Conceptual Physics (12th Edition)

Published by Addison-Wesley
ISBN 10: 0321909100
ISBN 13: 978-0-32190-910-7

Chapter 24 - Think and Discuss: 77

Answer

Zero. A net torque aligns the needle with the magnetic field.

Work Step by Step

The net force on the needle is zero. Though the north pole of the needle is attracted northward, the entire needle doesn't feel a net force because the other end is attracted southward. If the needle is not parallel to the external magnetic field lines, a net torque acts on it and rotates it until it aligns along the field lines. This is shown in Figure 24.3.
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