Page 44 - Teach Yourself Electricity and Electronics

24 Electrical units

2-1 Electric lines of flux always
exist near poles of electric
charge.

often 12 V to 14 V. In household utility wiring, it is a low-frequency alternating current
of about 117 V for electric lights and most appliances, and 234 V for a washing machine,
dryer, oven, or stove. In television sets, transformers convert 117 V to around 450 V for
the operation of the picture tube. In some broadcast transmitters, kilovolts are used.
The largest voltages on Earth occur between clouds, or between clouds and the ground,
in thundershowers; this potential difference is on the order of tens of megavolts.
In every case, voltage, EMF, or potential difference represents the fact that charge
carriers will flow between two points if a conductive path is provided. The number of
charge carriers might be small even if the voltage is huge, or very large even if the volt-
age is tiny. Voltage represents the pressure or driving force that impels the charge car-
riers to move. In general, for a given number of charge carriers, higher voltages will
produce a faster flow, and therefore a larger current. It’s something like water pressure.
The amount of water that will flow through a hose is proportional to the water pressure,
all other things being equal.

Current flow

If a conducting or semiconducting path is provided between two poles having a poten-
tial difference, charge carriers will flow in an attempt to equalize the charge between
the poles. This flow of electric current will continue as long as the path is provided, and
as long as there is a charge difference between the poles.
Sometimes the charge difference is equalized after a short while. This is the case,
for example, when you touch a radiator after shuffling around on the carpet in your
hard-soled shoes. It is also true in a lightning stroke. In these instances, the charge is
equalized in a fraction of a second.
The charge might take longer to be used up. This will happen if you short-circuit a
dry cell. Within a few minutes, or maybe up to an hour, the cell will “run out of juice” if
you put a wire between the positive and negative terminals. If you put a bulb across the
cell, say with a flashlight, it takes an hour or two for the charge difference to drop to zero.