If the current constantly changes direction it is called alternating current, or a.c. Mains electricity is an a.c. supply. The UK mains supply is about 230 V. It has a frequency of 50 Hz, which means that it changes direction and back again 50 times a second. The diagram shows an oscilloscope screen displaying the signal from an a.c. supply.
If the current flows in only one direction it is called direct current, or d.c. Batteries and solar cells supply d.c. electricity. A typical cell or battery may supply 1.5 V. The diagram shows an oscilloscope screen displaying the signal from a d.c. supply.