20. There are four wires viewed end-on in the drawing. They are long, straight,

Question

20. There are four wires viewed end-on in the drawing. They are long, straight, and perpendicular to the plane of the paper. Their cross sections lie at the corners of a square. The magnitude of the current in each wire is the same. What must be the direction of the current in each wire (into or out of the page), so that when any single current is turned off, the total magnetic field at P (the center of the square) is directed toward a corner of the square? Wire 1 O- Wire 1 Wire 2 Wire 2 Wire 3 Wire 4 (a) out of out of into out of (b) into out of into into (c) out of into out of out of into (d) into into into Wire 4 Wire 3

Explanation / Answer

All should point into the page

Now if any of the currents is turned off, The total magnetic field at the center is

directed towards a corner of the square.

This is by considering the Biot Savart Law for the Magnetic fields due to each wire and looking

at the directions of the field, we come to the conclusion that all the wires must have currents pointing into the page.

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