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The wires which connect the battery of an automobile to its starting motor carry a current of $300 \mathrm{~A}$ (for a short time). What is the force per unit length between the wires if they are $70 \mathrm{~cm}$ long and $1.5 \mathrm{~cm}$ apart? Is the force attractive or repulsive?
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Two wires connecting battery of an automobile to the starting motor carry 300 current in opposite direction. So, the force is repulsive between them. Force per unit length
$$
F / l=\frac{\mu_0}{4 \pi} \frac{2 I_1 I_2}{r} 10^{-7} \times \frac{2 \times 300 \times 300}{1.5 \times 10^{-2}}=1.2 \mathrm{~N} / \mathrm{m}
$$
$$
F / l=\frac{\mu_0}{4 \pi} \frac{2 I_1 I_2}{r} 10^{-7} \times \frac{2 \times 300 \times 300}{1.5 \times 10^{-2}}=1.2 \mathrm{~N} / \mathrm{m}
$$
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