Eric's Beijing Diary

This video gives a good description of power lines and how they work. But it occurred to me as I was watching this video that some of you may need some basic explanation of the various electrical entities, and how they interact and affect each other. The video gives several formulas, but to make it simple, I want to focus here on just three basic electrical realities:

E	Electromotive force (emf)	The force pushing electricity through the wire	Volts
I	Current	From the French phrase intensité du courant, (current intensity)	Amps
P	Power	The overall power flowing through the wire	Watts

To help understand how these three work together, I will use a river as an analogy. Many years ago, I was a riverboat pilot on the Mississippi River (for about 10 minutes). I was trucking at the time, and I had a load going into the Twin Cities. I got there on a weekend and could not get my load off until Monday, so I basically had nothing to do. I was at a truck stop which was located right next to the river. There was a guy there offering drivers a free ride to a casino which was also situated on the river. I am not much of a gambler, but I thought it might be nice take a free ride on the river.

When I got on the boat, I started asking about it. It turned out that the owner of the boat was on board, and he was very congenial about showing me the two Cat engines that powered the craft. Then he took me up to the bridge where the captain was steering the ship. He told the captain to go ahead and take a break. I don’t know if the owner of the ship was authorized to navigate the craft, but he was, after all, the owner, so the captain obeyed and went down to the main deck to get a snack or something. As soon as the captain left, the owner turned to me and beckoned me to come over and tahe the wheel. He told me to keep the ship between the buoys on either side. I thought I was doing that pretty well, but a call came from the captain on the main deck to move the ship more toward one side. I did, but apparently not to the captain’s satisfaction. Remember, all this time he thought the owner was steering the ship. He became concerned and came up to the bridge. As soon as he saw me steering the ship he said, “I’ll take over now,” and took the wheel. That ended my career as a river boat pilot on the Mississippi.

I said all that to say that if you read Twain’s Life on the Mississippi, you will get a feel for the awesome power of the Mississippi River. But believe me, there is nothing like actually doing it. The immense power of that river is really extraordinary. Incredible power. Why? Because it was flowing so fast? No. It looked to be flowing very slowly. But there was just so much water. So, to complete the analogy, if we let the width of the river represent the electrical current in a wire, and the force which is pushing the river along represent the electromotive force (voltage), then the over all power of the river would represent the electrical power (P) measured in watts:

P = I * E, or Watts = Amps * Volts

There is a problem with my analogy, because we use the word "current" when talking about rivers, too. But it's different. The current of a river is really more analagous to electrical voltage, because it refers to the force of the flow. When we talk about a river having a swift current, we are not commenting on how wide it is. But with electricity, the word current refers more to magnitude. So just follow through with my analogy. A river flowing down a mountain would not be nearly as wide as the Mississippi, but since it is flowing down fast, the force with which it flows would be much greater per unit volume than the Mississippi. So a river narrower than the Mississippi could have the same amount of overall power if it is flowing downhill at a much faster rate. Easy to talk about, but actually it would be pretty tough to find a river that could compete with the Mississippi, because it is just so vast. But you get the idea.

But even if you have never been a riverboat pilot on the Mississippi like Mark Twain and I, I think you will be able to follow this video. It's pretty straightforward, and very informative.

As you watch it, you will see that if there is too much electricity flowing through a wire, the wire will melt. To move the same amount of power through the wire without heating the wire too much, we can greatly increase the voltage so that the current can be decreased (note the formula above). For example, 4 * 5 is 20. But 20 times 1 is also 20. You see, if we need one of the factors to be really low (in this case, current). We can raise the other factor up high to get the same value. But isn’t that very high voltage dangerous? Yes, and that’s why we need to keep the power lines up high so that little kids can’t touch them. Also, super high voltage electricity can sometimes jump through the air. But only for a short distance, so as long as the power lines are separated from other metal (ground) the electricity will stay in the wire and be taken to its destination with very little loss. Final Question: But is that really high voltage useful for every day type of stuff like powering household appliances? No, that’s why you have to have another transformer at the other end to bring the voltage back down so that it can be used.

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Labels: Electricity