AP POLYCET Study Material for Physics

Refraction of Light at Plane Surfaces
⦁ Refraction of light: The bending of light ray when it is travelling from
one medium to another medium is called Refraction.
⦁ In refraction, speed of light changes at the interface.
⦁ The direction of light ray changes from one medium to another
medium.

Fig: shows light travelled from lighter to denser medium.

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Fig: shows light travelled from denser to lighter medium.

⦁ The refractive index of water is 1.33,where as ice is 1.31. Hence ice
is less denser than water.

⦁ The refractive index of Diamond is 2.42 is the highest.
⦁ When light travels from rarer medium (air) to denser medium (glass),
Light bends towards the normal in the denser medium, i.e wavelength
decreases as speed of light decreases.
⦁ Angle of refraction(r) is lesser than angle of incidence (i).
⦁ When light travels from denser medium (glass) to rarer (air) then, Light
bends away from the normal in rarer medium. i.e wavelength increases
as speed of light increases.
⦁ Angle of refraction(r) is greater than the angle of incidence (i)

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⦁ Examples of refraction:

(1) The pencil appears to be bent when it is placed in a bucket filled with water.
(2) A coin kept at the bottom of a vessel filled with water appears to be raised.

(3) A lemon kept in a glass of water appears to be bigger than its original size.

(4) The printed letters below the glass slab appears to be raised.

⦁ No bending of light when incidents normally at the interface of two
mediums.

⦁ Refractive index is a property of transparent medium, and
dimensionless quantity, and it has no units.
⦁ Absolute refractive index = Speed of light in vacuum(c)/Speed of light
in medium(v)

n = c/v
⦁ If refractive index is high, speed of light in medium is low.
⦁ Refractive index depends on 1) nature of material, 2) wavelength of
light
⦁ Relative Refractive index(nr) = Speed of light in medium 1
speed of light in medium 2

⦁ nr = n2 / n1 = Refractive index of second medium (n2)

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Refractive index of first medium (n 1)

⦁ Laws of refraction:

⦁ The incident ray, the refracted ray and the normal to the
interface of two transparent media, all lie on the same plane at the
point of incidence
⦁ Light follows Snell’s law in refraction.

⦁ Snell’s law:
Sin i / Sin r = n2 / n1 = constant

⦁ Critical angle: The angle of incidence is called critical angle i.e
(i=c) when the angle of refraction is 900.

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⦁ From snells law, Sin C/ Sin 900 = n2/n1
NOTE: this occurs only when light ray travels at the interface of denser
medium to rarer medium
⦁ Total internal reflection(TIR): If the angle of incidence is greater
than the critical angle (when the light ray passes from denser medium
to rarer medium) then the light ray totally internally reflected back
into the denser medium. This is called “total internal reflection”.

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NOTE: Incident angle and reflected angle may or may not be equal.
Examples: 1) Formation of Mirages is due to Total internal
reflection.
2) The brilliance of diamonds is also due to TIR as its
critical angle is low (24.40)
⦁ Optical fibres: It is a cylindrical wave guide which transports light
energy. Its works on the basic principle of Total internal reflection.

⦁ Applications of Optical fibres:
1) Laparoscopic surgery 2) Endoscopic surgery 3) Decorative
flower vases 4) Internet cables

Multiple Choice questions:

⦁ For critical angle, the angle of refraction is [ ]

a) 900 b) 450 c) 600 d) 1800
2)Which of the following is Snell’s law
⦁ n 1 sin i = sin r/ n 2 b) n 1/ n2 = sin r/sin i
⦁ n2/ n1 = sin r/ sin i d) n2 sin i = constant
3)The refractive index of glass with respect to air is 2. The critical angle of

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glass - air interface is [ ]

⦁ 00 b) 450 c) 300 d)600

4).Total internal reflection takes place when the light ray travels from
[ ]
a) Denser to rarer medium b) Denser to denser medium
a) Rarer to denser medium d) rarer to rarer medium

