Question

Lab Homework - Force on a Current-Carrying Wire This homework will be due when you come to lab the week of November 4. A horseshoe magnet, with its north and south poles indicated, is shown in the figure below. Between the poles of the magnet is a wire perpendicular to the page, with current out of the page. a) Draw magnetic field lines for the magnet. Explain why you drew the field lines the way you did. b) If there is a force acting on the wire, indicate the direction of the force in the diagram. Explain why there is or is not a force and how you determined the direction of the force. c) Does the magnet experience a force? If so, in which direction? If the magnet does experience a force, explain why it experiences a force. If the magnet does not experience a force, explain why it does not experience a force.

Answer 1

a) The magnetic field lines are as shown below.

The magnetic field lines move from north to south pole, do not intersect each other and is normal to the surface. That is why I drew it this way. The exact field line will have some curve. But for a horseshoe magnet, we can say the field lines are approximately parallel in the region inside it, near the poles.

b) The force acting on a current carrying conductor due to a magnetic field is in the direction

(cross product)

Here, the current I is in the out of page direction. Call it +Z

The magnetic field is in the +X direction.

So, the direction of force is

$\hat z X\hat x = \hat y$

Thus, the force on the wire is in the +Y or upward direction.

c) According to Newton's third law, every action force has an equal and opposite reaction force. Here, the magnet is applying a force on the wire. So there will be an equal and opposite force acting on the magnet by the wire. The direction of this force is -Y.