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Illustration 31.2: AC Voltage and Current


Please wait for the animation to completely load.

Assume an ideal power supply. The graph shows the voltage (red) across and current (blue) from the power supply as a function of time. Note the 10-3 exponent on the timescale when the animation starts (voltage is given in volts, current is given in hundredths of amperes, and time is given in seconds). Restart.

Start the low voltage animation. As you change the frequency, describe what happens to the bulb and the graph. Note the factor of 10-3 for the timescale when the animation starts. As you close the switch, notice that the voltage does not change, but the current increases. This is because closing the switch adds more resistors in parallel to the power supply.

Since the voltage is positive as much as it is negative, we do not talk about the average voltage (which would be zero) but instead describe the voltage either by the amplitude (the size of the peak voltage. What is it in this case?) or the rms (root-mean-square) voltage (= Vpeak/√2). For this power supply, the peak voltage is 5 V and the rms voltage is 3.5 V.

Household voltage is 120 V rms. What is the peak voltage? In order to plot the current on the same graph, the current shown is 100 times the actual current. What is the average power of one lightbulb? (P = IrmsVrms = VpeakIpeak/2 for a purely resistive load). You should find that these lightbulbs are 60 W bulbs.

With alternating current (AC), fluorescent lights in your room are flickering on and off 120 times/second (frequency in the US is 60 Hz), but you simply don't notice it (just like a movie is made of separate frames, but to you it looks continuous). In Europe the standard frequency is 50 Hz, so the fluorescent lights in Europe go on and off 100 times in one second.

Illustration authored by Anne J. Cox.
Script authored by Wolfgang Christian and Anne J. Cox.

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