5)The angle of deviation produced by glass slab is [ ]
a) 00 b) 200 c) 900 d) Depends on the light ray and
normal to slab.
6) Mirage is an example of [ ]
a)Total internal reflection b) Refraction c)
Reflection d)None of the above.
7) Refractive index of glass relative to water is 9/8. What is the
refractive index of water relative to glass? [ ]
a) 9/8 b) 8/9 c)1/9 d) None
8) Optical fibre works on [ ]
⦁ Reflection b) Refraction
⦁ c) Total internal reflection d) All of these

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Refraction of light at Curved Surfaces
⦁ The refraction at curved surfaces is an interesting phenomina. For
example, spectacle used by humans to see objects, rearview mirrors,
and optical telescopes to gauge stars.
⦁ A curved surface is a part of a sphere,The centre of the sphere is centre
of the curvature (C)of curved surface.
⦁ The centre of curved surface is called the pole (P) of curved
surface.
⦁ The line that joins the centre of curvature and the pole is called
‘principal axis’.

⦁ The equation for refraction of light at curved surfaces is

(n2/v) - (n1/u) = (n1- n2) / R

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Where n1, n2 = the refractive indices of two material media of curved
surfaces,u = object distance, v = image distance, and R = Radius of
curvature

⦁ Focus or Focal Point: The point of convergence of rays (or) the point
where the rays appear to be emanating iscalled Focus(F) or focal point.
⦁ Every lens has two focal points.
⦁ The distance between optic centre and focal point is called “focal length
(f)”

⦁ The focal points are equidistant from the centre, i.e., pole of the lens.

⦁ The distance between two focal points = 2F1 = 2F2 = Twice the focal
length.

⦁ Behaviour of certain light rays When they incident on a lens:

⦁ A ray is undeviated when it passes through principal axis.
⦁ A ray is undeviated when it passes through the optic centre.
⦁ The rays travelling parallel to principal axis converge at the
focus ordiverge from the focus.
⦁ The light rays obey the principle of least time,i.e they travel
along shortest optical paths.
⦁ The ray passing through the focus after refraction will
take a path parallel to principal axis, this is called principle
of reversibility, i.e if we imagine the ray is moving in
opposite to the indicated direction then it reverses its path.

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⦁ The light rays incident on a lens at an angle appear to be
converge or diverge from a point lying on focal plane.
⦁ Centre of curvature:

⦁ It is the centre of sphere contains lens part. It is
denoted by C.
⦁ The distance between curved surface and centre of

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curvatureis called “radius of curvature (R)”.
⦁ If the lens contains two curved surfaces, it will
have two centres of curvatures namely C1 and
C2 .
⦁ The line joining two centres is called principal
axis.
⦁ The mid point of the lens is called optic centre (O).

⦁ Lenses: A lens is made up of transparent material, Bounded by two
spherical surfaces both or one is spherical surface.

Types of lenses:
Convex lens
plano convex lens
Concave lens
plano concave lens
.Convex Lens:

⦁ It may have two spherical surfaces bulging outside.
⦁ It is called double convex lens or biconvex lens.

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⦁ It is thick at the middle and thin at the edges.
⦁ These lenses are also called as converging lenses, i.e
light rays are being focussed or converges to a point.
Plano convex lens:

⦁ One side of the surface of the lens is plain and the other
surface is spherical in shape .
⦁
⦁
Concave lens:

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⦁ A double concave lens has two spherical surfaces.
⦁ It is thin at middle and thick at the edges.
⦁ Each curved surface of a lens is a part of a sphere.
⦁ These lenses are also called as diverging lenses.

Plano concave lens:

⦁ One side of the surface of the lens is plain and the other surface

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is spherical in shape and bulges inside.

Image Formations of Convex Lens for various positions of an object:

⦁ Object at infinity: A point sized image at focal
point will be formed.

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⦁ Object beyond the centre of curvature of Principal
axis: Imagewill be real, inverted and diminished formed
on principal axis between the points F1 and 2F1.
⦁ Object at the centre of curvature (at 2F2): The image
will be onanother side at 2F1, and a real, inverted and of
same size as the object.
⦁ Object between Centre of curvature (2F2) and Focal
point(F2):The image will be beyond 2F1 which is real,
inverted and magnified.
⦁ Object at focal point (F2): Image at infinity.
⦁ Object between Focus (F1) and optic centre:
⦁ Virtual, erect and magnified which can be
seen witheyes.
⦁ Cannot be caught on the screen.
⦁ This behaviour of Convex lens is useful to
construct a microscope.

