To really understand what is happening in an electrical circuit,
we have to appreciate that something is actually moving in the wires,
and be able to imagine it. The direction of the current is an
important part of the mental model.
Two circumstances help hide this from us:
the easiest thing to study is a light bulb, which doesn't
care about the direction of current at all;
household electricity is alternating current, which changes direction
so fast that the current seems to be flowing both ways at once.
The 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.
In this section we will meet several new devices that strongly
care about the direction of current.
Appliance plugs frequently are designed so that they can only
go into the receptacle one way. Is this also an example of the
direction of current? No, it is a safety feature -- a way to
keep the hazards of electricity as far away as possible.