HPP Activity 74v1
Magnetic Torque
Magnetic forces are the basis for most electric motors in use today. Here, we will develop an
explanation for how magnetic forces produce rotational motion. Obtain
Rectangular wire
Suspension device
Battery pack
3 alligator clip wires
switch
U-shaped manet
Exploration
Set up the rectangular wire on the
suspension device so that it can rotate
freely. Connect the battery pack so that a
current will flow around the wire when the
switch is closed. Figure 1 shows the set-up.
Position the U-shaped magnet so that the
magnetic field is in the plane of the loop.
Position your eye so that the orientation of
the field and current direction are as shown
in Figure 2.
Figure 1.
Figure 2.
Activity Guide
 2010 The Humanized Physics Project
Supported in part by NSF-CCLI Program under grants DUE #00-88712 and DUE #00-88780
HPP Activity 74v1
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GE 1.
Close the switch briefly.
1. Describe what happens to the loop.
2. What is the direction of the magnetic force on each side of the loop, as
shown in Figure 2?
3. Should there be a net torque about the AB axis?
4. Is the torque experienced by the loop consistent with the observed motion
of the loop?
Now orient the magnet so that the field is perpendicular to the plane of the
loop. Turn on the current.
5. What happens?
6. Should there be a net torque about the AB axis in this case?
Invention
Any current loop, not just a rectangular one, in a magnetic field can experience a magnetic
torque that will cause rotational acceleration. The magnitude of the torque is given by
  IABsin 

where I is the current in the loop, A is the loop area, B is the magnitude of the magnetic field, and
 is the angle between the field and the normal to the plane of the loop.
Application
GE 2.
Suppose the loop in Figure 2 rotates 180 .
1. Would there still be a net torque about the AB axis?
2. What direction would the loop rotate now?
Activity Guide
 2010 The Humanized Physics Project
HPP Activity 74v1
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3. Normally, an electric motr requires continuous rotation. Would the set-up
used in Exploration section be a good electric motor?
4. What would have to change to make it a better motor?
Activity Guide
 2010 The Humanized Physics Project