Electrical Power & Mains Electricity

Direct current (d.c.)

• A direct current (d.c.) is a current that keeps flowing in just one direction around a circuit, starting at the source's positive end and arriving back at its negative end
• The two terminals of a d.c. source keep their roles unchanged: one stays at the higher potential (the positive end) and the other stays at the lower potential (the negative end) for the whole time the source is connected
• Sources of d.c. include cells, batteries, solar panels and d.c. power packs used in school labs

Alternating current (a.c.)

• An alternating current (a.c.) is a current whose direction continually reverses, flowing one way, then the opposite way, then back again, many times every second
• The supply's two terminals swap polarity in step with the current; neither terminal is permanently positive
• The frequency of an a.c. supply is the number of complete cycles per second, measured in hertz (Hz)
• In the United Kingdom, the mains supply is a.c. at a frequency of 50 Hz and a voltage of about 230 V
• Sources of a.c. include power-station generators and the sockets in any home

Comparison of d.c. and a.c. at a glance