# 2.2.4 AC & DC

### 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

• The potential difference across a cell in a d.c. circuit travels 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

#### Exam Tip

All the circuits you have studied so far are d.c. circuits. Don’t be put off by an exam question if you are asked to calculate the current, potential difference or resistance in a d.c. series circuits, you don’t have to do anything different from what you have already learned!

### AC & DC

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

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

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

Two graphs showing the variation of current with time for alternating current and direct current

• 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 times the current changes direction back and forth each second
• 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.

#### Exam Tip

If you are asked to explain the difference between alternating and direct current, sketching and labelling the graphs shown above can earn you full marks.

### Author: Katie

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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