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How Everything Works  
Page 24 of 160 (1595 Questions and Answers)

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

232. If you keep batteries in your car-where it gets really hot on a summer day-will the batteries "die" faster? (I got brand new batteries and have them in a flashlight in my car and they are almost dead, yet I never really used the flashlight but for a couple of minute.)
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Yes. Thermal energy spoils everything and the hotter you heat an object, the more thermal energy it contains. Keeping batteries or photographic film cool preserves them against aging.

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

234. What happens when a battery dies?
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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.

235. Can you explain power surges?
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Sometimes lightning strikes a power line and deposits a large amount of charge on it. This charge has considerable electrostatic potential energy so its voltage is very large (a large positive voltage if the lightning carried positive charge, a large negative voltage if the lightning carried negative charge). A the charge flows outward along the wires, it raises the local voltages of the wires. This sudden, brief increase in the local voltages is what you mean by a power surge. Many devices (e.g. computers and televisions) can be damaged by such a surge in voltage. Even a light bulb can be damaged because the extra voltage pushes too much current through the filament and can burn it out.

236. How can we talk about positive particles flowing through wires when it is really negatively charged electrons?
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The fiction of current being carried by positive charges really does work nicely. If a wire is carrying negatively charged electrons to the east, then the east end of the wire is becoming more and more negative and the west end is becoming more and more positive. The same would happen if that wire were carrying positively charged particles to the west. Even though these positively charged particles aren't really there, we can pretend that they are. By pretending that current is carried by positive particles, we don't have to worry about the arrival of a positive number of negatively charged electrons lowering the voltage of an object.

237. How does hydroelectric power work?
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Hydroelectric power begins with water descending from an elevated reservoir, such as a lake in the mountains. While it's in the elevated reservoir, this water has stored energy—in the form of gravitational potential energy. As this water flows downward through a pipe, its gravitational potential energy becomes either kinetic energy or pressure potential energy or both. By the time the water arrives at the hydroelectric power plant, it is either traveling very quickly or has an enormous pressure or both. In the power plant, the water flows past the blades of a huge turbine and does work on those blades. The blades are shaped somewhat like airplane wings and they "fly" through the moving water. Since the blades are attached to a central hub, they cause this hub to rotate and allow it to turn the rotor of a huge electric generator. The rotor of this generator typically contains a giant electromagnet. The electromagnet turns within a collection of stationary wire coils and it induces electric currents in those coils. These electric currents carry power out of the generator to the homes or business that need it.

238. How does power get from the plant to my house? Where do the voltages go up and down?
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The voltage is stepped up at the power plant so that a small current of very high voltage charges (high energy per charge) can carry enormous power across the countryside. When this current arrives at your city, its voltage is stepped down so that a medium current of medium high voltage charges can carry that same enormous power through your city. Finally, near your house, its voltage is again stepped down so that a large current of low voltage charges can carry this power into your house. Naturally, you do not use all of the power from the power plant yourself, so it is distributed among all of the buildings in the city.

239. How is AC converted in certain items to DC?
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These devices use diodes, which are one-way devices for current. They only allow the current to flow a certain direction and block its flow the other way. With the help of some charge storage devices called capacitors, these diodes can stop the reversals of AC and turn it into DC. Those little black battery eliminators that you use for household electronic devices contain a transformer, a few diodes and a capacitor or two.

A step-up transformer has a secondary coil with many, many turns. As the current in the primary circuit flows back and forth, it creates a reversing electric field around the iron core of the transformer. This electric field pushes charges through the secondary coil so that it travels around and around the core. Each charge goes around many times, picking up more energy with each passage. By the time the charge leaves the transformer, it has lots of energy so its voltage is very high.


240. How is AC current (alternating current) made?
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Usually with alternating current generators, which we will discuss next. It can also be made by electronic alternators, such as those found in the uninterruptible power supplies that provide backups for computers.

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