CIE A Level Physics (9702) 2019-2021

Revision Notes

26.1.2 Mean Power

Mean Power

  • In mains electricity, current and voltage are varying all the time
  • This also means the power varies constantly, recall the equations for power:

Mean Power equation 1

  • Where:
    • I = direct   current (A)
    • V = direct voltage (A)
    • R = resistance (Ω)
  • The r.m.s values means equations used for direct current and voltage can now be applied to alternating current  and voltage
  • They are also used determining an average current or voltage for alternating supplies
  • Recall the equation for peak current:

Mean Power equation 2

  • Therefore, the peak (maximum) power is related to the mean (average) power by:

Mean Power equation 3

Mean Power equation 4

Mean Power equation 5

  • Therefore, it can be concluded that:

 The mean power in a resistive load is half the maximum power for a sinusoidal alternating current or voltage

Worked example: Calculating mean power

Mean_Power_Worked_example_-_Calculating_Mean_Power_Question, downloadable AS & A Level Physics revision notes

Step 1:            Write down the known quantities

Resistance, R = 40 Ω

Peak voltage, V0 = 240 V

Step 2:            Write out the equation for the peak power and calculate

Mean Power Worked Example equation 1

Mean Power Worked Example equation 2

Step 3:            Calculate the mean power

        • The mean power is half of the maximum (peak) power

Mean power = 1440 / 2 = 720 W

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