Alternating Currents & Potential Differences (Edexcel A Level Physics) : Revision Note

Katie M

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Katie M

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Alternating Currents & Potential Differences

  • An alternating current (a.c) is defined as:

    A current which periodically varies between a positive and negative value

  • This means the direction of an alternating current switches every half cycle

  • The variation of current, or p.d., with time can be described as a sine curve ie. sinusoidal

    • Therefore, the electrons in a wire carrying a.c. move back and forth with simple harmonic motion

  • As with SHM, the relationship between time period T and frequency  f for a.c is related by the equation:

Time Period Equation
  • Where:

    • T = time period (s)

    • f = frequency (Hz)

  • Peak current (I0), or peak voltage (V0), is defined as:

    The maximum value of the alternating current or voltage

  • Peak current, or voltage, can be determined from the amplitude of a current-time or voltage-time graph

  • The peak-to-peak current or voltage is the distance between a positive and consecutive negative peak. This means:

peak voltage V0 = peak-to-peak voltage ÷ 2

AC labelled graph, downloadable AS & A Level Physics revision notes

Graph of alternating current against time showing the time period, peak current and peak-to-peak current

Root-Mean-Square Current & Voltage

  • Root-mean-square (rms) values of current, or voltage, are a useful way of comparing a.c current, or voltage, to its equivalent direct current (d.c), or voltage

  • The rms values represent the direct current, or voltage, values that will produce the same heating effect, or power dissipation, as the alternating current, or voltage

  • The rms value of an alternating current is defined as:

    The equivalent direct current that produces the same power

  • In other words, an rms current is 'equivalent', in a sense, to a DC current, because they produce the same overall effect in a circuit

  • The rms value of an alternating voltage is similarly defined as:

    The equivalent dc voltage that produces the same power

  • Rms current is equal to 0.707 × I0, which is about 70% of the peak current I0

    • This is also the case for rms voltage

RMS v Peak grap, downloadable AS & A Level Physics revision notes

Vrms and peak voltage. The rms voltage is about 70% of the peak voltage

Worked Example

The variation with time t of the output voltage V of an alternating voltage supply is shown in the graph below.

Worked example voltage graph, downloadable AS & A Level Physics revision notes

Use the graph to calculate the frequency of the supply and the peak voltage.

Answer:

Peak Voltage Worked Example

Examiner Tips and Tricks

Remember to double-check the units on the alternating current and voltage graphs. These are often shown in milliseconds (ms) instead of seconds (s) on the x-axis.

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Katie M

Author: Katie M

Expertise: Physics Content Creator

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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