Question Board -- Questions about Electricity and Magnetism

These are questions and reports sent in by participants in the Virtual Workshop on Electricity and Magnetism, and answers and comments. If you have a question or an answer or a comment, send it in!
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The questions (Click on a line to jump to the entry)
Questions about electricity and magnetism
What is the difference between a cell and a battery?
Thermal conductors and electrical conductors
What happens to the electrical current when the circuit is broken
Why 110 V?
Does hammering make magnets, or destroy them?
What is the importance of the e/m ratio?
The average value of the voltage in an AC circuit is zero. So how does anything happen at all?
Why is there a light-emitting diode on my TV remote?
What is the magnetic field good for?
How can I turn a magnet off?
Can I run an electromagnet on AC?
Can a magnet float in mid-air, or make something else float?

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My Question: Aren't poor electrical conductors also generally poor heat conductors? I know that the primary reason for coating wires with an insulator such as rubber is to insulate them for electrical purposes, but isn't it also to prevent thermal conduction?

Sally's answer
There are two different kinds of thermal conduction. Energy can be transported by having atoms bumping into atoms (vibrations of the material, is another way to describe it), and it can be transported by having electrons carry it around. Electrons also have a charge and are a part of electrical current. So good electrical conductors are in general good thermal conductors. But there are also materials that conduct energy by the atom-bumping route that are not electrical conductors at all.

While the insulation on a wire is not as good a thermal conductor as is the wire itself, that isn't why it is there -- it is there to keep the electrical current from taking the wrong path. Having warm wires is not desirable, and I'm sure that some electrical engineer is considering right now how to design some power line so that the electrical insulation is present and the thermal insulation is minimized.

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My Question: What happens to the electrical current when a circuit is broken? Sally's answer
Of course it stops. But the interesting part of the discussion, is how does it stop, and how fast. Let's compare two examples that we are familiar with, where a flow is halted:
*Consider a highway with cars zipping down it, when the road fixing people decide to have a humongous earth moving machine cross it. The car-circuit is broken and the cars come to a stop, but only near where the break is -- blocking route 64 in Charleston, WV does not immediately cause cars to stop on the Lexington KY bypass.
* Imagine a hose carrying water and you turn off the faucet. All the water in the hose and in the pipe leading to the faucet stops, more or less immedialely -- the news that the faucet has closed travels down the pipes at the velocity of sound (in water). This will be associated with a temporary rise in pressure in the line -- an effect called "water hammer" -- which exerts the backwards force that stops the water.

An electrical circuit behaves more like the hose than like the traffic. The current consists of electrons, which are charged particles. They interact with a long-ranged electrical force that is very sensitive to tiny imbalances in charge. So when the electron-traffic jam starts, all electrons in the wire find out about it at the speed of light. There is also an effect a little bit like water hammer. An electrical current gives rise to a magnetic field, and there is energy associated with it. When the current turns off, the field goes away, and a large electrical "force" occurs: this is why devices involving electromagnets will experience very large voltages when the current is turned off.
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My Question: Why do we use 110 and 220 V? Were these values chosen because they are convenient and are manmade and could have been different, or do they relate to some constant natural law of science? I mean, soft drink cans are 12 oz or 33 cl but this was chosen at random. Cans do not suffer if they are bigger. The minute is 60 sec by convention, not a natural law. Maybe electricity travels best in units of 110 V?

Sally's comments
We can't think of a physics reason for it. It surely is just random -- in fact more random than the can choice, which is a reasonable size (I finish my can, and don't feel the need for another). You will hear 110, 115, and 120 all quoted as the standard voltage -- I gather that the power company people can't control the voltage that arrives at the house more accurately than that. The value was chosen a long time ago -- probably in the early days of the Westinghouse Corporation (who were the first proponents of AC power distribution) -- and once chosen, we are pretty well stuck with it. All the light bulbs would blow out if the voltage went up to 150, and refrigerator motors will stall and burn out if the voltage goes too low.