⦁ Lens formula:

1/v- 1/u =1/f
For any lens with sign convention.
⦁ Focal length of a lens depends upon the surrounding medium.
⦁ Focal length of lens increases in water.
⦁ Lens maker’s formula: In the air medium, the relative refractive
index is the absolute refractive index(n) of the lens,

⦁ Where R1 and R2 are radii of curvature.

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⦁ Here the sign conventions of radius of curvatrure depends on the
direction of incident light rays impenging on the lenses.

Note: Always use sign convention.

⦁ If the refractive index of the medium is less than
convexlens, behaves as a convergent lens.
⦁ Convex lens behaves as a divergent lens, if the
refractiveindex of the transparent medium is
greater than lens.
⦁ Air bubble in water behaves as a diverging lens.

Multiple choice questions:

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⦁ The rays from the distant object falling on the convex lens pass
through
[ ]
a) Focus b) centre of curvature c) Pole d) Radius of
curvature
⦁ What is the focal length of the plano convex, when R is the
radius of the curvature of the surface, n is the refractive index of
the lens? [ ]
a) f = R b) f = R/2 c) f = R/(n-1) d) f = (n-1)/R
⦁ Real or virtual image is formed by which of the following lenses? [ ]
a) Bi convex lens b) Biconcave lens
c) Plano convex mirror d) all ofthese
⦁ The value of the focal length of the lens is equal to the value of
the imagedistance when the rays are [ ]
a) Passing through the optic centre b) parallel to the principal
axis
c) Passing through the focus d) In all these cases
⦁ Which of the following is the lens maker’s formula [ ]
a) 1/f = (n-1) (1/R1 + 1/R 2) b) 1/f = (n + 1) (1/R1 – 1/R2 )

c) 1/f = (n - 1) (1/R1 – 1/R2) d) 1/f = (n + 1) ( 1/R1 + 1/R2)
Questions on Concave Mirrors:
⦁ The image formed by concave mirror when the object is
held at adistance less than the focal length, is [ ]
a) Erect b) virtual and inverted c) inverted d) None
⦁ The property of which mirror when the object is held close

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less thanthe focal length is used as a shaving mirror [ ]
a) plane mirror b) convex mirror c)
concave mirror d) none
⦁ A mirror used by dentists is [ ]
a) concave mirror b) convex mirror c) Plane mirror d) None
⦁ Which mirror focuses the parallel sun rays at the focal
point of themirror
a) convex mirror b) concave mirror c) Any mirror d) plane mirror
⦁ Solar cooker works on the property of which mirror? [ ]
a) plane mirror b) convex mirror c) concave mirror d) none
⦁ Archimedes burnt the ships using which property of concave
mirror?
[ ]
a) Parallel rays converge at focal point of the mirror
b) Parallel rays diverge from pole
c) Deviated from centre of curvature after reflection
d) None of the above.
Q ) Watchmaker uses to repair.
a) Convex mirror b) concave mirror c) concave lens d) convex
lens
⦁ Pick the correct answer from the following two answers: [ ]
1. Focal length of a lens depends on the surrounding medium.
2. Focal length of a lens changes with object distance.
a) both (1) and (2) are true b) both (1) and (2) are false
(c) Only (1) is true (d) Only (2) is true
⦁ The size of the image formed by a convex lens is same as

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that ofthe object, when the object is placed [ ]
a) At the centre of curvature
b) Between the centre of curvature and focus
c) Beyond the centre of curvature
d) Between focusand centre.
⦁ The lens, which is thin, at the middle on both sides and
thicker, atthe edges is [ ]
a) bi- convex b) concavo- convex
c) plano – convex d) bi -concave
⦁ If 40 cm each is the object and image distances respectively
for aconvex lens, then the focal length is
a) 80 cm b) 40 cm c) 20 cm d) 25 cm

