Questions for Review
1. Magnets have at least 2 poles, typically designated North and South. North attracts South, and vice versa, while like poles repel each other. A magnet tends to align itself along a magnetic field line. A bar magnet lines up such that an arrow drawn from the S to N pole will line up parallel to the magnetic field line.
2. In order for a transformer to work there has to be a changing current, or ac current, in the primary coil. This induces a changing magnetic flux through the transformer core and thus a changing flux through the secondary coil, which induces a voltage across the secondary coil.
3. Faraday's law of induction says that a changing magnetic field inside a coil of wire creates, or induces, a potential difference (voltage) across the coil. The induced voltage is directly proportional to the number of turns in the coil. Cone speakers operate by this principal: the coil is attached to the cone and is near a fixed magnet. AC currents in the coil cause a changing potential across the coil which leads to a changing magnetic field in the coil. This field interacts with the field of the fixed magnetic and results in a time-varying force on the cone, causing it to vibrate as the direction of the coil's magnetic field oscillates with the direction of the current in the coil. It's also used in "dynamic" microphones, in which sound waves push on a diaphragm that moves a coil with respect to a fixed magnet, thereby inducing a current in that coil. Another application to hi fi is the cartridge of a phonograph. As the needle wiggles in the record groove, it moves a coil with respect to a fixed magnet, again inducing a potential across the coil.
4. Magnetism in a cone speaker...this is described above. There is a stationary magnet that produces a force on the coil attached to the cone, causing the cone to vibrate in and out, which then pushes on the air to make sound waves.
5. The direction of the induced current in a coil is such that the magnetic field created by this current opposes the changing magnetic field that induced the current in the first place. The conservation of energy requires this. If the direction of the induced current caused a magnetic field that was changing in the same direction as the applied field, this would increase the rate of change of that field, and thereby cause a larger induced current. This larger induced current would create a still greater rate of change in the field, and so on. In short, this is a "positive feedback" system in which the field would spiral up to an infinite value, and since the field is a form of energy, its energy would become infinite, which is impossible. Also, the conservation of energy requires that one "can't get something for nothing," i.e., the energy that would go into such an increasing field would have to come from some other source.
6. A piece of iron has iron atoms whose electron spins can be aligned parallel to an external magnetic field, when such a field is applied. Small regions of the iron called "magnetic domains" form, in which all the spins point in the same direction. This is an energetically favorable state of the iron that will persist after the external magnetic field is turned off, as long as the thermal energy of the system is not sufficient enough to cause the spins to reorient back to their nature, randomly aligned, state.
7. A transformer can step-up or step-down current or voltage, but not power, because of the Law of Conservation of Energy. Power enters the transformer through the primary winding, and the entering power is equal to the product of the current in the winding times the voltage across the winding. This power is transmitted to the secondary winding through the transformer core, which itself is not a source of energy, so the flow of energy, or power, out of the secondary cannot exceed the flow of energy into the primary....just as the flow of water in a river below an island cannot exceed the flow above the island, if the island itself is not a source of water. In fact, some of the energy circulating through the transformer core will be dissipated as heat, so the power out of the secondary will always be somewhat less than the power into the primary. If the voltage at the secondary is stepped-up by a factor of N2/N1 compared to the primary voltage, i.e. the ratio of turns in the two windings, then the current at the secondary must be stepped-down by the same factor to conserve energy.
8. A core in a coil is typically a ferromagnetic material which can be magnetized in the same direction as the coil magnetic field. This allow for build up of a larger magnetic field than if one had just the coil alone.
9. Magnetism arises from the intrinsic property of electrons and atomic nuclei known as spin, as well as from the motion of electrons inside of atoms. The "spin" of the electron is a quantum mechanical property that, combined with the motion of electrons in the atom, gives rise to the atom's magnetism. In ferromagnetic materials, the net effect of the spin and orbital motion is to create a magnetic moment, which feels a force from an applied magnetic field and can be lined up to point along the direction of this field. In unmagnetized iron, the magnetic moments of the atoms in small regions called "domains" may be parallel, create a net magnetism, but other similar domains will be oriented in random directions so the total magnetism of the material is zero. An applied magnetic field can align the domains and create a net "magnetization" of the material which may persist, in the absence of significant thermal motion, due to a property known as "hysteresis." Nuclear spins give rise to a nuclear magnetic moment, which is typically around 1000 times weaker than that due to the orbital motion and spin of the electron, but they are utilized in the technique of Magnetic Resonance Imaging, or MRI.
10. One could induce a current in a metallic ring using Faraday's law of induction, in which a changing magnetic field through the ring would induce an oscillating current. If you began with zero field and increased it to some final value, current would flow around the ring, but eventually the current will dissipate due to the resistance of the ring.
Exercises
1. B. A charged particle feels a force when moving perpendicular to a magnetic field.
2. A. The direction of the force experienced by a charged particle moving through a magnetic field is always perpendicular to both the magnetic field and the velocity of the charged particle.
3. B. 50 watts...transformers cannot create power
4. E. A magnetic field can be created by both C. an electric current and D. a charged particle in motion.
5. B. a magnetic domain
6. A. the greater the alignment of domains, the stronger the magnetism
7. A. A magnetic field goes from the north pole to the south pole exterior to a magnet.
8. A. the magnetic field reverses direction
9. A. 1 ampere, since IV = power = constant
10. E. all of the above are correct
11. D. all of the above increase the magnetic field - more current, more windings, or an iron core
12. B. 30 turns in the secondary
13. A. magnetic field
14. D. the force will be up, assuming the current direction to be that in which electrons flow
15. A. the speaker diaphragm moves to the left. The current comes from out of the page, so using the left hand to indicate its flow leaves the thumb pointing to the left. Thus the right side of the coil is like a S pole, which is repulsed by the S pole of the permanent magnet, creating a force on the diaphragm to the LEFT.
16. B. the magnetic field will be 6 times as strong, since it's proportional to both current and the number of turns
17. B. increase the voltage
18. B. counterclockwise, again with the current direction associated with the motion of electrons
19. B. 60 microvolts. The induced voltage is proportional to the rate of change of the magnetic field, which is directly proportional to the frequency. Two octaves higher frequency means the rate has increased four times.
20. B. like poles repel, unlike poles attract
21. A. 480 volts. This is a step-up transformer with a ratio of 2000/50 = 40.