Direct and alternating potential difference: Mains electricity is an ac supply. In the United Kingdom the domestic electricity supply has a frequency of 50 Hz and is about 230 V.

The idea

Mains electricity in the United Kingdom is an alternating current supply. In an ac supply the potential difference repeatedly reverses direction, so the current changes direction many times each second. The frequency of 50 Hz means the supply completes 50 full cycles every second, reversing direction 100 times per second. The stated value of about 230 V is the root mean square potential difference, which is the value that transfers the same average power as a 230 V direct supply; the peak value is higher, about 325 V. By contrast, a direct supply such as a battery keeps potential difference and current in one direction. You should be able to describe the difference between direct and alternating potential difference, read the frequency and voltage values from a UK mains label, and relate frequency to the number of cycles per second.

What earns the marks

• State that mains electricity is an alternating supply in which the potential difference and current repeatedly reverse direction.
• Recall that the UK domestic supply has a frequency of 50 Hz, meaning 50 cycles each second.
• Recall that the UK domestic supply potential difference is about 230 V.
• Describe direct potential difference as one that keeps the current in a single direction, as from a battery.
• Interpret a simple graph of potential difference against time for an ac supply by identifying the frequency from the cycle length.
• Distinguish between the stated 230 V value and the higher peak value of the alternating waveform.

Common mistakes and how to put them right

• Saying the current in an ac circuit flows only one way.

  Put it right: In an ac supply the current reverses direction repeatedly, completing 50 cycles per second at 50 Hz.

• Confusing frequency with the number of reversals per second.

  Put it right: 50 Hz means 50 complete cycles per second, which is 100 direction reversals per second.

• Treating 230 V as the peak potential difference.

  Put it right: 230 V is the value quoted for the UK supply and represents the effective value; the peak is higher, about 325 V.

• Describing direct current as simply 'safer' or 'steadier' without reference to direction.

  Put it right: Define direct supply by the fact that current flows in one direction only.

Exam tips

• Learn the two UK mains figures together as a pair: 50 Hz and about 230 V, and be ready to state both.
• When comparing ac and dc, always mention direction of current or potential difference rather than vague descriptions.
• If a graph is given, count the cycles and read the time for one cycle to find frequency using frequency = 1 ÷ time period.
• Use the phrase 'about 230 V' where appropriate, since the supply value is quoted as approximate.