KEY
1. a 2. c 3. a 4. c 5. c 6. a 7. c 8. a 9. b
10. c 11. a 12. d 13. c 14. a 15. d 16. c

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Human Eye and Colourful World



Summary:

⦁ The maximum angle, at which humans can see the whole object is
called
angle of vision.
⦁ The angle of vision for a healthy human being is about 600.
⦁ It varies from person to person and with age.
⦁ Humans can see an object comfortably and distinctly when
held at adistance of 25 cm.
⦁ This distance of 25 cm is called least distance of distinct vision.
⦁ 7he least distance of distinct vision for children below 10 years of
age is 7 to 8 cm. For old people, it will be 1 or 2 m or even more.

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⦁ The eye ball is nearly spherical in shape.
⦁ The front portion is covered by a transparent protective membrane
called the ‘cornea’.
⦁ Behind the cornea, filled with a liquid called aqueous humor and
behind this a crystalline lens. It is responsible for the image formation.
⦁ Iris, is the coloured part, is the muscular diaphragm, with a small
hole, called pupil.
⦁ Iris controls the amount of light entering the eye through ‘pupil’.
⦁ Iris makes pupil to act as a “variable aperture” for light to enter into
the eye.
⦁ The light that enters the eye forms an image on the
retina
⦁ The distance between the lens and retina is about 2.5
cm
⦁ The image distance is fixed and is 2.5 cm for any
position of object.
⦁ the focal length of a lens depends on its material and
radii of curvature of lens.
⦁ The eye lens can change its shape with the help of ciliary muscle
attached to it which change its focal length by changing the radii of
curvature.
⦁ The process of adjusting focal length of lens is called
“accommodation” of lens.
⦁ The eye-lens forms a real and inverted image of an object on the
retina.
⦁ Retina contains 125 million receptors called “rods and cones”
which receive the light signals.
⦁ Rods identify the Intensity of light,
⦁ Cones identify the colour.
⦁ These are transmitted to the brain through the opticnerve fibres.
⦁ The vision becomes blurred due to “ accommodation defects” of the
eye.
Defects of Eye:
There are mainly three defects of eye:

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⦁ Myopia
⦁ Hypermetropia
⦁ Presbyopia
Myopia:
Cannot see objects at long distances.
Also called- near or short sightedness.
For these people, focal length is < 2.5 cm.
Image forms before the retina.
A concave lens is used to correct myopia.

Hypermetropia:
Cannot see objects at short distances.
Also called far or long sightedness.
For these people, focal length is > 2.27 cm.
Image forms beyond the retina.
A biconvex lens is used to correct hypermetropia.

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Presbyopia:

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⦁ The ability of eye decreases with age.
⦁ Near point disappears.
⦁ Difficult to see the nearby objects clearly and distinctly,
Due to weakening of ciliary muscles and flexibility of
eye lens.
⦁ Its common in aged people.
⦁ A person can suffer from both Myopia and
hypermetropia with aging.
⦁ To correct this defect of vision, bi - focal lenses which
contain both concave and convex lenses.
⦁ Upper portion is concave part, and lower convex part.

Power of Lens:
⦁ It is the degree of convergence or divergence of lightrays by a
lens.
⦁ It is the reciprocal of focal length in metre, (P = 1/f).
⦁ Unit of power of lens is dioptre. It is denoted by D.

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Refraction of light through a prism:

⦁ Refractive index of prism,
⦁ n ={ (Sin (A + D)/2)/Sin (A/2) }
Where n = refractive index of the prism, A = Angle of prism,
D = Angle of minimum deviation.
Dispersion of Light through a prism:

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⦁ The splitting of white light into colours (VIBGYOR) is called
Dispersion.
⦁ The refractive index of red colour is low and hence it suffers low
deviation.
⦁ The colours of Rainbow are due to dispersion of the
sunlight bymillions of tiny water droplets.

⦁ Dispersion of light into different colours i.e wavelengths is
based on wave nature of light (light behaves as wave,) i.e
electromagnetic wave.

