 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Q8 In Fig. 3.20, in which case(s) the lamp/LED will not glow when the switch is closed?

(a) (b)

(c) (d)

Fig. 3.20, page 39 — four circuits. In a cell symbol the long line is the + terminal and the
short line the – terminal.

Only in case (c) the LED will not glow. In (a), (b) and (d) the lamp or LED lights up when the
switch is closed.

CASE WHAT THE CIRCUIT WILL IT REASON
CONTAINS GLOW?

(a) One cell and an electric Glows Complete circuit; an incandescent lamp works
lamp whichever way the cell is connected

(b) One cell and an electric Glows Same as (a) — reversing the cell only reverses the
lamp, cell reversed direction of current, and the filament still heats up

(c) Battery of two cells and Does not The LED is connected the wrong way round — its
an LED glow triangle points towards the + terminal of the
battery, so current is blocked

(d) Battery of two cells and Glows Here the triangle points away from the + terminal,
an LED, battery reversed so current can pass through the LED

Page 50 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Why it happens: An incandescent lamp does not care about direction — current
heats its filament either way. An LED conducts in one direction only. In the LED
symbol the triangle shows that direction, so trace the current from the long line (+)
of the battery: if it meets the triangle point-first it passes, if it meets the bar first it is
blocked.

Check it yourself: In (c), simply interchanging the two wires at the LED — or turning
the battery round — would make it glow, exactly as in Activity 3.7.

Q9 Suppose the ‘+’ and ‘–’ symbols cannot be read on a battery. Suggest a method to
identify the two terminals of this battery.

Use an LED, because an LED conducts in one direction only and so acts as a direction-detector.
Method:

1. Take an LED and note its two wires — the longer wire is its + terminal, the shorter one its –
terminal.
2. Connect the longer wire of the LED to one terminal of the battery and the shorter wire to the
other terminal, using two connecting wires.
3. If the LED glows, the terminal joined to the longer wire is the positive (+) terminal of the
battery, and the other one is the negative.
4. If the LED does not glow, interchange the two connections. When it lights up, read off the
terminals in the same way.

OBSERVATION CONCLUSION

LED glows The terminal joined to the LED's longer wire is +

LED does not glow, but glows after interchanging The terminal now joined to the longer wire is +

A second, simpler check — look at the cells themselves. In every cell the small protruding
metal cap is the positive terminal and the flat metal disc is the negative terminal. In a cell
holder the spring always presses on the flat disc, so the spring end of the holder is the negative
terminal of the battery.

Caution: Never try to test a battery by touching it with your tongue or by joining its
two terminals directly with a wire. Use an LED or a lamp — that is both safe and
reliable.

Page 51 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Q10 You are given six cells marked A, B, C, D, E, and F. Some of these are working and
some are not. Design an activity to identify which of them are working. (i) List the
items that you require. (ii) Write the procedure that you will follow. (iii) With the
items, carry out the activity to identify the cells that are working.

The idea is to build one test circuit and pass every cell through it in turn, changing nothing else.
Then the only thing that can change the result is the cell itself.
(i) Items required

The six cells marked A, B, C, D, E and F
One cell holder (for a single cell)
One torch lamp (incandescent bulb) and a lamp holder
Four short lengths of connecting wire, with about 1 cm of covering removed from each end
One cell that is known to be working, to check the test circuit itself
A notebook to record the observations

(ii) Procedure

1. Fix the lamp in the lamp holder and join two wires to the screws of the holder.
2. Join two wires to the two ends of the cell holder.
3. Connect one wire of the lamp holder to one wire of the cell holder. Keep the other two ends
free — these two free ends will be joined for each test.
4. First test the tester. Put the known good cell in the holder, with its negative terminal
towards the spring, and join the two free ends. The lamp must glow. If it does not, tighten
the joints and check the bulb before going further.
5. Take out the good cell. Put cell A in the holder the same way, join the free ends and watch
the lamp.
6. Record 'glows' or 'does not glow'. Take A out and repeat exactly the same steps with B, C, D, E
and F, one at a time.
7. A cell for which the lamp glows brightly is working; a cell for which the lamp does not glow
(or glows very faintly) is not working.

(iii) Carrying out the activity — record your results in a table like this. The entries below are
only a sample; write down what your six cells actually show.

Page 52 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

CELL DOES THE LAMP GLOW? CONCLUSION

A Yes, brightly Working

B No Not working

C Yes, brightly Working

D No Not working

E Yes, brightly Working

F Very faint glow Almost used up — treat as not working

Tip: Keep the same lamp, the same wires and the same joints for all six tests. If you
change the bulb halfway through, a dark lamp may be blaming the wrong thing.
Also do not leave a cell connected for long — it drains the cell.

