Electricity is a very important means of transport of energy
from one place to another --
because it is very easy to connect and disconnect energy-using
devices;
because it is easy to convert mechanical energy into electrical
energy and vice versa;
because it is easy to transport electrical energy;
because electrical energy is transmitted very rapidly (almost
at the speed of light);
and because there are many physical effects involving
electrical energy which make possible conversion into light, sound,
motion, force, heat, cooling, and rapid transmission of
information.
Of course, before we can make devices that use electricity, we have to
understand how electricity works.
To understand what is happening in an electrical system,
we have to appreciate that something is actually moving in the
wires, and be able to imagine it. The wires have to provide a path
for the current -- a circuit that goes from one terminal of
a source of electrical energy (like a battery) to the other terminal.
The current has a direction, which is
an important part of the mental model.
One place where current direction makes a difference in
everyday life is when we put batteries into something. If you put a
battery in backwards, the device most likely will not work.
However, most devices require that current pass in the right direction.
In this section we meet the light-emitting diode,
which cares strongly about which way the current is going.
The "simple" circuit
All the circuits discussed in the first module were simple, in the sense that
there is only one path for the current. All of the current goes through each wire
and any devices that are in the circuit. The current is everywhere the same.
You can do a lot with such circuits! Here's an example:
The refrigerator light is only on when the door is open.
How would you make a switch that works this way? You
could use your switch to make a "burglar alarm," that would cause a buzzer to sound
when a door is opened.
More about capacitors
The diagram for a
capacitor tries to describe what is inside. Some capacitors
actually look like this: a pair of conducting plates with a thin layer
of insulation between them. The "plates" can be large pieces of metal
foil, separated by insulating paper, and then rolled up to make it
compact. However, it would take many square miles of foil to make
a capacitor comparable to the one in the kit; the typical foil-and-paper
capacitor will store only 1 millionth as much energy as the one in
the kit.
Capacitors are used for several purposes in electrical
circuits.
They can store energy. The capacitor in your kit was originally
intended as a power supply that could keep a computer from
forgetting everything in a short power outage. Computer monitors
and automobile ignitions require a very high voltage, and this is
produced by pumping current into a capacitor, in much the same way
that high pressure air is made by pumping air into a tank.
Capacitors can store information. The fastest memory in a
computer is in the form of a large set of tiny capacitors.
Capacitors are a kind of filter: a constant current is blocked
when the capacitor charges up, but an alternating current is
transmitted. This in turn leads to their use in more complicated
filters; when you turn the dial on a radio you most likely are
changing the behavior of a capacitor.
This lets the signal from one station enter into the circuit,
so that you hear that broadcast and not that of all the other stations.
The diagram for a
capacitor tries to describe what is inside. Some capacitors
actually look like this: a pair of conducting plates with a thin layer
of insulation between them. The "plates" can be large pieces of metal
foil, separated by insulating paper, and then rolled up to make it
compact. However, it would take many square miles of foil to make
a capacitor comparable to the one in the kit; the typical foil-and-paper
capacitor will store only 1 millionth as much energy as the one in
the kit.