Page 6 - 12-phy-14 ELECTROMAGNETISM

14. Electromagnetism eLearn.Punjab

Combining all these factors,

F ∝ ILB sina

or F = kILB sina

where k is constant of proportionality. If we follow
SI units, the value of k is 1. Thus in SI units

F = ILB sin .....(14.1)a

Eq.14.1 provides a definition for the strength of
magnetic field. If I = 1 A. L = 1m and a = 90 , then
o
F = B. Thus B, the strength of magnetic field which is
also known as magnetic induction is defined as the

force acting on one metre length of the conductor Animation 14.4 : Magnetic field
placed at right angle to the magnetic field when 1 Source & Credit : Dyarrow

A current is passing through it. In SI units the unit
of B is tesla.

A magnetic field is said to have a strength of one tesla A it exerts a force of one newton on one
metre length of the conductor placed at right angles to the field when a current of one ampere

passes through the conductor. Thus
-1
Do You Know? 1T=1NA m − 1
It can be seen that the force on a current carrying

conductor is given both in magnitude and direction by
the following equation:
F = ILB
x .....(14.2)

where the vector L is in the direction of current flow.
The magnitude of the vector ILxB is ILB sina. where a

is the angle between the vector L and B. This gives the

magnitude of the force.

The direction of the force F (Fig. 14.3) is also correctly

given by the right hand rule of the cross product of

vectors of L and B i.e. rotate L to coincide with B through

(a) Two long parallel wires carrying currents I and the smaller angle. Curl the fingers of right hand in the
1
I in opposite direction repel each other. (b) The direction of rotation. The thumb points in the direction
2
wires attract each other when the currents are in of force. In some situations the direction of the force is
the same direction.
conveniently determined by applying the following rule:

6