Class 12 Physics
CBSE and UP Board
Moving Charges and Magnetism
Chapter-4 Exercise 4.22
Additional Exercise
The wires which connect the battery of an automobile to its starting motor carry a current of 300 A (for a short time). What is the force per unit length between the wires if they are 70 cm long and 1.5 cm apart? Is the force attractive or repulsive?
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Current in both wires, I = 300 A
Distance between the wires, r = 1.5 cm = 0.015 m
Length of the two wires, l = 70 cm = 0.7 m
Force between the two wires is given by the relation,
F = μ0 I²/2πr
Where,
μ0= Permeability of free space = 4π x 10⁻⁷ Tm A⁻1
F = 4π x 10⁻⁷ x (300 )² / (2 π x 0.015) = 1.2 N/m
Since the direction of the current in the wires is opposite, a repulsive force exists between them.