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Flashlights
Page 1 of 4 (37 Questions and Answers)

230. 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)?
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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.

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.

580. How does a battery work? How many different kinds of batteries are there? - BW
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Batteries use chemical reactions to move electric charges from one terminal to another. A chemical reaction is a process that rearranges molecules—you begin with a certain collection of molecules and end up with a different collection of molecules. As the atoms in those molecules rearrange, they stick to one another more tightly than before and they release some of their chemical potential energy. This released energy then takes another form. While some chemical reactions such as burning will turn this released energy into thermal energy, a battery uses this released energy to move electric charges from one place to another. The battery moves extra positive charges onto its positive terminal and extra negative charges onto its negative terminal. While you can't see those charges, you can tell that they're there. If you use wires to connect the terminals to the two sides of a light bulb, the charges will rush through the wires and the light bulb will glow.

There are many types of batteries, but two of the most important modern batteries are alkaline batteries (used in flashlights and toys) and lead-acid batteries (used in automobiles). An alkaline battery uses a reaction between zinc metal and manganese dioxide to move electric charges between its two terminals. The battery's negative terminal is made of powdered zinc and its positive terminal is surrounded by manganese dioxide. Between the two terminals is an alkaline paste of potassium hydroxide. As the chemical reaction proceeds, negative charges are transferred to the battery's negative terminal and positive charges are transferred to the battery's positive terminal. As these charges are used by the flashlight or toy, the battery replaces them with new charges. Since each transfer of charges consumes some of the battery's original chemicals, the more the battery's charges are used, the more its chemicals are consumed. Eventually the powdered zinc is gone and the battery stops working. Once the powdered zinc has been used up, it can't be replaced.

A lead-acid battery uses a reaction between lead metal, lead oxide, and sulfuric acid to move electric charges. It, too, consumes its original chemicals while transferring charges. However, a lead-acid battery can be recharged easily by pushing charges through it backward. When a car is running, its generator pushes charges backward through the lead-acid battery and converts the consumed chemicals back into their original forms. This recharged battery is almost as good as new, so it can be used over and over again and lasts for several years.


605. How can you make an electromagnet from a battery and copper wire?
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You'll need a large steel nail or bolt, too. Wrap about 100 turns of copper wire around the nail, keeping the turns fairly uniformly spaced. Make sure that both ends of the wire coil, start and finish, project out from the windings. When you're done winding the coil, strip off about 1 cm of the insulation from each end of the wire. Now connect one end of the wire to the positive terminal of a AA alkaline battery and the other end of the wire to the negative terminal of that battery. The nail will become a strong magnet and will be able to pick up other nails or paper clips with ease. Electricity will also heat the wire, so be prepared for the electromagnet to become uncomfortably hot. Detach the wires from the battery when you're no longer able to hold everything safely.

633. How does electricity work and is it possible to design a light bulb that will let you know when it is about to stop working? — LS, Chicago, IL
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Electricity involves electric charges. While static electricity involves stationary electric charges, the electricity you are probably referring to is dynamic: electricity in which the electric charges move. Most (dynamic) electricity is the movement of electrons—tiny negatively charged particles that form the outer part of atoms. The electricity in the wires leading to and from a lamp is the flow of electrons through those wires. A lamp has two wires attached to it because the electrons flow into the lamp through one wire and out through the other wire. However, because the electricity we normally use is alternating current, the direction in which the electrons flow through those two wires reverses 120 times a second (60 full cycles of reversal, over and back, each second).

As the electrons flow through the lamp's filament, they leave behind much of their energy. This energy is deposited in the tiny filament and the filament becomes extremely hot. It begins to emit much of its thermal energy as thermal radiation, part of which is visible light. So you can think of the electricity as a steady stream of tiny delivery trucks (the electrons), carrying energy to the lamp's filament, and then returning to the power company to pick up some more energy. The filament sends this energy into the room as heat and light.

When a light bulb burns out, it's because the filament has became so thin that a section of it has overheated and melted. This thinning process is caused by the slow evaporation (or actually sublimation) of tungsten atoms from the filament. A thinned filament usually fails as you turn the bulb on because that's the time of maximum power delivery to the filament and thus maximum stress. Unfortunately, it's very hard to tell in advance whether the filament will be able to tolerate the next attempt to turn it on. Probably the best predictor is the number of hours the bulb has been on. If you always replace a bulb after it has operated for 750 hours at full power, you'll probably avoid most outages.


666. What is a short in the electrical system of a car? What causes shorts? — BM, Puyallup, WA
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A short circuit is a conducting path that allows electric current to flow from its source (typically the positive terminal of a battery) to its destination (typically the negative terminal of that battery) without passing through the equipment that the current is supposed to operate. The conducting path is thus a short cut for the current that allows it to complete its circuit too quickly, hence the name "short circuit." In virtually all automobiles, the whole body of the car is connected to the negative terminal of the battery so that any accidental conducting path from the battery's positive terminal to the body of the car is a short circuit. Since a short circuit doesn't include a device that's designed to consume electric power, the wires of the short circuit must consume that electric power. They often become hot and may cause a fire.

680. What is the "memory effect" of a NiCad (nickel-cadmium) battery? Is it reversible or minimizable? - MF
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NiCad batteries are more rechargeable than most batteries because the chemicals that power NiCad batteries remain solid throughout the discharge cycle. The chemicals in most other batteries, including alkaline batteries, go into solution or otherwise change shape during the discharge cycle so that it difficult to reconstruct the original battery electrodes during recharging.

Unfortunately, the two solid electrodes in a NiCad battery are damaged by repeated charging and discharging. These electrodes work best when they are both fine powders (the positive electrode is nickel hydroxide powder and the negative electrode is cadmium metal powder). With repeated use, the powder particles grow larger and larger and they stop contributing to the battery's power. "Memory" appears during the discharge cycle when all the useful small particles have been used up and only the undesirable large particles remain. Repeated charging and partial discharging tends to convert many of the small particles into large particles. You can improve the battery by fully discharging it before recharging it, presumably because this deep discharge breaks up the larger particles so that the battery contains mostly small particles once again.


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