Word Power
can be charged to a potential of ten thousand volts, but
the current flow when you discharge it will be
minuscule. We can measure high pressure in a bicycle
tire, but there is relatively little air there. On the other
hand, a huge tractor tire can have a lot of air in it, but it
may be at a fairly low pressure.
Renewable Energy Terms
Volt—Unit of
electromotive
force
Ian Woofenden
Access
Author: Ian Woofenden, PO Box 1001, Anacortes, WA
98221 • ian.woofenden@homepower.com
MINI
12/8
MINI
12/8
©1998 Ian Woofenden
Derivation: Named after Alessandro Volta, the Italian
physicist who first invented the battery. The volt
received official recognition as the unit of electrical
potential in 1881.
Voltage is an electrical concept that is frequently
misunderstood. In simple terms, it is electrical
“pressure,” analogous to pressure in a garden hose or a
bicycle tire. It is also called “potential” or “potential
difference.”
If I said I might drop a piano on you from my 34th floor
apartment, you’d know that it would hurt more when it
hit you than if I only dropped it from my cousin’s
apartment on the 5th floor. However, knowing what floor
I’m on would not tell you whether I have a concert
grand, a spinet, or my child’s toy plastic piano ready to
drop. And you wouldn’t even be sure that I would
indeed drop it, only that it’s possible. Likewise, voltage
in itself tells us nothing about the flow rate (current) or
the total quantity of electricity (though it does have a
relationship to them). Voltage is roughly analogous to
how many floors up my piano is, compared to a piano
sitting on the sidewalk below.
Voltage is not flow rate, and it is not volume. We can
measure high voltage when there is very little electricity
available, and low voltage when there is a virtually
unlimited supply. A capacitor the size of your fingernail
82
Let’s not say that a PV module “produces” 17 volts,
since having voltage doesn’t necessarily mean you
have electricity flowing. When you put your multimeter
leads on the terminals of the PV, you are only reading
the potential difference between the two points. Voltage
alone doesn’t give you the whole picture, and should
not be confused with current, power, or energy. Current,
which is measured in amperes, will be the topic for next
time.
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Home Power #68 • December 1998 / January 1999