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This Week's Experiment - #272 The Speed of Electricity
This week's experiment is the result of a discussion that I recently had
involving a common misconception about the speed of electricity. Many books
say that electricity travels at the speed of light, but does it really? To
find out, you will need:
a garden hose
a water faucet
First, lets take a look at this misconception. Turn on the light in the room
where you are. After you flipped the switch, how long did it take before the
light came on? It seems to happen instantly. When you flipped the switch,
did the electricity rush past the switch, through the wires to the light, all
at the speed of light?
No. Electricity is caused by the movement of tiny pieces of atoms called
electrons. If you could see one of these electrons as it moves through the
wire, you would find that it moves only a few centimeters per second. Now
wait a minute! If the electricity is that slow, why doesn't it take several
seconds for the light to come on?
To understand that, go outside and get your garden hose. Make sure that it
is empty (no water in it) and connected to the faucet. Turn on the faucet
and notice how long it takes before water squirts out the end of the hose.
It probably took several seconds. OK that makes sense. It takes a while for
the water to move through the hose to the end.
Next, turn off the water. Wait a second and then turn it back on. How long
did it take this time for the water to squirt? It happened almost instantly!
Was the water in the hose moving at the speed of light this time? No, of
course not. What was different? The hose was already full of water. Water
coming past the valve on the faucet pushed on the water already in the hose.
All the water in the hose moved and some came out the other end as soon as
you turned on the faucet.
The same idea applies to electricity. The wires in your house contain lots
of electrons, just as the garden hose contained water. When you turn on the
switch, the electrons in the wire get a shove and they all move, just as the
water in the hose. Neither is moving at the speed of light, but both get
instant results at the far end. Just be sure that the hose is not pointed
towards anyone or you may have to move at "light speed" to get away.
These experiments are from Robert Krampf - The Happy Scientist
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