Q11 Using an LED that requires two cells in series to glow, Tanya made the circuit as
shown in Fig. 3.21. Will the lamp glow? If not, draw the wires for correct
connections.

– +
–

+
– +

Fig. 3.21, page 39 — the circuit Tanya made with two cells and an LED. Redrawn.

No, the LED will not glow. Tanya has made two mistakes at once.

The cells are not in series. In Fig. 3.21 the black wire joins the negative terminal of one cell
to the negative terminal of the other. Two cells joined – to – push current in opposite
directions, so their effects cancel and the battery gives no useful push at all.

Page 53 of 60

as e
a g l
Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

The LED is joined the wrong way round. The wire from a positive terminal of a cell reaches
co m
the negative (shorter) wire of the LED, and current can pass through an LED in one
e m.
m l as
.co g
direction only.

m a
l a se
Correct connections:

a g
m wire → longer (+) wire of LED
coyellow
. ag
cell 1
e m
g l a–s
+ a –

co m
m.
link:
+ to –
+ las
e
. com – + ag
m
ase
LED

agl cell 2
green wire → shorter (–) wire of LED
a s
.com agl
m
gl ase
Correct wiring: one cell is turned round so that the link wire joins the + terminal of one cell to the –

a
terminal of the other — that makes a battery of two cells in series. The free + terminal then goes to
the longer wire of the LED and the free – terminal to the shorter wire.

co m
m .
The two rules to follow while re-drawing the wires:
m as e
.co a g l
em
1. Join the + terminal of one cell to the – terminal of the other cell. The two terminals left

l a s free are then the + and – terminals of the battery.
ag 2. Join the + terminal of the battery to the longer wire of the LED and the – terminal of the
battery to the shorter wire of the LED.
se m
com g l a
m . a
ase
Why it happens: Two cells help each other only when they are joined + to –; joined –

agl
to – they oppose each other. And even a correct battery cannot light an LED that is
facing the wrong way, because an LED conducts in one direction only. Both
conditions must be satisfied together.
co m
m .
m as e
.co a g l
a s em
agl Exploratory Projects — Pages 39–40
.c
s e m
m a
e m . co agl
g l as
a

co m
m .
m ase
.co


a g l Page 54 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Interdisciplinary — Geography, History, Technology

EXPLORATORY PROJECTS

Q1 Suppose that due to some problem, the power supply is disrupted in your area for
two days. List out which actions from your daily life you would not be able to do.

Go through one ordinary day, hour by hour, and mark everything that needs electricity. That is
the honest way to make this list.
Sample answer — a list for two days without power:

TIME OF WHAT I WOULD NOT BE ABLE TO DO
DAY

Morning Pump water to the overhead tank, use the geyser for a hot bath, grind masala in the
mixer, toast bread, iron the school uniform, get news from TV

School / work Use fans and lights in the classroom, use a computer or projector, charge a phone, use
hours the lift, run the school water cooler

Evening Switch on lights to study, run the fan or cooler, watch TV, use the washing machine, use
the electric sewing machine

Night Keep milk and vegetables cold in the refrigerator, use street lights while walking, charge
the emergency lamp

Any time Draw money from an ATM, get a photocopy, use a shop's card machine, use a water
purifier

What still works — anything that runs on cells or batteries: a torch, a wall clock, a wrist watch,
a mobile phone until its battery runs out, a radio with cells. This is exactly why the chapter calls
a cell a portable source of electrical energy.

Try This: Also note down what you would do instead — study by daylight, cook on a
gas stove, store water in buckets. A good project compares 'what stops' with 'how
people manage'.

Page 55 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Q2 Using a solar panel (Fig. 3.22a) as a source of electrical energy, make a circuit to run
a toy fan (Fig. 3.22b) as shown in Fig. 3.22c.

Motor

(a) Solar panel (b) Toy fan (c)

Fig. 3.22, page 40 — (a) a solar panel, (b) a toy fan on a motor, (c) the two joined so that
the fan runs. Redrawn.

The solar panel simply takes the place of the cell or battery — everything else you have learnt
about circuits stays the same.
What you need: a small solar panel, a toy fan (a small motor with a plastic blade), two
connecting wires with the ends stripped, and bright sunlight.
How to do it:

1. Find the two terminals of the solar panel; they are usually marked + (red wire) and – (black
wire).
2. Join one terminal of the panel to one terminal of the toy fan's motor with a connecting wire.
3. Join the other terminal of the panel to the other terminal of the motor. The loop is now
complete.
4. Hold the panel flat in bright sunlight. The fan starts turning.
5. Now cover the panel with your hand or a piece of cardboard. The fan slows down and stops
— proof that the panel, not the wire, is the source.

Things worth observing and writing in your report:

The fan runs fastest when the panel faces the Sun squarely, and slows down when the panel
is tilted or shaded.
If you interchange the two wires, the fan turns the other way — the motor reverses because
the direction of current has reversed.
The fan does not run indoors under an ordinary bulb, or runs very slowly — sunlight is far
stronger than room light.