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⦁ Here the refractive index of the prism is different for
different colours, i.e different wavelengths, this implies
that the different colours of light move with different
velocities in a medium.
⦁ In refraction, the frequency of light wave is same in both
the media(rarer & denser), i.e, the frequency remains
unaltered (will not change), i.e frequency is the property of
the source from where light is being is generated and
frequency is equal to no. of light waves leaving the source
per second. This cannot be changed by any medium. but
wavelength changes.
⦁ To know the relation between speed of light wave (v), wave
length (λ) and its frequency(f).
v = f λ, where (v = speed of light in medium,
f = frequency, λ = wave length).
⦁ The refraction of light at any interfaace, Vα λ ,i.e speed of
the wave increases with increase in wavelength of light and
vice versa.

Scattering of light:

⦁ It’s a complex phenomenon.
⦁ The process of re-emission of absorbed light in all
directions withdifferent intensities by atoms or molecules,
is called “scattering of light”.
⦁ The blue colour of sea water and sky is due to scattering of
light.
⦁ The Sun appears as red in the Sun rise and Sun set due to
less scattering of red light and to travel long distance to
reach us.
⦁ The Sun appears as white during noon time because water
molecules rise into the atmosphere due to rise in
temperature.
⦁ Sir C.V. Raman discovered the Scattering of light.
⦁ Raman experimentally found that frequency of
scattered light is greater than the frequency of
incident light. This is called “ Raman Effect”.

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⦁ Raman effect is used to determine the shapes of molecules.

Multiple choice questions:
⦁ The least distance of distinct vision is about
a)25 cm b) 50 cm c) 30 cm d) 15 cm
⦁ The distance between eye lens and retina is about

a)10 cm b) 2.5 cm c)2 cm d) 5 cm
⦁ The maximum focal length of eye lens is about
a) 2.5 cm b) 2.2 cm c)3 cm d) 1.5 cm
⦁ The power of lens is 1D then focal length is
a) 100 cm b) 50 cm c) 25 cm d)75cm
⦁ Myopia can be corrected by which lens
a) concave lens b) convex lens c) concavo-convex
d) Plano convex
⦁ The size of the object is perceived by an eye depends on
a)size of the object b) distance of the object from the eye
c) aperture of the pupil d) size of image on retina
⦁ A doctor advised to use 4D lens. The focal length of the lens is
a)25 cm b) 400 cm c) 4 cm d) 40 cm
⦁ Which part of the human eye helps the lens to change its focal
length?
a) Retina b) Pupil c) ciliary muscle d) cornea
⦁ For every position of an object in front of the human eye,
the imagedistance is fixed at
a) 1 cm b) 1.5 cm c) 2.5 cm d) 0.25 cm
⦁ To correct one’s hypermetropia defect, the type of lens used is
a) biconvex b) biconcave c) concavo- convex
d) Planoconcave

⦁ With an increase in angle of incidence of light ray on a prism,
theangle of deviation
a) remains constant b) first increases and then decreases
c)first decreases and then increases d) first increases

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and thenremains constant.
The scientific work of C.V. Raman is on
a) dispersion of light b) total internal reflection
c) defection of vision d) scattering of light
Scattering of light involves the process of
⦁ bending of light at the interface of two media
⦁ splitting of light into different colours
⦁ convergence of light rays at the focus
⦁ re -emission of absorbed light
Blue of sky is explained by
a) scattering of light b) total internal reflection
c) refraction of light d) dispersion of light
The sun appears red colour during sunset and sunrise, due to
a) scattering of red light is very small b) scattering of red light is
high
c) scattering of other colours is high d) none of these

KEY
1. a 2. b 3. a 4. a 5. a 6. b
7. a 8. c 9. c 10. a 11.c
12.d 13.d 14.a 15.a

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Electric Current

Lightning is an electric discharge between two clouds or between cloud
and earth. This electric discharge through air as an electric spark or
lightning.
Lightning is the motion of charge in the atmosphere.
All metals are good conductors of electric current.
The nature of the substance plays an important role (connecting
wires) in the transfer of energy from battery to bulb.
Drude and Lorentz proposed that positive ions in a metal (lattice
points) are fixed and negative electrons are free charge carriers.
The fixed arrangement of positive ions is called lattice.

