Voltage and potential difference are different names for the same
thing. The potential difference is the amount of energy you get when
carrying a unit of electrical charge from one place to another. You
need to specify both places in order to define the voltage difference
between them. For example, the two ends of a battery have a potential
difference between them -- they are at different voltages.
Current is something else entirely. It is the rate at which
electrical charge flows. The standard unit for charge is the Coulomb,
and the standard unit for current is the Ampere. One Ampere is one
Coulomb per second. The unit for voltage is the Volt. All of these are
named after pioneers of electricity and magnetism. One Watt of power is
made available by flowing a current of one Ampere across a potential
difference of one Volt.
If you touch the battery with wet hands, then a lot of current may
flow from one of the terminals of the battery to the other. The
resistance of your body is low due to the fact that blood is mostly
saltwater, and saltwater conducts electricity rather well. The contact
with the skin is usually poor. Wetting your hands will make the contact
better, since the water will be salty from your skin's sweat. But the
battery can supply lots of current, and this can burn the tissue inside
the body.
For a few thousands of volts of static electricity built up on
your body by walking on a carpet on a dry day, the potential for damage
is less. This is because very little charge is involved, and once you
allow the current to flow (by touching a doorknob or a faucet, for
example), the voltage decreases rapidly. The whole spark is over in a
tiny fraction of a second and the voltage is gone. Power is voltage
times current, but the total energy deposited is power times time, and
if this is short enough, very little energy is deposited. The battery
can keep supplying current for a long time, depositing more and more
energy, making the burns worse.
Tom
(published on 10/22/2007)
