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?

Explanation

`1.2 N m^(-1)` , repulsive. Note, obtaining total force on the wire as `1.2 x× 0.7 = 0.84 N` , is only approximately correct because the formula `F = (mu_0)/(2pi r) I_1 I_2` for force per unit length is strictly valid for infinitely long conductors.