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NCERT Class XII Chapter
Moving Charges and Magnetism
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Question : 22 of 28
Marks: +1, -0
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?
Solution:  
Two wires connecting battery of an automobile to the starting motor carry 300 A current in opposite direction. So, the force is repulsive between them.
Force per unit length
F/l = μ04π2I1I2r\frac{\mu_0}{4\pi} \frac{2 I_1 I_2}{r} = 10−710^{-7} × 2×300×3001.5×10−2\frac{2 \times 300 \times 300}{1.5 \times 10^{-2}} = 1.2 N m−1m^{-1}
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