Electrical Definitions

Defintions (Electrical Systems)

Semi-Conductors

Semi-conductors is a general term for a class of substances which allow current to flow, but in a limited or controlled fashion. Silicon and Gemanium are generally poor conductors. But when they are "doped" with other elements (for example, Germanium doped with Arsenic) they become better conductors. This by itself would not be of much use since there are plenty of much better conductors. But when Ge crystals, doped with two different elements, are grown in layers, a junction is created that passes current easily in one direction but not in the reverse direction. This simple but amazing property is the basis for the entire solid-state industry. If you're interested in more information, PBS has a nice site. Start with the discovery of the p-n junction and explore from there.

For a simple cylindrical wire of length L and cross sectional area A, the resistance is given by the formula

R = ^{r L} / _A

The symbol r represents the resistivity of the material. Good conductors have a low resistivity while poor conductors have a high resistivity. Note that if we increase the cross sectional area A, the resistance becomes less. A thicker wire has less resistance and will carry more current. A longer wire has more resistance, since the electrons must move through a greater distance and hence will undergo a greater number of collisions.

In physics, energy and work are strictly defined. Work is done by a force when it moves an object through a distance. If the force is parallel to the direction of motion, it is given by

Work = Force x Distance

If you push a box with a force of 100 N (that's about 22 lb) over a distance of 10 m (about 32 ft), you will do work on the box equal to 100 N x 10 m = 1000 Joules. Compare this a 100 W light bulb, which consumes 100 Joules per second. To produce the same power as consumed by the light bulb, you would have to push the box over the 10 m distance in 10 seconds.

Here's another example to give you some perspective. The energy associated with motion is called kinetic energy. It is given by

KE = ¹/₂ m v²

A 20 kg (44 lb) block with a speed of 10 m/s (22 mph) has KE = ¹/₂ x 20 x 10² = 1000 Joules! Look at the first example. It would take a force of 100 N pushing this 20 kg block over a 10 m distance to take it from rest to 10 m/s ... or to bring it to rest again.

Heat is also a form of energy. At first glance it may seem totally different from the work or KE discussed above, but it isn't. Heat is a measure of the vibrational motion of a substance. Rub your hands together and they will heat up. You do work on your hands (forcing your hands through a distance) that is converted to heat. If a car crashes into a wall, the KE is converted into several forms ... work that causes deformation, sound, and heat. The heat in the toaster or light bulb filament ultimately comes from the work done by the potential source as it pushes charge over a distance through resistance. Despite it's many varied forms, energy is energy.

Standard Electrical Symbols

Below are a few of the standard symbols for electrical devices that you will find in schematic diagrams.

Device	Symbol	Comments
DC battery		The first symbol is the most common.
Switch		Single and double pole.
Resistor		Fixed and variable.
Capacitor		A capacitor stores charge.
Inductor		An inductor impedes AC current.
Diode		Current flows only in direction of arrow.
Transformer		Transforms AC potential.
	Common Household Circuit Symbols
Lamps		Any light fixture.
Switches		Single pole and double (three way).
Fuses		Normal fuse and GFCI fuse.