Page 2 - LN

Stretch the forefinger, middle finger and the thumb of your left hand mutually perpendicular to each
other as shown in figure. If the forefinger indicates the direction of the magnetic field and the
middle finger indicates the direction of current, then the thumb will indicate the direction of motion

(i.e., force) on the conductor.

(b) Magnitude of Force:
Experimentally it is found that the magnitude of the force acting on a current carrying conductor kept in a

magnetic field in direction perpendicular to it, depends on the following factors :
(i) The force F is directly proportional to the current flowing in

the conductor, i.e. F α l. (ii) The force F is directly

proportional to the intensity of magnetic field, i.e. F α B.

(iii) The force F is directly proportional to the length of the conductor (inside

the magnetic field), i.e. Fα l. Combining these we get, F = IBI or F = K I B l
Where K is constant whose value depends on the choice of units. In S.I. units K = 1 and the unit of

-2
magnetic field is tesla (T). 1 tesla is equal to 1 newton ampere -1 metre -1 or 1 weber metre .
Force is directly proportional to sinΘ where A is the angle between current and the direction of magnetic field.
I.e., F α sinθ
Combining all we have F = BIl sinθ

Special cases:

0
0
(i) When θ = 0 or 180 , sinθ = 0 ⇒ F = 0
Force on a current - carrying conductor placed parallel or antiparallel to magnetic field is zero.
(ii)If θ = 90°, sin θ =sin 90° = 1, F= BIl is the maximum force. Force experienced by the conductor is
maximum when placed perpendicular to magnetic field.

(iii)If B = 0, F = 0 i.e. the coil placed in field free area doesn't experience any force.