Flashlight batteries are 1.5 V for a physics reason: it is determined by the amount of energy that is released per electron transferred in the chemical reactions inside the battery. A 9 V battery actually has 6 little cells stacked up inside!
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My Question: Does hammering make magnetism, or destroy it? If a bar magnet can have its magnetism removed by hammering, why is it that ships or bridges can acquire magnetism through hammering? In the bar the small magnetic fields are disrupted, but in the ship they are somehow organised instead of being continually disturbed, how is this?

Joe's comments
It is a matter of degree. In the absence of an applied field, a magnet doesn't really want to be magnetized: it would lower its energy if it could randomize the orientation of the domains (if you put two bar magnets next to each other, they will stick together so that if one points north, the other points south -- they try to cancel each other out. This is equally true of the internal parts of a magnet -- they also want to point in opposite directions of each other, down to fairly small length scales. Somewhere below a micron in size, it begins to cost energy to subdivide the magnet more). So if you hammer on a good magnet, you will produce a less good one; the highest strength magnets are treated as if they were very fragile.

However, when there is an external field, then the domains want to align with it. But they probably can't change easily (and if they could, they would equally well change back as soon as the field went away. To make a permanent magnet we don't want anything wishy-washy). Hammering on a completely unmagnetized object will allow the earth's magnetic field to align it a little bit. In the case of a bridge or a fence post that maintains a constant orientation for much of its history, this will produce a weak magnet; in the case of a ship, I suppose it matters which way it is oriented when they build it or work on it. The amount of magnetism that can be acquired this way is noticeable (the first magnets were found objects) but not great.

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My Question: What is the importance of the e/m value?

Joe's answer
By studying how a beam of electrons behaves in a magnetic field, we can show that is made of particles with a definite ratio of charge to mass.

100 years ago scientists did not know what electricity was. In some respects, it acts like a fluid running down the wires, but it could also be interpreted in terms of stretching a spring (where voltage corresponds to the force on the spring and current is how much the spring is stretched. In this analogy, there isn't anything moving in the wires at all). Other possibilities were that there are many kinds of charged particles, or that charge doesn't have mass (light also seems to flow from one place to another, but the mass associated with the flow of energy is immeasurably small). The e/m measurement showed that the charge is carried by particles, of a definite, single kind: electrons. Showing that charge is carried by particles gave science the understanding of electricity that made possible vacuum tubes and transistors. It's quite likely that you are reading this on a computer screen that is being "written" by a beam of electrons -- if there was not a definite e/m ratio, the device wouldn't work at all.

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My Question: The average value of the voltage in an AC circuit is zero. So how does anything happen at all?

Joe's answer
The voltage changes rapidly from our point of view, but it is actually very slow. In 1/120 of a second the fastest electrons in the wires have time to run from the power plant to your house and back to the power plant. For all of this "very long" time, the voltage is driving the current through the wires, and it is hardly different from direct current. So your light bulb lights up. Then later, the voltage changes direction, but the current changes direction, too. Again the light bulb lights up.

Here is a different version of the explanation: electrical power is the product of current and voltage difference. So long as the current is flowing from high voltage to low voltage, electrical energy is being turned into other forms (i.e. the light bulb is on, the motor is running, ...). What matters is the average of current x voltage, and not the averages of the quantities themselves; voltage alternates between plus and minus and the current alternates between plus and minus but the power stays positive.

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My Question: Are the red light on the TV remote and the red light on the TV light emitting diodes? Why do they use them there?

