Lately things electrical have been coming to mind. Namely

• the drift of electrons in electric current
• the operation of inductors
• Electricity -- a mixture of electricity and magnetism
• Electrical potential

A drift of electrons

When I was first studying electronics, I asked an electrical engineer about the flow of electricity. He said it was comparable to water. So for a long time I thought of electrical flow as being like water flowing in a river.

Then, when studying electricity in physics, I came across this equation for electric current:

Rearranging the members of this equation so that the relationships between the quantities is unchanged in order to find

Hang in there, I know this can be boring. If you must, skip to the last paragraph. I think you'll find it interesting.

Where

• I  = current in amps
• n  = number of electrons per cubic meter (using a value of 1 electron per atom contributing to the current flow)
• e  = electronic charge
• A  = cross-sectional area of the wire carrying the current
• v_d = drift velocity of the electrons travelling throught the wire

Using the following values (after computing them where necessary)

• I  = 1 amp (value selected for convenience)
• n  = 8.45 X10²⁸ electrons per cubic meter
• e  = 1.602 X 10^-19 Coulomb per electron
• A  = 1 mm² or 10^-6 meters² (value selected for convenience)

And the answer is --
v_d = 7.39 X 10^-5 meters per second, less than 0.1 millimeter per second.

If electrons drift along lazily about the width of a hair per second, why does the light in my room go on as soon as I flip the switch?

According to my physics textbook (University Physics, Alvin Hudson and Rex Nelson, Harcourt Brace Jovanovich, 1982)

...when a circuit is connected to a source of [power], the electric field is established in all parts of the circuit a nearly the speed of light. So when the final connection is made, forming a complete closed path with the [power source], electrons start to flow more or less simultaneously in all parts of the circuit. Even though the average drift speed of each electron is slow, all parts of the circuit feel the effects of the current almost instantaneously. ...

Journal entry dated 23 March 2007