Question

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?

Answer 1

The force between two wires is given by the relation,

$F=\frac{{\mu }_o{I}^2}{2\pi r}=1.2N/m$

Since the direction of current in two wires is opposite, a repulsive force exists between them.