Are Electrons the Buckets or the Water?

[holly]

I am working on E&M chapters. My professor said to think of electricity as a long line of firemen, an old-time bucket brigade. The guy at the well yells "Go!" and as soon as he yells, the electrical field is established at the speed of light or thereabouts. The fireman way at the end of the line, nearest the fire, throws his bucket. He is the first to throw. He throws the WATER, rather. But, there was already water in the bucket. He didn't need to get the water passed to him. They ALL have water in their buckets to start. Every time you look in a bucket, there's water already there.

Are the electrons the buckets, or the water? Is the charge the water?

One of my ex's says NO NO NO, that's all wet. The electricity in the wire is like the firemen holding hands. One squeezes the other one's hand, then he "passes" the squeeze along to each fireman, all along the whole length. The firemen have their feet firmly on the ground. They are not moving about. They pass the squeeze at almost the speed of light.

But, my professor says they ARE moving about, about a centimenter a second. Am I to believe an electron from Chicago is going to run down the wire all the way to Texas? Some questions in the chapter review ask where the electrons come from in your lightbulb, and the answer is, THEY ARE ALREADY THERE. I hope I'm using Texas electrons, personally.

I HAVE read three other textbooks about this. I have tried to do Google searches such as "electricity very simple explanations" and so forth. The books are just not beginning at a simple enough level. USING THE FIREMEN, or maybe even ants or something, can someone please give me an understandable explanation of the electricity running down the wire? Thank you.

 

[cookiemonster]

Okay! One of the more confusing and misunderstood concepts in E&M.

Electrons are the water in your example.

The electrons do move. They have a calculable (usually) drift velocity that, if I recall, is a lot like diffusion. Normally this drift velocity is very very very small compared to the actual transfer of information.

Another way to think of it is like a faucet connected to a hose. You turn on the faucet and it takes some time to get all the way through the hose. That time will be the length of the hose divided by the drift velocity. But once water starts coming out the end, it's coming out continuously. So water is pushing water is pushing water, etc. It's a big chain reaction.

So what happens if we turn off the faucet? Is the water going to keep going until the hose is empty? No, it stops pretty much immediately! That's because there is no more water being pushed into the hose, so there is no more water pushing it out the other end, so water stops coming out quite quickly.

In the E&M analog, the wire (hose) is already full of water (electrons). Once you start pushing water (electrons) into one end of the hose (wire), water (electrons) starts coming out the other end of the hose (wire).

Another analogy. Put two 2x4's on a table such that there's a path between them. Put a bunch of balls inbetween them so that the entire path is filled and all the balls are touching. Now push one ball at one end. What happens to the other end? The ball at the other end starts moving right off! But the ball you pushed is still at the first end! Hm...

cookiemonster

 

[holly]

Thanks, cookiemonster! I think I understand it now, and right in time for class, too. So they ARE moving but it's much like dominoes...I hope you are planning to be a professor somewhere once you get your PhD. Gosh, are you ever needed in this modern world! Thank you again.

 

[turin]

A dangerous analogy in the hands of those trying to disprove relativity.