A few sample questions and problems for Exam II are below. The exam will cover chapters 4, 6, 7, 9 and 10. Don't wait for the weekend to study! We covered Chapter 9 during Thursday's tutorial. Answers to chapter 10 questions can be found after the sample exam questions.
1. The origin of magnetism is current. The electrons orbiting about the nucleus constitute a current loop which produces a magnetic field. For most materials, the atoms are randomly oriented and no NET magnetic field is observed. But in some substances, such as iron, nickel, colbalt, etc., a small percentage of the atoms can be aligned to produce a net magnetic field.
3. The electron must be moving (which constitutes a current) to feel another magnetic field. A moving electron creates its own magnetic field which feels a force from any other magnetic field.
4. The earth's core is molten. That means the atoms are not fixed in place and hence don't "line up" to create a net magnetic field.
7. Nothing special happens at the equator. The north pole of the compass still points towards the earth's geomagnetic south pole...which is near the earth's geographic north pole.
10. The force the magnet exerts on the clip is equal and opposite to the force the clip exerts on the magnet. Newton's III law applies to EVERYTHING!
13. The force a magnetic field exerts on a moving electron (or any electric current) is always perpendicular to the direction of motion of the electrons. The field never changes the speed (but it can change the velocity!) of the electrons. Hence, the field can't do work on the electrons. You need an electric field to change electron speed.
16. If the generator is not connected to anything, the only work you do is against friction. But if it is connected, a current can flow. This current will do work...light a bulb, turn a motor, etc. YOU are supplying this work as you rotate the armature.
18. Your metal car will alter the earth's magnetic field through the loop. Changing the magnetic field through any wire loop will create a current. Take a look at the road near most of the traffic lights on STT, particularly in the right turn lane for those intersections that have advance right turn lights. Have you ever noticed that if no car is in that lane, the right turn light doesn't come on?
21. For a motor, an outside voltage source provides the current that flows through the motor's armature, which causes the armature to rotate. In a generator, rotational motion is mechanically applied and the rotational motion produces a current. All motors behave like generators as they start to rotate. The rotation always creates a "back" current in the opposite direction of the original current that cause the rotation in the first place.
23. I did this demo in class! The alternating magnetic field induces a current in the metal ring...so it heats up.
27. The falling magnet induces currents around the pipe (just like in #23 above). This current creates its own magnetic field. As you might guess, the direction of this field opposes that of the falling magnet that created the current in the first place. What would happen if this were not the case? Why can't the falling magnet create a current that results in a magnetic field that attracts the magnet?