Sally's answer
The light on the remote surely is a light-emitting diode, and the one on the television probably is. LEDs are low voltage devices -- they work well on 2 or 3 volts, and are very efficient (all of the electrical energy is turned into light), and don't burn out like light bulbs (I'm not sure what they would ever die of. There's a question for an investigation! -- but you probably will need to wait several years to find out the answer!). The remote is operated by a few batteries, and we don't want them to run down any sooner than necessary, so LEDs are a natural application here. The TV could use an ordinary light bulb with a red cover over it; the only advantage to an LED would be that it doesn't burn out. You will see LEDs in the brake lights on cars (especially the ones in the back window of the newer cars) and in some stop lights around town. For that matter, the TV remote "talks" to the TV by blinking an infrared light at it, and that is also a LED -- just not a visible light one. Light emitting diodes are not common just yet, but eventually they will be everywhere.

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My Question: What is the magnetic field good for?
Joe's answer
The magnetic field has three important properties, which causes it to be used all over the place:
*We can create it with an electromagnet. That means it can be turned on and off; we can make things happen when we want it to happen.
*It causes a force on certain materials that are at a distance. The magnet doesn't actually have to be touching the thing that is attracted.
*the field penetrates most materials. So a coat of paint doesn't matter.

Here are some applications:
* Chemists and biologists can stir a liquid with a magnet, by setting up a rotating magnetic field outside the container. The machinery that causes the rotation isn't in contact with what is being stirred -- which is very useful if the contents are dangerous bacteria, poisonous chemicals, or radioactive stuff. Only the magnet and the container get contaminated, and they can be replaced.
* The keys on a computer need to cause a switch to open and close when you push on them, and these switches need to continue to work when dust, bread crumbs, and an occasional splash of soda get in. A switch that required direct metal-to-metal contact would soon stop working, but instead the keys contain a magnet and are over a magnet-detector. It is enough that the magnet gets closer -- it doesn't actually have to touch for the key to work.
* At the recycling plant, steel cans are separated from the aluminum cans (and the paper and glass objects) by passing the stuff that has been collected under a magnet. The can can be underneath or inside something and the magnet will still find it.

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My Question: How do you turn a magnet on and off?
Sally's answer
What you want is an electromagnet. For this, you need a piece of wire that is 1/2 meter (18 inches) long or longer. The kind of wire is not very important, but it doesn't have to be very thick, and doesn't need thick insulation on it. We use wire that is made for this purpose; it is about 1/2 mm (1/64") thick and is insulated by a coat of varnish.
Remove the insulation from the last cm (1/2 inch) of each end, by scraping with a knife or sandpaper. To make the magnet, wrap the wire around a drink straw, always going around in the same direction, making a neat tight coil, remembering to leave 10 cm (4 inches) at each end not wrapped, so that you have a pair of wires to connect the device to a battery.
Now touch the two wires to the two ends of a flashlight battery, and while it is connected, see what effect the magnet has on a compass or some paper clips.

Be aware that the electromagnet runs down the battery pretty fast. Don't leave it connected while you are writing up your results or looking for something else to do with it.

Here are some experiments you should try:
1. Compare the behavior of the electromagnet to its behavior when you push a piece of iron or steel (a nail or a screw or a bolt) inside the drink straw.
2. Compare what happens with one battery and with two batteries connected end-to-end, like in a flashlight.
3. Compare an electromagnet made with a long wire and with a short wire.
4. Compare an electromagnet made with thin wire and with thick wire.

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My Question: I need a very strong magnet. Can I use an AC electromagnet for this?
Joe's answer
You can connect an electromagnet to 110 V AC, but there are two important consequences:

*The field will rapidly change direction, so that a magnet will not be attracted by it -- 60 times a second, the electromagnet will pull and then push, with not much net effect.

*There is an effect call inductance, which can be interpreted as meaning that there is energy associated with a magnetic field. 120 times per second, the field will build up and all the energy has to be added; but then 120 times per second, the field goes back to zero and all the energy comes back out again. Another way to think of this is to say that there is a "magnetic inertia" that keeps the magnetic field from changing rapidly. Sometimes this is of interest, but in your application it probably means that you will not get as large a magnetic field as you would with a DC current. However, you could put a device called a rectifier in the circuit, which only lets current go one way. The voltage will still be varying a lot, but now the inductance effect means that you will see the (constant) average magnetic field, and the variations due to the changing voltage will be greatly reduced. So you don't need a very fancy power supply, if you don't mind a certain amount of variation.

