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Alternating Currents

Physics: AQA A Level, topic 7.5.7

Alternating Current

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

Alternating current

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.
Measurement

Measurement

  • 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}

Oscilloscopes

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

Screen

Screen

  • 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).
Time-base

Time-base

  • 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.)
Y-gain or y-scale

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.
Example

Example

  • 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.
Example cont.

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.

Test yourself on Alternating Currents

  • What is the equation for the root mean squared current?

  • How do you find the period from an oscilloscope?

  • If the voltage gain is set to 5 volts and the peak of a wave is two boxes high what is the peak voltage?

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