Alternating Currents

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Alternating Current

Just like voltage, current can alternate (i.e. switch directions).

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Alternating current

  • Current flow can change over time just like voltage.
  • Alternating current goes like a sine wave.
    • This means that current oscillates from one positive value to the same negative value.
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  • The peak to peak value is double the maximum positive value.
  • The current has a root mean squared value that is calculated as:
    • Irms=I/2{I_{rms}}=I/\sqrt{2}


The Mains supply is alternating current (a.c). An oscilloscope is a convenient way of measuring the amplitude and period of a.c. supplies.

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  • The screen on an oscilloscope displays the p.d. across the input terminals as a function of time (i.e. a sine wave for mains a.c).
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  • This is the control to change the x-axis scale of the waveform.
  • The setting changes the amount of time per division across the screen.
    • A time base of 10 ms per division displaying 50 Hz mains would record a complete cycle of the waveform every 2 divisions across the time-axis. (Since the period of the mains p.d. is 1/50 = 0.020 s.)
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Y-gain or y-scale

  • This sets how much p.d. is displayed for each vertical division.
  • A y-gain of 5V per division would record a d.c p.d. of +20V as a horizontal line that was 4 divisions above the time axis of the display.
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  • In this oscilloscope trace, the period of the wave is 4 squares across.
    • The time base setting is 5 ms per division, which means that the period of the wave is 5 × 4 = 20 ms.
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Example cont.

  • The peak-to-peak voltage is 6 squares. This is the vertical measurement from the top of the wave to the bottom of the wave.
    • The y-gain setting is 100 V per division, so the peak to peak voltage is 600 V.
    • The peak voltage is the vertical measurement from the 0V line to the peak. In this case, the peak voltage is 300 V.

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12Option: Turning Points in Physics (A2 only)

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