1. Alternating Current voltage time 0 maximum positive voltage (the peak voltage) maximum negative voltage (peak voltage) Direction of current and size constantly change. Mains frequency is 50 Hz

2. Alternating Current 2 Voltage (V) Time (s) 0 peak voltage effective voltage The quoted value ( rms ) or Effective voltage of an a.c. supply is the same as the steady d.c. voltage which would produce the same heating effect in a resistance wire. The quoted value of the mains voltage in the UK is 230 V. The peak voltage is 325 V. Effective voltage is measured with a voltmeter. Peak voltage is measured with an oscilloscope.

3. Alternating Current 3 The quoted value of an a.c. supply is also known as the r.m.s. value (root mean square). The peak value is 2 larger than this r.m.s. value. This applies to both voltage and current. Voltage Time peak value r.m.s value

4. Alternating Current 4 Peak and r.m.s. values: Example The period of a sinusoidally varying current is 0.005 s. The peak value of the current is 4·0 A. Calculate a) the frequency b) the root mean square value of the current. Frequency = 1 / Period Frequency = 1 / 0.005 Frequency = 200 Hz

5. Calculation of frequency timebase (s cm –1 ) y input 0.02 Find the time for one wave. This is the period. frequency = 1/period Example Time base = 0.02 s cm -1 Period = time base setting x no of boxes for 1 cycle Period = 0.002 x 4 = 0.08 s Frequency = 1 / period Frequency = 1 / 0.08 = 12.5 Hz.