Alternating Currents

Current That Is Changing All the Time
A direct current travels in one direction only.
In alternating currents the direction of charge
flow is changing all the time.
The change in direction from forwards to
backwards to forwards again is called a cycle.
In Europe there are 50 cycles every second.
DC from a
AC waveform
Time (s)
One complete alternation is called a cycle (NOT wavelength).
The frequency is the number of cycles per second. Units are hertz (Hz).
The period is the time taken for one cycle. It is measured in seconds.
f = 1/T.
The current follows exactly the same wave form as voltage.
AC and DC are equally good at heating, lighting,
or running motors. DC is essential for chemical
processes such as electrolysis.
 Low voltage DC is used in electronic devices.
 AC is much more easily distributed than DC.
The values of voltage and current are
constantly changing in AC, unlike in DC in
which they are steady. We can measure AC
voltages in two ways:
Measure the peak to peak voltage, easily
done on a cathode ray oscilloscope (CRO).
Measure the root mean square (rms) value,
or the effective value.
We use the rms value
because its use allows
us to do electrical
calculations as if they
were direct currents.
 We measure the rms
value with a
voltmeter or
Irms = I0
Vrms = V0
Peak Voltage
Vrms = 0.71 Vpk

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