Page 56 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Why it happens: A solar panel changes the energy of sunlight directly into electrical
energy, just as a cell changes chemical energy into electrical energy. Both give a
direct current (DC), which is why the same simple circuit works with either.

Q3 Visit an electrical items shop. With the help of the shopkeeper, identify the various
types of cells available. For each cell, also find out which device(s) it is used for.
Prepare a report.

Carry a notebook, ask the shopkeeper to show you each type, and note its shape, size, marking
and use. Do not touch or open any cell.
Sample report — cells commonly found in a shop:

TYPE OF CELL WHAT IT LOOKS LIKE DEVICES IT IS USED IN

AA (pencil cell) Cylindrical, about 5 cm long Torch, wall clock, TV remote, toys

AAA Cylindrical, thinner and shorter Small remotes, computer mouse, digital
than AA thermometer

C and D cells Thick cylinders Big torches, emergency lamps,
transistor radios

9 V battery Small rectangular block with two Multimeters, smoke alarms, small
studs on top experiment kits

Button cell Flat and round like a coin Wrist watches, hearing aids, calculators,
car keys

Rechargeable battery Flat pack or cylinder, marked Mobile phones, laptops, power banks,
(Li-ion) 'rechargeable' electric vehicles

Lead-acid battery Large, heavy box with two thick Cars, inverters at home
terminals

Also find out and write about: the voltage printed on each cell (1.5 V on most pencil cells, 9 V
on the block, 3 V on many button cells), the price, how long it lasts, and which of them can be
recharged.

Did you know? Used cells should never be thrown into ordinary dustbins. The
chemicals inside can harm soil and water, so they must go to a battery-collection or
e-waste bin. Add a line about this in your report.

Page 57 of 60

Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Q4 Prepare a list of objects in your home under three categories: (i) Objects which are
electrical insulators only (ii) Objects which are electrical conductors only (iii)
Objects which are made of both, whose some parts are insulators and some
electrical conductors

Walk through the house and ask one question for each object: what material is it made of? Then
place it in the right column.
Sample answer:

(I) INSULATORS ONLY (II) CONDUCTORS (III) BOTH PARTS TOGETHER
ONLY

Plastic bucket Steel spoon Screwdriver — metal rod with a plastic handle

Wooden chair, wooden ruler Iron nail, safety pin Electric wire — copper inside, plastic outside

Glass tumbler, glass bangle Aluminium foil, copper Plug top — brass pins in a plastic body
coin

Rubber eraser, rubber slipper Brass lock, steel scale Torch — metal contacts inside a plastic body

Candle, cotton cloth, paper, Iron key, steel plate Switch board, iron (press) with plastic handle, pressure
ceramic cup cooker with bakelite handles

Why it happens: Category (iii) is the most interesting one. Wherever we must use
electricity but also touch the device, the designer puts a conductor where the current
has to go and an insulator where our hand has to go. A screwdriver used by an
electrician, a plug top and an electric wire are all built on that one idea.

Try This: Pick up any object from column (iii) and mark with your finger exactly
where the conductor ends and the insulator begins. That boundary is what keeps
the user safe.

Chapter at a glance
An electric cell is a portable source of electrical energy. Its metal cap is the positive (+)
terminal and its flat metal disc is the negative (–) terminal.

Page 58 of 60

as e
a g l
Class 7 Science Chapter 3 Electricity: Circuits and their Components AglaSem · NCERT Solutions

Two or more cells joined so that the + terminal of one touches the – terminal of the next
co m
se
make a battery. A battery gives energy for a longer time and/or more energy.
m.
o m l a
g wire is the +
c current can pass through it in one direction only — itsalonger
.and
An incandescent lamp glows because current heats its thin filament. An LED has no

se
filament, m
l a
ag
terminal.
A lamp glows only when the circuit is complete, that is, when one terminal of the lamp is

m
joined to one terminal of the cell and the other to the other terminal. The direction of
. co ag
m
current is taken from + to – of the cell.

as e
g l
A switch simply completes (ON, closed circuit) or breaks (OFF, open circuit) the path. It can
a
be placed anywhere in the circuit.
Materials through which current flows easily are conductors (metals such as copper, silver,

co m
m.
gold); those through which it cannot pass are insulators (rubber, plastic, glass, wood,
ceramics).
m as e
.co a g l
se m
g l a
a
m a s
m .co agl
l a se
a g

co m
m .
m as e
.co a g l
se m
g l a
a
se m
com g l a
m . a
ase
agl

co m
m .
m as e
.co a g l
se m
g l a
a c
m .
m a s e
e m . co agl
g l as
a

co m
m .
m ase
.co


a g l Page 59 of 60