slow electrons fast electricity

[ensign siegel]

Electricity travels at the speed of light 'along' a wire we are taught. But isn't the electron drift in a wire very slow even when the battery is connected? What is the 'electromagnetic wave' that travels along a wire if it isn't electrons. Or is it electrons? I'm confused. And since I'm a primary school teacher, if I'm confused so are the pupils. Has anyone an explanation I can understand with some basic physics?

[Cap'n Refsmmat]

Depends on whether you're talking about alternating or direct current. Direct current relies on the speed of the electrons. Alternating current, however, moves at the speed of electric field propagation through the wire; because each electric charge is accelerating, it emits an electromagnetic field that propels the other electrons in the wire. Electromagnetic fields travel at the speed of light.

 

Perhaps an analogy would help. Suppose I have a solid steel bar and I push on the end, so that it slides through an opening. The rate at which it passes through the opening depends on how hard I push it. However, if I hit the bar with a hammer, the force is transmitted from my end to the far end incredibly rapidly -- at the speed of light. The atoms I hit push on the atoms next to them, which push on the next atoms, and so on, all through electrostatic forces, which travel at light speed.

 

Of course, you can also point out that the average speed of a single electron in an alternating current wire is 0, since it oscillates back and forth...

 

If you know differential equations, here's how I learned AC current speed:

 

http://farside.ph.ut...ves/node37.html

 

(disclaimer: I'm not entirely certain of my description of how AC currents work)

[lemur]

I was under the impression that the explanation is simply that particles don't have to move (as much) as waves that travels through them. E.g. an ocean wave does not carry its own set of water molecules. The water molecules push each other transmitting the energy of the wave through the water as a medium. I thought that electrons in a conductor work as a medium for the current, only I think gas molecules are more accurate analogy than water waves because the waves of electric current travel as pockets of compressing and decompressing electrons, much like a sound wave travels through air. Is this false?

[Cap'n Refsmmat]

That is exactly true for alternating current. Direct current is a real current flow, though.

[swansont]

Depends on whether you're talking about alternating or direct current. Direct current relies on the speed of the electrons. Alternating current, however, moves at the speed of electric field propagation through the wire; because each electric charge is accelerating, it emits an electromagnetic field that propels the other electrons in the wire. Electromagnetic fields travel at the speed of light.

 

 

But the DC interaction still moves at the speed of light (in the material). You don't have to wait for electrons from the battery to make it to the light bulb to light it up.

[TonyMcC]

One analogy uses the idea of a long train of carriages on a railway line. If you push the last carriage the effect is felt almost immediately by the first carriage, which moves. For an a.c. circuit imagine the train is as long as the circuit of rails. Pushing and pulling the train over a small distance will be felt at any point around the circuit.

Edited January 8, 2011 by TonyMcC

[skyhook]

I think the teacher thread starter need to keep it simple for the kids, sometimes they do not need to know about some stuff at the primary school level.

 

These questions do make me think a bit.

 

So you have a wire circuit, and a current flows in it when you on a switch. When the current flows, it generate electromagnetic waves around it. This EM radiation is a form of energy radiation. More specific, how will you say it ? This is a bit complex for me. Photons ? or some particles ? When you off the switch, the EM wave is not detected anymore, (I think).

 

There is this electronic tool, that is a non contact ac/dc meter. It is like a test pen. You just need to touch the cable on the outside or just point the tip near to the cable. If a current is flowing, it will be able to detect it. Quite cool, unfortunately, the one I have couldn't detect voltage lower than 12v. I would think this tool make use of the principle that an EM wave occurs with a flowing current, to detect and measure it.or if it not, pls say so anyone...<br>

Edited January 11, 2011 by skyhook

[Schrödinger's hat]

There is this electronic tool, that is a non contact ac/dc meter. It is like a test pen. You just need to touch the cable on the outside or just point the tip near to the cable. If a current is flowing, it will be able to detect it. Quite cool, unfortunately, the one I have couldn't detect voltage lower than 12v. I would think this tool make use of the principle that an EM wave occurs with a flowing current, to detect and measure it.or if it not, pls say so anyone...<br>

 

For AC this sounds right. I can also see it working quite well on auto-electronics, as there is generally a fairly large fluctuation there, but for DC, if the current is steady, there is no travelling wave.

It could work on magnetic and electric fields, or there could be more thermal fluctuation on a DC load than I thought.

 

In reply to the OP the best analogy I have encountered for DC electricity is water.

 

Think of a water tower with a long hose.

Almost as soon as you turn on the tap (after a speed of sound-in water delay), the water level falls. But it can take a long time for a pebble, or bit of muddy water to reach the end.

One way to show the distinction may be to get a hose,

fill it with water so it turns on or off as soon as you turn the tap.

Then unplug it from the tap (hold both ends roughly level so you don't spill too much) and add a few drops of dye.

You can then turn the tap on and off, starting and stopping the water flow, but you have to wait some time before the water changes colour.

The wave, or the energy or motion of the particles travels quicker than the water.

Another thing to explore is that the wave always takes the same amount of time (there will be small variations due to effects of elasticity of the hose, and this will be quite small unless you have a very very long hose), but the flow depends on pressure, (how open the tap is, or the height of the reservoir)

 

Other things you can explore:

Gravitational potential is analogous to electrical potential.

If you do not use a hose, and let water fall (a hose can store energy in elastic potential) then the amount of work you can do is proportional to how far the water falls, it can spin things, or push someone over.

You have to bear in mind that water can also store energy in its velocity.

 

Depending on your budget you can explore analogies to other elements too.

 

A T-junction in a hose with a balloon on it acts much like a grounded capacitor. you can put a tap on one end, a nozzle or constriction on the other end and show how the capacitor (balloon) stores charge (water) and energy (pressure/elastic potential).

 

As I stated, energy can be stored in velocity. This is somewhat analogous to an inductor, more accurate would be a turbine with a flywheel (it will resist flow until velocity is gained, and resist reduction of flow until it slows down)

 

Constrictions are analogous to resistors.

 

Make sure you make it clear that water isn't exactly the same as electricity. For one electrons don't run out the end if you leave the plug on because they do not have enough energy to leave the metal. (some analogy could be drawn with surface tension, i guess) Also magnetic fields are an important aspect of electronics which is completely absent.

 

Hope this helps.

 

I guess I forgot the physics. Although others appeared to answer this adequately.

To simplify slightly, the same thing keeping the electrons moving is keeping the water moving.

As one electron is moved, the field around it exerts a force on the next electron, this one then accelerates until the field from the one after balances it out. Eventually (after a few milli or micro seconds) they are all moving and slightly closer together (well the ones before a resistance are closer than those after, but this is not necessary for the basic theory).

If there is a switch, you can imagine the electrons are bunched up (at a higher voltage) ready to push on other electrons they encounter when the switch is closed.

The same thing happens in the water. The electrons on one molecule get pushed close to those on another, etc etc.

If we have a tap, the water is at a higher pressure (bunched up) so it comes out immediately.

The main difference is that it's not just pressure (How much the water is pushed together) that stores/transmits the energy in the water, but velocity, height (gravitational potential) and so on.

Edited February 22, 2011 by Schrödinger's hat

[Klaynos]

I like the marble analogy...

 

If you have a tube of red marbles and you push a blue one in the end, a red one pops out the other end nearly immediately, so the energy you've used to push that marble in has travelled down the line much much faster than the marble itself.