The wires which connect the battery of an automobile to its starting motor carry a current of 300a (for a short time). What is the force per unit length between the wires if they are 70cm long and 1.5 cm apart? IS the force attractive or repulsive?

Explanation

1. Magnetic force exerted per unit length of either of two extremely long, thin parallel wires, separated by perpendicular distance y and carrying currents `I_(1)andI_(2)` is,
`F/l=(mu_(0)I_(1)I_(2))/(2piy)`
= `((4pixx10^(-7))(300)(300))/((2pi)(0.015))`
`thereforeF/l=1.2N/m`
2. Total magnetic force on either of two equally long wires is,
`F=1.2xxl`
= `1.2xx0.7`
`thereforeF=0.84N`
3. Above force is repulsive because of opposite directions of electric currents in the given wires.