IGCSE Physics / Mains electricity
Alternating and direct current
Edexcel International GCSE Physics (4PH1), chapter 6: Mains electricity, specification point 2.6, 2.1. Free lesson.
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Direct current flows in one direction, while alternating current repeatedly changes direction. You can identify each from an oscilloscope trace and should know the units used for electrical quantities.
How direct and alternating current differ
The difference between direct current (d.c.) and alternating current (a.c.) is the direction of flow. A cell or battery supplies d.c., while the mains supply from a wall socket is a.c. The mains values vary between countries. D.c. flows in one direction only. Its voltage can be steady, such as the 1.5 V from a cell. A.c. changes direction continuously, flowing forwards and backwards. In many countries, the mains is about 230 V at 50 Hz. A frequency of 50 Hz means 50 complete cycles each second. Some countries use mains at about 120 V and 60 Hz.
How to recognise current on an oscilloscope
An oscilloscope draws a graph of voltage against time. The centre line represents zero voltage. A trace on one side of the centre line shows a current flowing in one direction; going below it shows the current flowing in the opposite direction. A d.c. supply gives a flat line because its voltage stays steady and on one side of the centre line. An a.c. supply gives a wave that goes above and below the centre line. The height of a trace can be read using the voltage scale, and its width using the time scale.
What a.c. does below the centre line
A trace below the centre line does not mean the current has switched off. It shows that the current is flowing in the opposite direction. So a.c. repeatedly reverses direction rather than simply turning on and off. A repeating trace that goes above and below the centre line is a.c. To explain why, say that the current changes direction continuously.
Units used in electricity
Each electrical quantity has its own unit and symbol. Time is always measured in seconds in electricity equations, so convert minutes or hours into seconds before using them. Current: ampere (amp), A. Voltage: volt, V. Resistance: ohm, Ω. Charge: coulomb, C. Energy: joule, J. Power: watt, W. Time: second, s.