Direct Current and Alternating Current

Direct Current

  1. Direct current is a uniform current flowing in one fixed direction in a circuit
  2. Direct current (d.c) is usually supplied by acid-based batteries or dry cells. 
  3. A common example of acid-based (electrolyte) batteries is the car battery. 
  4. The figure below shows the graph of current supplied by a dry cell over time.

Alternating Current

  1. Alternating current is an electric current in which the direction of flow of the electrons reverses periodically
  2. Alternating current (a.c) is generated from alternating current generators such as hydroelectric power generators. 
  3. The electricity supplied to households is alternating current.
  4. Household electricity (alternating current) changes direction 50 times every second. Its magnitude also changes with time.

Period And Frequency

  1. The time taken for one complete cycle is known as the period, T. 
  2. The frequency f is defined as the number of complete cycles in 1 second. 
  3. The relationship between the frequency and the period is:
  4. In SPM, you need to know the effect of both the direct current and alternating current on
    1. a bulb
    2. a capacitor
    3. a moving coil loudspeaker
  5. The table below gives the summary of the comparison of the effect of direct current and alternating on a bulb, a capacitor and a moving coil loudspeaker.
Direct CurrentAlternating Current
Effects on a bulbThe bulb lights upThe bulb lights up
Effects on a capacitorCurrent is detected at the very beginning and then ceased to become zero afterwards.Current is detected
Effects on moving coil loudspeakerNo sound producedSound produced