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Flashlights
Page 3 of 4 (37 Questions and Answers)

1003. How does electricity work?
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I'll assume that you are asking about moving or dynamic electricity, the type that lights the bulb in a flashlight (as opposed to static or stationary electricity). In that case, you are referring to a flow of electric charges that is generally called an electric current. This movement of electrically charged particles carries with it energy, both as kinetic energy (energy of motion) in the charged particles and as potential energy in the electrostatic attractions and repulsions of these particles. The particles typically acquire this energy from a battery. The battery pulls opposite charges away from one another and pushes like charges together. These actions increase the energy of those charges. The charges then rush through electrically conducting materials, generally metals, in order to bring opposite charges closer together. This flow of charges releases the energy given them by the battery.

In a flashlight, the batteries provide the charges with power and the light bulb makes use of the power. The charges first flow through the battery (which gives them energy), then through wires to the light bulb, then through the light bulb (where they give up their energy), and finally back through wires to the battery. The charges move in a loop—a circuit—so that they don't accumulate anywhere. They travel endlessly between battery and bulb, shuttling energy from the battery to the bulb. As is always the case in electric circuits, two wires connect the battery and bulb—one wire to carry charges to the bulb and one wire to return them to the battery to begin their trip over again.


1049. Is it possible to charge batteries using static electricity? Can lightning or atmospheric charges be stored in a capacitor and then released into a cell for charging? — JM, Lafayette, NT
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Yes, static electricity has energy associated with it and that energy can be used to charge batteries, at least in principle. Static electricity is literally stationary separated electric charges—essentially separated charges stored on capacitor-like surfaces. As you suggest, it may be easiest to transfer these separated charges into a real capacitor and then to use this charged capacitor to recharge an electrochemical cell. Whether such a procedure can be carried out efficiently and in a cost-effective manner isn't clear to me. The charges involved in lightning have so much energy per charge—so much voltage—that they're hard to use for anything. Even the charges that you accumulate when you rub your feet on a wool carpet on a cold, dry winter day acquire an enormous amount of energy per charge. To charge most batteries, you need lots of low energy charges, not the small numbers of high-energy charges that are typical of static electricity. Using this tiny current of high-energy charges to charge a battery is equivalent to trying to fill a swimming pool with water from a high-pressure car-washing nozzle—too little water under too much pressure. You can do it, but there are better ways.

1050. What are some unusual conductors of electricity?
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How about graphite and cadmium sulfide? Graphite, such as that in the lead of a pencil, conducts electricity even though it's not formally a metal. If you draw a dark line on a sheet of paper, that line can act as a wire for sensitive electric circuits. Cadmium sulfide is a photoconductor—a material that is electrically insulating in the dark but that conducts electricity when exposed to light. Photoconductors of this sort are used in some light sensors, as well as in xerographic copiers and laser printers.

1073. What is the most effective way to electronically measure the level of charge of a lead acid battery? — RS
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The voltage of any battery—the amount of energy it gives to each positive charge that it transfers from its negative terminal to its positive terminal—increases slightly when the battery is fully charged. That's because when the battery is fully charged and its chemicals are highly ordered, the laws of thermodynamics that encourage the development of disorder act to increase the battery's disorder through effects that also increase the battery's voltage. But as the battery discharges, these thermodynamic effects fade and the battery's voltage diminishes slightly. So the easiest way to determine the battery's charging status electronically is to look at the voltage rise across the battery when little or no current is flowing through it. The higher the voltage, the more fully charged the battery is.

1135. How does a relay work? — CS, Fairfax, Virginia
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A relay is an electromagnetically operated switch. It contains a coil of wire that acts as an electromagnet. Since electric currents are magnetic, this coil of wire develops north and south magnetic poles whenever current passes through it. A metal core is often placed inside the coil of wire to enhance its magnetism. Adjacent to the coil of wire is a moveable piece of iron. While iron normally appears nonmagnetic when it's by itself, it becomes highly magnetic whenever it's exposed to a nearby magnetic pole. The iron piece becomes magnetic as current flows through the coil and the two are attracted toward one another. As the iron piece shifts toward the coil, it moves various electric contacts that are attached to it. These contacts close some circuits while opening others. The coil remains magnetic and continues to hold the iron piece near it until current stops flowing through the coil. When the current does stop, the coil loses its magnetism and so does the iron piece. A spring in the relay then pulls the two apart and the electric contacts return to their original positions.

1190. What is the speed with which electric power is transmitted through the power grid? Believe it or not, the education center at an important nuclear power plant claims that "electrons travel at the speed of light," an obvious impossibility for current in a copper wire. What is the maximum speed of an electron in a commercial electric power grid? in a superconductor? — AW, Alexandria, VA
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Amazingly enough, the speed at which electric power travels through a wire is very different from the speed at which electrons move through that wire. In most wires, electric power travels at very nearly the speed of light while the electrons themselves travel only millimeters per second! This statement is true whether the electricity is traveling in a copper wire or a superconductor!

