The Backyard Library · Why, Dad? · Episode 19

How does electricity get to your house?

The video says two surprising things: the electricity you are using was made less than a second ago, and the electrons in your lamp have never left the wire. Here are the two models behind those lines. Drag things and see.

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Lab 1

Flip the switch: what actually travels?

The wire is full of electrons already. When you flip the switch, a push races from the plant to your lamp at nearly the speed of light. The electrons themselves only wiggle in place. Slow the wiggle down to see it.

Slow motion 60× slower

Distance to the power plant 50 miles

0.00036 sfor the push to reach your lamp

0.13 µmhow far each electron wiggles (a bacterium is about 1 µm)

0electrons that travelled from the plant to your lamp

The wiggle on screen is magnified about a million times. In real time it happens 60 times every second, which is why your eye never sees it.

Lab 2

Up for the trip, down for your house

A town needs about 100 million watts. Push that through a wire at a low voltage and most of it turns into heat on the way. Step the voltage up for the trip and the same wire barely warms. Try it.

Voltage for the trip 345,000 V

Length of the trip 100 miles

0.8%of the town's power lost as heat on the trip

290 Acurrent in the wire

barely warmthe wire on the trip

Same wire, same town, same 100 million watts. Only the voltage changes. That is the whole reason the towers are so tall and the gray can on your pole exists: up for the trip, down for your house.

Try this at home. Walk outside and find your own transformer. On a pole it is a gray can; in newer neighborhoods it is a green box on the ground. That is the last step down to 120 volts, and you have walked past it a thousand times.

Electron drift speed in a household wire is roughly 0.05 mm per second at 1 ampere; at 60 Hz the back-and-forth wiggle is about 0.13 micrometers.
A signal travels along a power line at roughly 70% of the speed of light.
Heat lost in a wire is current squared times resistance; halving the current quarters the loss. Line resistance here is 0.1 ohm per mile, a typical order of magnitude.
US transmission voltages: 69, 115, 138, 230, 345, 500 and 765 kV. Distribution 12 kV. House 120 V.
These are the same models that drew the diagrams in the video.