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?

Given:

Current through the wire, I = 300 A

Length of wire, L₁ = 70 cm

Length of wire, L₂ = 70 cm

Distance between the wires, d = 1.5 cm

We understand that current carrying conductors create magnetic fields which in turn applies force on nearby current carrying conductor(s).

We know that,

…(1)

Where,

F = force applied

�₀ = permeability of free space

I = current through the conductors

d = separation between the conductors

Now, putting the values of F, �₀, I and d in equation (1)

⇒

⇒ F = 1.2 Nm^-1

Hence, the force applied on the wires is 1.2 N per metre. The force is repulsive in nature because the current is carried in the opposite direction.