To understand how this difference in speeds is possible, think about what happens when you turn on the water to a long hose. If that hose is already filled with water, water will immediately begin pouring out of the hose's end even though the water is flowing quite slowly through the hose. While the water itself moves slowly, the water's effects travel through the hose at the speed of sound in water—several miles per second! Water at the end of the hose "knows" that you have opened the faucet long before new water from the faucet arrives.

Similarly, when you turn on a flashlight, electrons begin to flow out of the battery's negative terminal at speeds of only a few millimeters per second. But these electrons don't have to travel all the way to the light bulb for the bulb to light up. When these electrons leave the battery, they push on the electrons in front of them, which push on the electrons in front of them, and so on. They produce an electromagnetic wave that rushes through the wire at an incredible speed. As a result, electrons begin flowing through the light bulb only a few billionths of a second after the first electron left the battery. So while the electrons that carry electricity through the power grid flow rather slowly, the power they deliver moves remarkably fast.


1201. How does lightning damage electrical appliances that are properly grounded and have their power switches in the off position? Doesn't that eliminate a path for the electricity? — RDU, Atlanta, Georgia
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When lightning strikes a power line, it pours enormous amounts of electric charge onto that wire. These like charges repel one another and they quickly spread out all over the wire. If this wire enters your home, the charges traveling along it will flow into any appliance that's plugged in, whether it's turned on or not. But if the appliance is turned off, this charge will reach the open switch and it will come to a stop, at least temporarily.

What matters then is just how much charge enters the appliance. The open switch would normally block the passage of electricity, which is why the appliance doesn't operate while it's turned off. But as charge accumulates on one side of the switch, the voltage at that point rises higher and higher. When the voltage becomes high enough, as it easily does after a lightning strike, the charges can leap into the air and travel to the other side of the switch even though the two sides don't touch one another. Another view of this disaster is that the like charges on one side of the switch repel one another so vigorously that some of them are pushed through the air to the other side of the switch. As a result of this movement of charges through the air—an electric arc—current passes through the appliance as though it were turned on. If this current exceeds what the appliance can tolerate, the appliance will be destroyed. Even grounding the appliance may not help—charges can flow uncontrollably through the appliance and, while some charges take paths to ground, others flow through sensitive components and destroy them.


1213. I understand how computer information travels as bits from source to destination, but how does each bit travel through the wire at a molecular level? — JCC, Atlanta, Georgia
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At the simplest level, a bit travels as a packet of positive or negative charge through a wire. To start this movement, the source injects a small amount of charge onto the end of the wire. Since like charges repel and opposite charges attract, this new charge pushes on charges further down the wire, and those charges push on charges still farther down the wire, and so on. Overall, a wave of forces and responses rushes along the wire until it reaches the destination end of the wire. There charges flow off the wire and into the destination device. While these charges aren't really the same ones that were put on the wire by the source, they have the same charge and one can imagine that charge has simply moved from the source device to the destination device by way of the wire. The destination device can examine this charge to determine whether the source was sending a 0 or a 1.

1214. It is difficult for me to understand current flowing from a battery through a circuit. A battery has both a positive end and a negative end. Which direction does the current flow? — SK
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When you connect a battery in a circuit, negatively charged electrons flow away from the battery's negative end and they return toward the battery's positive end. The battery then pumps the electrons back to its negatively charged end and they begin the journey all over again (hence the name "circuit"). But because the electrons have a negative charge, current does not flow in their direction. Instead, current is defined as flowing in the direction of positive charge flow. In the present case, current flows from the battery's positive end, through the circuit, and back to the battery's negative end. Current is thus flowing in the direction opposite to the direction of electron movement! If you want to know which way current is flowing, you can normally find the direction in which electrons are flowing and then reverse it. Life for physicists and electrical engineers would be so much simpler if Benjamin Franklin hadn't made an unfortunate choice that gave electrons—the principal carriers of electricity—a negative electric charge. We have been living with the consequences of that choice ever since.

1234. I am doing a science fair project on conductors and insulators. What are some of the best and worst conductors of electricity? — LM
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The best conventional conductors are silver, copper, gold, and aluminum. What makes them good conductors is that electrons move through them for relatively long distances without colliding with anything that wastes their energy. These materials become better conductors as their purities increase and as their temperatures decrease. A cold, near-perfect crystal is ideal, because all of the atoms are then neatly arranged and nearly motionless, and the electrons can move through them with minimal disruption. However, there is a class of even better conductors: the so-called "superconductors." These materials allow electric current to travel through them will absolutely no loss of energy. The carriers of electric current are no longer simply independent electrons; they are typically pairs of electrons. Still, superconductivity appears because the moving charged particles can no longer suffer collisions that waste their energy-they move with perfect ease. We would be using superconductors everywhere in place of copper or aluminum wires if it weren't for the fact that superconductors only behave that way at low temperatures.

As for the best insulators, I'd vote for good crystals of salts like lithium fluoride and sodium chloride (table salt), and covalently-bound substances like aluminum oxide (sapphire) or diamond. All of these materials are pretty nearly perfect insulators.


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