MadSci Network: Engineering

Re: Speed of an electron in a current.

Date: Wed Oct 28 07:33:28 1998
Posted By: Lawrence Skarin, Faculty, Electrical Engineering, Monroe Community College
Area of science: Engineering
ID: 909472726.Eg

Hello, Danny.

The subject you are asking about is "electron drift velocity." "Drift" is electron motion under electric field influence.

Your question is dealt with admirably by Bill Beaty at:
, where he computes electric current in a lamp cord going to a 100 watt lamp at 8.4 centimeters/hour! Talk about slow!!

Now, how do we believe this, and yet see a lamp come on "instantly" when we shove the plug into the socket? The answer is the speed of the electric field traveling from the socket to the lamp. The field gets there quickly whereas the electrons may never! Bill did his computation with direct current (DC). With alternating current (AC), electrons go back and forth within a tiny space.

Let me use an analogy. A brand-new 20 meter garden hose begins spurting a few seconds after you open the valve. Thereafter, (because the hose is full of water), the water spurts from the nozzle "instantly" when you open the valve. This is because the pressure wave generated by valve opening travels much more quickly than the cubic centimeter of water emerging from the valve. Well, a wire is already full of free-to-move electrons. You don't even have to fill it.

To find out more on how speed of drift can be measured, put "electron drift velocity" into your search engine and see what you get. Most interest these days is in drift within semiconductors because this affects speed in the computer sense.

So you see, the individual electrons don't do much traveling at all!

Good luck in your studies. Please bookmark Bill Beaty's site. His insights and explanations will help you greatly.

Larry Skarin

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