A magnetic resonance imaging (MRI) machine uses an enormous and extremely strong magnet to study a patient's body. The magnet, which has its north pole at the patient's head and its south pole at the patient's feet, is actually a coil of superconducting wire through which electric charges flow.
1. This fancy electric system seems unnecessary; why can't the technicians simply put a large number of north magnetic poles near the patient's head and an equal number of south magnetic poles near the patient's feet?
2. The needle of your magnetic compass has its north magnetic pole painted red and its south pole painted white. If you stand a few meters from the MRI machine, at the end where the patient's head is, why does the white end of the compass turn toward the patient's head?
3. The compass is a magnetic dipole, with no net magnetic pole. So why do you feel it pulled toward the patient's head more and more strongly as you get closer to the magnet?
4. Aluminum isn't normally magnetic, but as you carry a large aluminum tray toward the magnet, you find that the magnet repels the aluminum. Explain.
5. You eventually manage to get the aluminum tray up to the magnet. As long as the tray doesn't move, it experiences no magnetic forces. But when you drop it, it falls past the magnet remarkably slowly. What slows down its fall?
You and your friends are at an isolated beach for Spring Break and you want to listen to music out on the dunes. You have a little portable cassette recorder that's normally powered by four 1.5 volt "AA" batteries. It also has a small socket in which to plug a 6 volt power adapter--a small black cube that attaches to a wall outlet and provides 6 volts to the recorder.
Unfortunately, you are out of "AA" batteries and there are no power outlets on the beach itself. The local grocery store was cleaned out by last week's crowd and the only batteries left are two 1.5 volt "D" batteries and a box of 9 volt batteries. You buy all of these batteries and head back to the beach, certain that you can get the recorder to work.
6. You carefully disassemble the power adapter so that you can use its wires to connect batteries to the recorder. Someone suggests connecting a 9 volt battery directly to the recorder. You point out that the recorder needs 6 volts, not 9 volts. In a sentence or two, use the phrase "energy per charge" to describe why it would be a bad idea to power the recorder with 9 volts.
7. You really need 6 volts for the recorder but you have only two 1.5 volt batteries, not the usual four. How can you arrange some of the batteries you do have to make a source of 6 volt electric power?
8. The recorder works nicely when powered by your arrangement of "D" and 9 volt batteries. However, a 9 volt battery doesn't seem to last very long in this scheme. What is it about 9 volt batteries that makes them go "dead" so quickly when used with a device, like the cassette recorder, that carries a large amount of current?