This is a very simple set of definitions to understand electrical
terms and notation. For a more complete list, see
Electricity.
(Note: the Wikipedia links provided
on this page are often much more technical than the treatment here.)
Many of the properties of electricity can be introduced by
analogy
to water (charge) flowing in pipes (conductors)
under pressure (potential) from high points to low points
(from higher to lower voltages). Small diameter pipes
(resistors) resist the flow of water (current) and
cause pressure drops (voltage drops). Reservoirs (capacitors)
store water (charge). The amount of water (charge)
stored in a reservoir (capacitor) is the product of its
surface area (capacitance) and the height of the water
(voltage / potential). The amount of work that can be done by
emptying a reservoir (capacitor) by letting its water flow
out (current) depends on the quantity of water
(charge) and its height (voltage). For both
electricity and hydraulics the product of these quantities is the
energy stored therein. The amount of energy released per unit time is
power. Releasing the energy slowly (low power) takes longer, but the
same work is generally accomplished as releasing the energy quickly
(high power). Power is the product of water flow (current)
and its pressure (voltage).
A capacitor stores charge much as a reservoir stores water. If a
reservoir feeds a pipe, the higher the water level, the more
pressure on the pipe. Similarly, the more charge on a capacitor,
the more voltage the capacitor presents to the system.
Capacitance is then similar to the surface area of the reservoir.
Energy effects changes in a system. The change may happen quickly
by expending a lot of energy in a short period of time (high
power), or it may happen more slowly by using proportionally lower
energy for a longer period of time (low power). For many systems,
the energy required to effect change is independent of whether it
is done quickly or slowly (i.e. by high power or low power), and
so energy just measures the capacity for change in the system.
In Physics, the SI unit for energy is the joule. In electrical
calculations a joule is a watt⋅second (Ws). More common units in
electrical work are watt⋅hours (Wh), which are 3600 Ws, and the
Kilowatt-hours (KWh), which are 3,600,000 (Ws).
Imagine a quantity of water at a given height (a given potential).
The higher it is, the more work it can do running downhill. The
more water there is, the more it can do. The work it can do is
represents its energy, which is the product of the quantity and
potential.
An inductor stores energy much as a water wheel
does. When the water speeds up, the wheel
resists, but eventually speeds up to match the new
speed. When the water slows down, the water wheel
transfers some of its energy back to the water.
Electric potential is analogous to water pressure. Where electric
potential (pressure) is uniform, there is no force or push, just
as we do not feel the tremendous atmospheric pressure at sea
level. However, in places where potential (pressure) varies, it
produces a force that can push charged objects to different
locations (i.e. create a current). A potential difference is
called voltage, and is measured in volts.
It is harder to move water through a narrow pipe than a wide one.
Similarly it requires more effort to move current through a high
resistance conductor than a low resistance one.
An electric potential difference—voltage—may exist
between two points. (Often the earth is used as one point, and is
considered to be 0V.) The voltage between two points measures the
work it is to move charge between them.
