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Higher Physics – Unit 2 2.2 Alternating Current and Voltage AC Voltage Supply The size of an AC voltage supply is constantly changing. peak voltage Vmax voltage/V time/s - Vmax The maximum voltage is called the peak voltage, VP. Since the size of the voltage is changing, we use an average value when doing calculations. The average value used is the “root mean square” or rms voltage. It is calculated using: Vrms Vpeak 2 The rms voltage equals the equivalent dc voltage. Example 1 The rms voltage in the UK is quoted as 230 V. Calculate the peak voltage. Vrms 230 V Vp ? Vpeak 2 Vrms Vpeak 2 230 325 V AC Current An AC current also has a peak and rms value. To calculate the rms current: Irms Ipeak 2 The rms current equals the equivalent dc current. Components in AC circuits however, must be able to withstand the peak voltage. AC and Ohm’s Law The peak or rms values can be applied to Ohm’s Law. Vpeak Ipeak R Vrms Irms R Example 1 Calculate the peak current for a 100 Ω resistor connected to a mains supply. Vrms 230 V R 100 Ω Ipeak ? Vpeak 2 Vrms 2 230 Vpeak 325 V Vpeak Ipeak R Ipeak Vpeak R 325 100 3.3 A Ipeak Must use ALL peak values or ALL rms values. You may need to use the following equations from standard grade: E P t V IR PIV P I2 R V2 P R When asked to calculate the output power, or heat produced (energy), you must use rms values. Worksheet – 2.2 Alternating Current and Voltage Q1 – Q6 Peak Voltage on Oscilloscope y gain 10 V per division amplitude 2 divisions peak voltage 2 10 20 V 2 5 10 1 0.5 20 Once the peak voltage is known, the rms voltage can be calculated. y-gain (V div-1) Vrms Vpeak 2 20 14.1 V 2 AC Frequency on Oscilloscope The timebase control tells you how long it takes the spot to move 1 division along the screen. 5 2 1 The time taken for one complete wave to pass a point is the PERIOD. 10 20 50 Timebase (ms div-1) period (s) 1 T f frequency (Hz) The wave on the oscilloscope is 8 divisions long. T 8 5 10 3 0.04 s The frequency is calculated to be: f 1 T 1 0.04 f 25 Hz Resistors and AC A resistor is connected to an AC voltage supply as shown. R A A graph of current against frequency in a resistor circuit is: As the frequency of the AC voltage is increased, the current in the circuit is unchanged. current/A frequency/Hz Worksheet – 2.2 Alternating Current and Voltage Q7 – Q9