AC & DC (AQA GCSE Combined Science: Trilogy: Physics): Revision Note

DC Series Circuits

• A direct current (d.c.) is defined as

A current that is steady, constantly flowing in the same direction in a circuit, from positive to negative

• A direct potential difference is one where the polarity (direction) of the potential difference does not change

• The potential difference across a cell in a d.c. circuit is in one direction only

• A d.c. power supply has a fixed positive terminal and a fixed negative terminal

• Electric cells, or batteries, produce direct current (d.c.)

• A series circuit consists of a string of components connected in a loop between the ends of the power supply

• In a series circuit:

  • The current is the same at all points

  • The potential difference of the power supply is shared between the individual components

• Two disadvantages of series circuits are:

  • If one of the components breaks, all of the others will stop working

  • The components cannot be controlled (switched on and off) separately

AC & DC

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

A current that continuously changes its direction, going back and forth around a circuit

• An alternating potential difference is one where the polarity (direction) of the potential difference keeps reversing

• On an oscilloscope, direct current and alternating current are represented in the following way:

• An a.c. power supply has two identical terminals

• The potential difference between the terminals varies as a sine curve, or sinusoidally

• The frequency of an alternating current is the number of complete cycles (one forwards and one backwards) it makes each second, measured in hertz (Hz)

• In the UK, mains electricity is an alternating current with a frequency of 50 Hz and a potential difference of around 230 V

Comparing Direct & Alternating Current

• The following table summarises the differences between d.c. and a.c.