Electrical - Wind Energy: Renewable Energy and the Environment - page 129

7 Electrical

7.1 F U N DA M E N TA L S
Electricity and magnetism are concerned with charges and the movement of charges. The funda-
mental ideas of electricity and magnetism are discussed in introductory physics texts. The following
terms are given as a background for generators and controls.
Current: The current is the flow of charge, q (electrons in most cases), past some point. Charge
is measured in coulomb. Direct current (DC) is when the flow is in one direction, and alternating
current (AC) is when the flow changes direction. The frequency, number of cycles per second, is
measured in hertz (Hz).

$
$

q
t

, ampere (A) (7.1)

I

For electric utilities in the United States, the voltage and current change sixty times per second,
60 Hz. Other countries use 50 Hz for their utility systems. If the utility voltage or current is plotted
versus time, it looks a sine curve ( Figure 7.1 ) .
Volt age: It takes energy to move charges around, and the potential energy (PE) to move charge
divided by the charge is called the potential difference and is measured in volts. For AC, the voltage
also changes with time, just like the current.

PE
q

, volts (V) (7.2)

V

Resistance: There is a resistance to the flow of charge across different elements in a circuit. A
circuit consists of a source (voltage), current through the wires, and a load or resistance.

V

I , ohm (Ω)

(7.3)

R

In metals the amount of current is linearly proportional to the voltage, a relationship known as
Ohm's law.

V IR (7.4)

Also, in metals the resistance increases with temperature, which means more energy is lost as the
temperature increases, because of the current.
Power: The power in a circuit is the voltage times the current:

P VI (7.5)

The power lost due to heating of the conductor (metals) depends on the square of the current:

P VI I 2 R (7.6)

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