Electric current = electric charge/time
I=Q/t
The SI unit of electric current is ampere denoted by A.
1Ampere=1Coloumb/1Second
The free electrons in a conductor are accelerated by the electric
field.
The movement of positive and negative charges in an uniform
electric field is shown below,

+ere top plate indicates negative charge, bottom plate indicates
positive charge.
Electrons move in a direction opposite to the direction of the electric field.

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The Electrons in the conductor move with a constant average speed
called drift speed or drift velocity.

ǁŚĞƌĞ
ŝƐ
ůĞĐƚƌŝĐ&ŝĞůĚ

I & E in the same direction , while electron flows opposite to the both
I&E
Drift velocity vd = I/nqA ,
I.e (q=e), hence vd=I/neA.
(where I=current, n=charge density, q=charge of electron, A=Area of
cros section ).

Ammeter
An ammeter is a device used to measure electric current.
An ammeter is always connected in series to the circuit.

$indicates ammeter and I indicates current including its direction.
The work done (W) in moving a charge(q) from one point to another point
in an electric field is defined as potential ,
V=w/q=Fl/q,
(where F is the force due to electric field and l is the distance between the
two points).
This potential difference between the two points is also called as voltage.
The SI unit of potential difference is“Volt” and it is denoted by V.
1Volt=1Joule/1Coulomb (1V=1J/C)

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⦁ Electromotive Force (emf): It is defined as the work done by the
chemical force to move unit positive charge from negative terminal to
positive terminal of the battery.

Ohm’s law :

⦁ Current passing through a conductor is directly proportional to the
potential difference between the two ends of it.
i.e., I α V , I = cV where c is constant ( c = 1/R )
I = V/R
V=IR, Where R is resistance of the conductor.
SI unit of Resistance: Ohm.
The symbol of Ohm is Ω.
1 Ohm = 1 Volt / 1 Ampere
1Ω = 1V/A
⦁ Ohm’s law for materials as classified into two categories.
⦁ Which obey Ohm’s law are called ohmic materials.
Ex: metals.
⦁ Which do not obey Ohm’s law are called non ohmic materials.
Example: LEDs.
NOTE: Ohm’s law is valid if the temperature of the material
(conductor) remains constant.

⦁ The resistance of the material changes with temperature.

⦁ V-I graph is non-linear when temperature changes.

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where I depends on V.

Ohm’s law is not applicable to gaseous conductors.
Ohm’s law cannot be applied to semi conductors.
Example: Germanium and silicon.

The resistance is the property of a conductor is defined as the
obstruction to the motion of the electrons in a conductor.
The material which offers resistance to the motion of electrons
is called resistor.

Factors affecting the resistance of a material:
Temperature, length, area of cross-section of the conductor, and nature of
the conductor.
The resistance (R) of a conductor is directly proportional to its length (l)
R α l (at constant temperature)
The resistance of a conductor inversely proportional to area of its
crosssection.
i.e Rα1/A(atconstanttemperature)
R= ρl/A ,(Where, ρ is a proportionality constant and is called
specific resistance or resistivity of the conductor).
The SI unit of resistivity is ohm-metre, Symbolically Ω-m.

The reciprocal of resistivity is called conductivity(σ), unit of conuctivity is
mho.
The value of resistivity of a material determine their conductivity.

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Equivalent Resistance of a Series Connection:
V=IReq
IReq=IR1+IR2+IR3

Req=R1+R2+R3
i.e. The equivalent resistance is equal to sum of individual resistances
when the resistors are connected in series.
⦁ One of the resistors in series breaks down, the circuit becomes
open, hence current flow does not take place.
⦁ Hence, household electrical appliances cannot be connected in series.

Equivalent resistance of a parallel connection
The equivalent resistance of a parallel combination is less than the
resistance of any one of the resistors.

⦁ Let two resistors R1 and R2 are connected in parallel,

1/Req = 1/R1+1/R2

Req = R1R2/(R1+R2)

⦁ Kirchhoff’slaws:
1.Current law (or) Junction law: At any junction in a circuit,the sum of
the currents entering into the junction must be equal to the sum of the currents

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leaving the junction.

