What is the hum you hear when walking under large power lines?

The electric currents in those lines are reversing 120 times a second. That means that the electrostatic forces between the charges they carry and anything nearby reverse 120 time a second and the magnetic forces that they exert on one another when currents flow through them turn on and off as well. You hear all of the motions that are caused by the pulsating electric and magnetic forces.

What materials are magnets made of?

They are mostly iron, cobalt, or nickel, which are intrinsically magnetic metals (as we'll discuss in the lecture on Tape Recorders). But to help them retain their magnetic alignments, permanent magnets have other elements in them, too. Iron is magnetic at the microscopic scale, but that magnetism is broken up into lots of tiny regions that all point in random directions. To make a whole piece of iron magnetic, something must help those tiny regions stay pointing in the same direction. The good permanent magnets have structures that keep all the tiny regions pointing in one direction.

What is the difference between an electrostatic charge and an electromagnetic charge?

The word "electrostatic" refers to the forces that occur between electric charges that are essentially stationary or "static." Instead of "electrostatic charge," we normally just say "stationary electric charge." The word "electromagnetic" refers to effects associated with both electricity and magnetism. By "electromagnetic charges," you probably mean magnetic poles. If both of electric charges and magnetic poles existed, then their behaviors would be remarkably similar. The electric charges would attract or repel one another, depending on whether they were like or opposite and the magnetic poles would attract or repel one another, too. Whenever the electric charges moved, they would begin to affect the magnetic poles and whenever the magnetic poles moved they would begin to affect the electric charges. Everything would be symmetrical. However no one has ever proven that magnetic poles exist all by themselves. So there is an asymmetry to our universe. The only way in which to create magnetic effects is to have moving electric charge.

How can a battery lose energy when it's not being used (like when it sits in a flashlight that's not turned on for months or years)?

The battery maintains a steady positive charge on its positive terminal and a negative charge on its negative terminal, month after month. These opposite charges attract one another and they do manage to get back together occasionally. They usually travel right through the battery itself, assisted by thermal energy. When that happens, the battery has to pump additional charge from the negative terminal to the positive terminal to make up for the lost charge and consumes a little more of its chemical potential energy. You can slow down this aging process by refrigerating the batteries. With less thermal energy available, the accidental movements of charge through the battery become less frequent.

How do collisions with tungsten atoms in the filament convert current's electrostatic and kinetic energies into thermal energy?

When the electrons moving through the tungsten filament collide with the tungsten atoms, they do work on those tungsten atoms. Although the atoms are very massive and the electrons bounce off of them like Ping-Pong balls from bowling balls, the atoms do jiggle about after being struck. Bombarded by a steady stream of electrons, the atoms in the tungsten begin to vibrate harder and harder and soon become white hot. The electrons leave the tungsten filament with relatively little energy left-they use almost all of their kinetic and electrostatic potential energies to get through this gauntlet of tungsten atoms.

What exactly are fuses and why do people change them or blame them if something short circuits?

A fuse is a weak link inserted into a circuit to break the circuit if too much current flows through it. The electric resistance of the fuse is large so that the current deposits a fair amount of thermal energy into it as it passes through. When the current exceeds the designated amount, the fuse melts and burns out. A short circuit usually blows out the fuse because it causes an enormous increase in the current flowing through the circuit. When that happens in your house, you should be thankful for the fuse because it saved you from the fire that might occur if it weren't there. You sure don't want the wires in your wall to melt and burn out because they might take the whole building with them. A circuit breaker is just an electromagnetic variation on the fuse. As the current through the circuit break increases, an electromagnet inside the circuit breaker becomes stronger and stronger until it eventually flips a switch that opens the circuit.

What happens when a battery dies?

A battery uses its chemical potential energy to pump electric charges from its negative terminal to its positive terminal. Eventually it runs out of chemical potential energy. In an alkaline battery, the chemical potential energy is mostly contained in zinc powder and this powder oxidizes as the battery operates; in effect, it burns up in a very controlled manner. By the time the battery is dead, there just isn't much pure zinc metal left.

If current times voltage equals power, this makes it seem that high current times low voltage would equal low current times high voltage; but this is not true because of resistance. How is resistance taken into account in the current times voltage equal power equation?

Your first observation, that high current times low voltage would equal low current times high voltage is true; it means that electricity can deliver the same power in two different ways: as a large current of low energy charges or as a small current of high energy charges. That result is critical to the electrical power distribution system. The resistance problem is a side issue: it makes the delivery of power as a large current of low energy charges difficult. If you could get this current to peoples' houses without wasting its power, there would be no problem, but that delivery isn't easy. The wires waste lots of power when you try to deliver these large currents. So the electric power distribution system uses small currents of high energy charges instead.

Why have all the electromagnets we have used in class repelled whatever they came in contact with? I have seen electromagnets actually attract objects before.

Electromagnets can and do attract. While it's true that I've only shown repulsion so far, I'll show attraction soon. All I need to do is make sure that the opposite poles of the electromagnets approach one another. Note: when I suspended the magnetic ball in midair with a device that used an electric eye and feedback, the support was attraction from an electromagnet above it.

Where does the exact reversal occur in the circuit (where the energy diminishes completely and then turns the opposite way)?

The reversal of the current in an alternating current (AC) circuit occurs everywhere in the circuit at once. The whole current gradually slows to a stop and then heads backward. At the moment it comes to a complete stop, the electric power company isn't supplying any power at all and the circuit isn't consuming any. Because the power delivery pulses on and off in this manner, devices that operate on AC power are designed to store energy between reversals. Motors store their energy as rotational motion. Stereos store energy as separated electric charge in devices called capacitors, or as magnetic fields in devices called inductors.

Did you say that if you doubled the current then you doubled the voltage loss and if you halve the current then you halve the voltage loss?

Yes. When you try to push current through a wire, the voltage drop across that wire (i.e. the energy lost by each charge passing through that wire) is proportional to the number of charges flowing through that wire each second (i.e. the current through the wire). If you double the number of charges flowing through the wire each second, then each charge will lose twice as much energy (the voltage drop across the wire will double). If you halve the number of charges flowing through the wire each second, then each charge will lose half as much energy (the voltage drop across the wire will halve).