Alternating Current & Voltage (CIE A Level Physics)

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

A current which periodically varies from positive to negative and changes its magnitude continuously with time

• This means the direction of an alternating current varies 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 time period of an alternating current is defined as:

• 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 the graph

Time period and peak current on a current-time graph

Graph of alternating current against time with a time period of 20 ms and peak current of 2 A

• Mains electricity is supplied as alternating current
  • Power stations produce alternating current
  • This is the type of current supplied when devices are plugged into sockets

• The equation representing alternating current which gives the value of the current I at any time t is:

• Where:
  • I = current (A)
  • I₀ = peak current (A)
  • ⍵ = angular frequency of the supply (rad s^-1)
  • t = time (s)

• Note: this a sine function since the alternative current graph is sinusoidal
• A similar equation can be used for representing alternating voltage:

• Where:
  • V = voltage (V)
  • V₀ = peak voltage (V)

• Recall the relation between the equation for angular frequency ⍵:

• Where:
  • f = frequency of the supply (Hz)