We have I1+I2+I3=I4+I5

Kirchoff’s voltage law (or) Loop law:

The algebraic sum of potential differences in a closed loop of a circuit is
equal to zero.

ELECTRIC POWER

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⦁ Workdone: Let a charge q coloumb passing through a conductor from
one point to another point in an electric field having a potential
difference v and charge is travelling through length l in a time t.
The work done by electric field is given by:
W=F×d (where F=electric force,d=distance travelled)
W=E×q×d = E×l×q = V×q , (where E is the electric
field in the conductor through which charge q is travelling).
Energy lost by the charge per second =Work done per second = W/t
W/t =qV/t (we know q/t = I)
W/t = VI
(I= current flowing through the conductor, W/t = work done per second).

The work is equal to the energy lost by the charge when passing through the
conductor.

⦁ Electric power (P) : Power is the rate of doing work. (W/t).
P=W/t=VI
This equation can be used to calculate power consumption by any
electric device that is connected in a circuit.
According to the Ohm’slaw,
V=IR
P=I2R=V2/R (as P=V×I)
The equation P=VI can also be used to calculate the power which be
extracted from a battery or any source.
In this case modified equation P=VI

Example:

A bulb is marked 60W and 120V. This means that if this
bulb is connected to 120V source, it will able to convert 60w of
electrical power into heat or light in one second.

From the marking of bulb,we can measure the resistance of the bulb.
From the relation P=V2/R, i.e R=V2/P
Substituting the values V and P in above equation,

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We get R = 120x120/60 = 240 Ω
To calculate power as we knew P = V2/R = 1202/240 = 60 watts,
now according to the problem one should find the relation between watt and
joule by using following steps
⦁ Kilowatt is generally used to express power consumption.
⦁ 1 KW= 1000 W= 1000 J/S
The unit of electric power consumption is equal to 1 KWH (one
KiloWattHour).
1KWH= (1000) J/S/(60x60 )S=3600x 1000J=3.6x105 J

Multiple choice questions:
⦁ The kilowatt hour is the unit of..................
a.Power b. work c.energy d.None ofthese

⦁ A thick wire has a-------- resistance than a thin wire.
a.High b.low c.does not depend on thickness d.higher

⦁ The SI unit of current is
a.ampere b.volt c.ohm d.coulomb
⦁ A unit form wire of resistance 50Ω is cut into five equal parts.
These parts are now connected in parallel.Then the
equivalent resistance of the combination is
a.650Ω b.12Ω
c.250Ω d.2Ω
⦁ Check the following statements.
A. In series connection,the same current flows through each element.
B. In parallel connection,the same potential difference gets applied
across each element.
a)both A andB are correct b)A is correct but B is wrong
c)AiswrongbutB is correct d)bothAandBarewrong

KEY
1.a 2.b 3.a 4.d 5.a

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Electromagnetism

H.C. Oersted first observed that magnetic compass needle is deflected by
current carrying conductor.
Oersted concluded that electricity and magnetism are related phenomena.
The unit of magnetic induction field strength is named as Oersted
in his honour.
Magnetic field:
The region (or) space around a magnet where its influence is felt is called
“magneticfield”. The magnetic field varies with the distance from the
magnet and is characterized by strength and direction. It exists in all
directions i.e., it is three dimensional.

Magnetic field lines/Magnetic lines of force:
The path traced by a unit north pole in moving it near a magnet is called
magnetic field lines (or) magnetic lines of force.
All magnetic lines of force start at north pole and ends at the south pole
outside of a bar magnet, but inside the bar magnet, magnetic lines of
force appear at the south pole move towards to the north pole, further
these magnetic lines of force are continuous and closed loops
The tangent drawn to the field line at a point gives the direction of
the magnetic field.
The field is strong when the lines are crowded (near the poles of
the magnet) and if weak when the lines are apart.

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The magnetic field is said to be non-uniform, when strength or direction
changes from point to point.
The magnetic field is said to be uniform, if both strength and direction
are constant throughout the field.

