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?

(A) Current in both wires,$I = 300\; A$.
Distance between the wires,$r = 1.5\; cm = 0.015\; m$.
The force per unit length between two parallel current-carrying wires is given by the formula:
$f = \frac{F}{l} = \frac{\mu_0 I^2}{2 \pi r}$
Where $\mu_0 = 4 \pi \times 10^{-7}\; T\cdot m/A$ is the permeability of free space.
Substituting the values:
$f = \frac{4 \pi \times 10^{-7} \times (300)^2}{2 \pi \times 0.015}$
$f = \frac{2 \times 10^{-7} \times 90000}{0.015}$
$f = \frac{0.018}{0.015} = 1.2\; N/m$.
Since the current in the wires flows in opposite directions (one to the motor and one returning),the force between them is repulsive.