Page 45 - Teach Yourself Electricity and Electronics

Current flow 25

In household electric circuits, the charge difference will essentially never equalize,
unless there’s a power failure. Of course, if you short-circuit an outlet (don’t!), the fuse
or breaker will blow or trip, and the charge difference will immediately drop to zero. But
if you put a 100-watt bulb at the outlet, the charge difference will be maintained as the
current flows. The power plant can keep a potential difference across a lot of light bulbs
indefinitely.
You might have heard that “It’s the current, not the voltage, that kills,” concerning
the danger in an electric circuit. This is a literal truth, but it plays on semantics. It’s like
saying “It’s the heat, not the fire, that burns you.” Naturally! But there can only be a
deadly current if there is enough voltage to drive it through your body. You don’t have
to worry when handling flashlight cells, but you’d better be extremely careful around
household utility circuits. A voltage of 1.2 to 1.7 V can’t normally pump a dangerous cur-
rent through you, but a voltage of 117 V almost always can.
Through an electric circuit with constant conductivity, the current is directly propor-
tional to the applied voltage. That is, if you double the voltage, you double the current; if
the voltage is cut in half, the current is cut in half too. Figure 2-2 shows this relationship
as a graph in general terms. But it assumes that the power supply can provide the neces-
sary number of charge carriers. This rule holds only within reasonable limits.

2-2 Relative current versus
relative voltage for
different resistances.

When you are charged up with static electricity, there aren’t very many charge
carriers. A dry cell runs short of energy after awhile, and can no longer deliver as much
current. All power supplies have their limitations in terms of the current they can pro-
vide. A power plant, or a power supply that works off of the utility mains, or a very large
electrochemical battery, has a large capacity. You can then say that if you cut the resis-
tance by a factor of 100, you’ll get 100 times as much current. Or perhaps even 1000 or
10,000 times the current, if the resistance is cut to 0.001 or 0.0001 its former value.