Magnetic flux : The number of magnetic lines of force passing through
the plane of area ‘A’ perpendicular to the field is called “magnetic flux.”
It is denoted by ‘ɸ’.

The S.I. unit of magnetic flux is “weber”.

Magnetic flux density(B): It is defined as the magnetic flux passing
through unit area taken perpendicular to the field. It is also known as
magnetic field induction.
Magnetic flux density = Magnetic flux /Area.
B = ɸ/A or ɸ = BA

If plane makes an angle Ɵ with field then ɸ = BAcosƟ=B.A
(here B,A are vector quantities, i.e they have both direction and
magnitude).

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Unit of magnetic flux density is weber/(meter)2 or Tesla.

Magnetic field due to current carrying straight wire:

Current carrying in a wire produces magnetic field. The direction of
the magnetic field, around a current carrying wire is determined by
right hand thumb rule.
Thumb indicates the direction of current.
The curled fingers show the direction of magnetic field.

Magnetic field due to a circular coil:

The direction of the field is perpendicular to the plane of the coil.

With Right Hand Thumb rule, the thumb points the direction of

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magnetic field, the curled fingers show the direction of current.

Magnetic field due to solenoid:

One end of the solenoid behaves as north pole and the other end
behaves like a south pole.

Magnetic force on moving charge
Magnetic force on the charge = Charge x speed x magnetic flux
density

F = qvB

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Here direction of the velocity of charged particle is perpendicular
to the direction of the magnetic field ‘B’,
this indicates magnetic field is perpendicular to the plane of paper and
into the paper. The force experienced by the charged particle in above
figure is in the direction of Y axis.
If the angle(ɵ) between the velocity direction (V) and magnetic field
of direction(B), then the force experienced by the charged particle is
given by

NOTE: since Yɭƒ†%ɭ are vector quantities and the symbol x represents as
cross product and not as x what we read.

Magnetic force acting on current carrying conductor :
Magnetic force acting on current carrying conductor place in a magnetic
field is given by,
where I = Q/t where Q= total charge
I = Current in the wire, L = Length of the wire, B = Strength of uniform
magnetic field.
The force on the current carrying wire when angle between current and
magnetic field is Ɵ , given by (at any angle)

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F = ILBsinɵ
F=ILB (where ɵ = 90 degrees), sinɵ =1.

To find the radius of the path and time period of a charged particle:
We know that F = qvB,
r = radius of the circular path,
centripetal force = mv2/r
qvB = mv2/r

then,Time Period of the particle( T )= 2πr/v

The above equation after substitution becomes, T = 2πm/Bq

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Electric motor:
In an electric motor, electrical energy is converted into mechanical
energy.

Faraday’s law:

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When there is a continuous change in magnetic flux linked with a
closed coil, a current is generated or induced in the coil.
(OR)
“The induced EMF generated in a closed loop is equal to the
rate of change in magnetic Flux passing through it”.
Induced EMF = Change in magnetic flux/time
The consequence of Faraday’s law is the conservation of energy

Lenz’s law:
The law states that “the induced current will appear in such a direction
that it always opposes the changes in the flux of the coil.”

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Applications of Faraday’s laws of electromagnetic induction:
For security check
Tape recorder
ATM
Induction stove
In generators, mechanical energy is converted into electrical energy.

Multiple choice questions:

Which converts electrical energy into mechanical energy
Motor b) Battery c) Generator d) Switch
Electrical energy is converted into mechanical energy by which
device
Battery b) Motor c) Generator d) Switch
Mechanical energy is converted into electrical energy by which
device
Generator b) Motor c) Battery d) Switch
The magnetic force on a current carrying wire placed in uniform
magneticfield if the wire is oriented perpendicular to magnetic
field, is
0 b) ILB c) 2ILB d) ILB/2
If a conductor is moving with a speed of 10 m/s in

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the directionperpendicular to the direction of the
magnetic field of induction
0.8T, and induces an EMF of 8 V between the ends of coil, the
length ofthe coil is
a) 10 m b) 20 m c) 1m d) 100 m

KEY

1. a 2. b 3. a 4. b 5. c

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