NCERT Book Class 12 Physics Chapter 4 Moving Charges and Magnetism

Chapter Four

MOVING CHARGES
AND MAGNETISM

4.1 INTRODUCTION
Both Electricity and Magnetism have been known for more than 2000
years. However, it was only about 200 years ago, in 1820, that it was
realised that they were intimately related. During a lecture demonstration
in the summer of 1820, Danish physicist Hans Christian Oersted noticed
that a current in a straight wire caused a noticeable deflection in a nearby
magnetic compass needle. He investigated this phenomenon. He found
that the alignment of the needle is tangential to an imaginary circle which
has the straight wire as its centre and has its plane perpendicular to the
wire. This situation is depicted in Fig.4.1(a). It is noticeable when the
current is large and the needle sufficiently close to the wire so that the
earth’s magnetic field may be ignored. Reversing the direction of the
current reverses the orientation of the needle [Fig. 4.1(b)]. The deflection
increases on increasing the current or bringing the needle closer to the
wire. Iron filings sprinkled around the wire arrange themselves in
concentric circles with the wire as the centre [Fig. 4.1(c)]. Oersted
concluded that moving charges or currents produced a magnetic field
in the surrounding space.
Following this, there was intense experimentation. In 1864, the laws
obeyed by electricity and magnetism were unified and formulated by