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22. 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? 

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Since the distance between the wires is much smaller than the length of the wires we can calculate the force per unit length on the wires using the following relation.

F=\frac{\mu _{o}I_{1}I_{2}}{2\pi d}

Current in both wires=300 A

Distance between the wires=1.5 cm

Permeability of free space=4\pi \times10-7 TmA-1

F=\frac{4\pi \times 10^{-7}\times 300\times 300}{2\pi \times 0.015}

F=1.2 Nm-1

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