Electric and Magnetic Fields: Alternating Currents

Alternating Currents

Alternating current (AC) is a type of electrical current in which the direction of the flow of electrons switches back and forth at regular intervals or cycles.
AC is produced by most power stations and is used in home and office supplies.

A basic AC generator, also known as an alternator, can be constructed using a coil of wire rotating in a magnetic field.
As the coil rotates, the magnetic field induces a current which switches direction as the relative orientation of the coil changes. This leads to an electrical output that varies sinusoidally over time.
The rate at which the direction of the current changes is the frequency, usually measured in hertz (Hz).

An AC waveform can be represented graphically with time on the x-axis and current or voltage on the y-axis.
The peak or maximum value is the maximum voltage or current delivered in one cycle.
The period (T) is the time taken for one complete cycle.
The frequency (f) is the number of cycles per second.
The root mean square (rms) is used to define a type of average of a waveform. It refers to the square root of the mean of the squares of all the instantaneous values during one cycle.

Transformers only work with AC, not DC.
Transformers can step up (increase) or step down (decrease) the voltage. This is vital for efficient power distribution.
The turns ratio of a transformer determines the factor by which it will step up or step down the voltage.

The instantaneous power, p, in an AC circuit is given by the product of the instantaneous voltage and the instantaneous current.
The average power can be represented as P_av = I_rmsV_rms cosΦ where I_rms is the rms current, V_rms is the rms voltage and cosΦ is the power factor.
The power factor represents the phase angle difference between the current and the voltage.