How Everything Works
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MLA Citation: Bloomfield, Louis A. "How Everything Works" How Everything Works 24 Apr 2018. Page 23 of 160. 24 Apr 2018 <http://www.howeverythingworks.org/prints.php?topic=all&page=23>.
221. How can currents and electromagnets encounter frictional effects without touching?
When you slide a strong magnet quickly above a metal surface, there is a friction-like magnetic drag effect. This effect occurs even when the two objects don't touch. The origin of this effect lies in the repulsions between the metal and magnet: it's strongest slightly in front of the moving magnet so the magnet encounters some difficulty heading forward. The reason why the magnetization of the metal is strongest slightly in front of the moving magnet is related to the loss of energy by current moving in the metal. The magnetization (of the metal surface) in front of the moving magnet is fresher than the magnetization behind it. The current responsible for the magnetization behind the magnet has been flowing for long enough to have lost energy. But the faster you move the magnet across the metal surface, the less time the currents in it have to lose energy and the less friction-like force the magnet experiences.

222. How does a magnet induce a metal to become attracted to the magnet? Does the metal become a magnet also?
A steady, motionless magnet can't induce a piece of normal metal (not iron, cobalt, or nickel) to become magnetic. Only a moving or changing magnet can do that. When a metal is exposed to a changing or moving magnet, it does become magnetic. That metal becomes a type of magnet; an electromagnet. The metal itself isn't really the magnet; the electric charges inside it are. These charges move in response to the changing or moving magnet nearby and they become magnetic, too. The effect is always repulsive, not attractive. The temporarily magnetic metal repels the magnet that is making it magnetic.

223. How does running current through a coil cause a magnetic field?
Electricity and magnetism are interrelated in a great many ways. At the very basic levels, they are manifestations of the same fundamental physical concepts. As a result, electricity can produce magnetism and magnetism can produce electricity. One way in which electricity can produce magnetism is for charged particles to move. When an electric current passes through a coil (or any wire, for that matter), it creates a magnetic field. The coil develops a north magnetic pole and a south magnetic pole. I can't really explain why because the answer is simply that moving charges create magnetic fields; that's the way our universe works and no one has ever seen otherwise.

224. If magnetic trains are to work, wouldn't friction on the bottom of the train create thermal energy which would destroy the magnetism of the train?
When a magnetically levitated train is operating properly, it doesn't touch the track and experiences no friction. In principle, it shouldn't get hot at all. The magnetic drag effect will warm the track slightly, but that won't matter to the train's magnets. Actually, the train's magnets will almost certainly be superconducting wire coils with currents flowing in them. That type of magnet doesn't depend on the magnetic order of permanent magnets. It's the magnetic order of permanent magnets that is destroyed by heat. An electromagnetic coil will stay magnetic as long as current flows through it, even if it's so hot that it's ready to melt.

225. If you have more volts is it more energy (like a stun gun—is it better to have one with more current or volts or both)?
Volts is a measure of energy per charge. Thus if you tell me how much charge you have and the voltage of that charge, I can tell you have much energy that charge contains. I simply multiply the voltage by the amount of charge. Current is a measure of how many charges are moving through a wire each second. If you tell me how much current a wire is carrying and for how long that current flows, I can tell you how much charge has gone by. I just multiply the current by the time. To figure out how much energy electricity delivers to something (such as a person zapped by a stun gun), I need to know the voltage, the current, and the time. If I multiply all three together, the product is the energy delivered. In a stun gun, the voltage is important because skin is insulating and it takes high voltage to push charge through skin and into a person's body. But current is also important because the more charge that passes by, the more energy it will carry. And time is important because the longer the current flows, the more energy it delivers. So all voltage and current are both important. I can't guess which one is most important.

226. What is the dangerous part of electricity: charge, current, voltage, or what?
Current is ultimately the killer. A current of about 30 milliamperes is potentially lethal when applied across your chest. But your body is relatively insulating, so sending that much current through your chest isn't easy. That's where voltage comes in. The higher the voltage on a wire, the more energy each charge on the wire has and the more likely that it will be able to pierce through your skin and travel through your body. Thus it's a combination of voltage and current that is dangerous. Current kills, but it needs voltage to propel it through your skin.

227. What is the difference between current and voltage?
Current measures the amount of (positive) charge passing a point each second. If many charges pass by in a short time, the current is large. If few charges pass by in a long time, the current is small. Voltage measures the energy per charge. If a small number of (positive) charges carry lots of energy with them (either in their motion as kinetic energy or as electrostatic potential energy), their voltage is high. If a large number of charges carry little energy with them, their voltage is low.

228. What is the difference between fields and charges (magnetic and electric)?
Electric charges themselves push and pull on one another via electrostatic forces. Magnetic poles push and pull on one another via magnetostatic forces. We can also think of the forces that various electric charges exert on one charge that you're hold as being caused by some property of the space at which that one charge is located. We call that property of space an electric field and say that the charge is being pushed on by the electric field. We could do the same with magnetic poles and a magnetic field. But these two fields are more than just a useful fiction. The fields themselves really do exist. You can see that whenever moving electric charge creates a magnetic field or when a moving magnetic pole creates an electric field. Light consists only of electric and magnetic fields.

229. What materials are magnets made of?
They are mostly iron, cobalt, or nickel, which are intrinsically magnetic metals. 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.

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)?
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.

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