Edexcel A Level Physics

Revision Notes

7.21 Alternating Currents & Potential Differences

Test Yourself

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

Peak Voltage Worked Example

Exam Tip

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.

You've read 0 of your 0 free revision notes

Get unlimited access

to absolutely everything:

  • Downloadable PDFs
  • Unlimited Revision Notes
  • Topic Questions
  • Past Papers
  • Model Answers
  • Videos (Maths and Science)

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Katie M

Author: Katie M

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.