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Moving Charges And Magnetism

Question
CBSEENPH12037633

Two straight wires A and B of lengths 10 m and 12 m carrying currents of 4.0 A and 6.0 A respectively in opposite directions lie parallel to each other at a distance of 0.03 m. Estimate the force on a 15 cm section of the wire B near its centre.

Solution
Given,
Length of wire A = 10 m
Length of wire B = 12 m
Current carried by A= 4 A
Current carried by B = 6 A
Distance between the two parallel wires, r = 0.03 m 

Force per unit length acting on the wire, 

Fl  = μ0I1I22πrNm-1        = 4π×10-7×4×62π×0.03        = 1.6 × 10-4 Nm-1 
 
Force on 15 cm section of wire B (near its centre), 
F= 1.6 x 10
–4 x 0.15 N
 = 2.4 x 10
–5 N

The force is repulsive as the currents are in opposite directions; the direction of force is normal to the wire away from A.