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?

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.