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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 battery +V0 AC waveform Time (s) -V0 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 ammeter. Irms = I0 2 Vrms = V0 2 Voltage RMS voltage Peak Voltage Time Vrms = 0.71 Vpk