Direct current flows one way and never turns round
Definition
Direct current (d.c.)
A current that always flows in the same direction, produced by a supply whose terminals keep a fixed polarity.
- In a d.c. circuit one terminal of the supply stays permanently positive and the other stays permanently negative.
- The charge therefore drifts steadily one way round the loop the whole time and never reverses.
- A graph of current against time for a steady d.c. supply is a flat horizontal line sitting entirely above the time axis.
- Cells, batteries, solar cells and USB supplies all give d.c., which is why a battery has to go into a torch the right way round.
- Any component that only works one way round, such as a diode or an LED, needs d.c. and will simply not light if the supply is reversed.
Alternating current keeps reversing
Definition
Alternating current (a.c.)
A current that repeatedly changes direction, because the supply reverses its polarity many times each second.
- In an a.c. supply each terminal takes its turn at being positive and then negative.
- The charge in the wire therefore moves one way, slows to a stop, and then moves back the other way, over and over.
- A graph of current against time is a wave that alternates above and below the axis, crossing zero each time the current turns round.
- The number of complete cycles per second is the frequency, measured in hertz, and the UK mains runs at 50 Hz50\ \text{Hz}50 Hz.
- At 50 Hz50\ \text{Hz}50 Hz the current changes direction 100100100 times every second, far too fast to notice, so a lamp filament has no time to cool between reversals and looks steadily lit.
- Mains electricity and generators produce a.c., so this is what comes out of every socket in the house.
Where each one is used, and why the difference matters
- A.c. is used for the mains supply and for distributing electricity across the country.
- D.c. is what electronic devices run on, which is why a laptop charger exists at all: its job is to turn the mains a.c. into the d.c. the laptop needs.
- A lamp filament or a heating element works on either supply, because the wire warms up whichever way the charge is moving through it.
- A d.c. motor and a diode do care about the direction, so they behave quite differently on a.c.
- An oscilloscope connected across each supply settles the question at a glance: a flat trace for d.c. and a repeating wave for a.c.
Common Mistake
- A.c. does not flow both ways at once; it flows one way, then the other, in turn.
- The mains frequency of 50 Hz50\ \text{Hz}50 Hz counts complete cycles per second, and the current actually reverses twice per cycle.
- A battery cannot supply a.c., and a wall socket does not supply d.c., so do not mix the sources up in an answer.
Exam technique
- For a "difference" question, write one sentence covering both, as in "d.c. flows in one direction only, whereas a.c. repeatedly changes direction".
- Sketching the two current-time graphs is the quickest full answer, so label both axes and show the a.c. trace crossing the time axis.
- Name a source alongside each type, a cell for d.c. and the mains for a.c., because that is often the second mark.
Self review
- What is direct current?
- What is alternating current?
- Sketch a current-time graph for each type and label them.
- Name one source of d.c. and one source of a.c.
- State the frequency of the UK mains supply and say what that number counts.