If all you need is a magnetic field, a big permanent magnet would eventually be the best -- it doesn't cost anything to run, and won't burn out.

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My Question: Can I make magnets repel off the earth's magnetic field, or create a strong magnetic field to make objects float in mid-air?
Joe's answer
The short answer is No.     The long answer is Yes: there are several ways to make a magnet hover, but each is complicated or tricky or just barely works.

If you try to make one magnet hold another up, you will find that one or the other always finds a way to turn over, so that now they attract.   There is a law of physics hiding in this observation, having to do with how the force between two magnets behaves.    It says that the magnetic energy never has a minimum, which implies that there always is a "downhill" direction for the magnet to go.

Method #1 to avoid this is to prevent the magnet from turning over.   There is a toy you can buy at Edmund Scientific, (see http://scientificsonline.com/product.asp_Q_pn_E_3053435 ) in which a cylinder is supported by a magnetic field.  But it is touching a glass plate at the right end (in the picture on the Edmund page), so it isn't really hovering unsupported.

Method #2 is to prevent the magnet from turning over by having it spin really fast, like a top.   Edmund Scientific offers this, too
( http://scientificsonline.com/product.asp_Q_pn_E_3081525 ).   I have one of these;   they are very tricky to get going.   The weight of the top has to be adjusted very precisely (depending on the temperature and on the presence of iron or steel or any other magnet anywhere in the room) and the base has to be exactly levelled, and the top only spins for a minute or two before it is slowed by air friction enough that the top falls over.   Still, it really is hovering.

We won't even discuss the methods in which there is a detector that watches the hovering object and changes the magnetic field in response to its motion (it is the magnetic equivalent to balancing a ruler on your finger.  You can only do it with your eyes open.), because this is totally cheating.

There is a second set of ways to make a magnet hover, which depend on the existence of materials that respond to the presence of a magnet and change their properties.   It is sort of like a mirror:  you can't walk through one because the mirror-person is always in your way (you can't even put your finger through the mirror, because she puts her finger at exactly the right place to block it, no matter how fast you move your hand.   How does she do that?).   

The very best "magnetic mirror" is a superconductor.   You can easily suspend a small magnet over a superconductor.   If the magnet tries to turn over or move, the superconductor changes its magnetic properties to oppose it.   Unfortunately, superconductors only work at low temperatures -- -170 C (-300 F).    Here at the physics department we make things this cold all the time, but you can't do it at home. Even then, this seems a miracle to older physicists, like me -- when I was a student, superconductivity didn't exist above -260 C (-440 F) and required very special refrigerators.

A superconductor is a "perfect diamagnet" which means that no magnetic field enters it at all (just as no light enters a mirror).   However, there are also weak diamagnets, that  only slightly oppose a magnetic field (just as a glass window acts like a mirror sometimes, even though most of the light that hits it goes on through).   At http://scitoys.com/scitoys/scitoys/magnets/suspension.html , you will read a description of a way to make a magnet hover using pieces of bismuth, which is a diamagnet.   The way this man's device works is that he has the hovering magnet carefully balanced between gravity and the attraction of another magnet, and the bismuth is giving a tiny nudge if the hovering magnet gets out of place.   I'll bet it is really tricky to set up.

Many things are diamagnetic, including most things made of organic molecules.   So some people in the Netherlands who have a tremendously strong magnet made a frog hover (  http://www.hfml.sci.kun.nl/froglev.html ).   This also is not a very practical way to hold up something (you need a multimillion dollar magnet machine). It doesn't seem like a great experiment, but they had fun (I don't know what the frog thought).

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