Direct current     Electricity from batteries and solar cells flows in one way, this is direct current: d.c.                                                                          The horizontal line in the oscilloscope                                     shows constant value of d.c flowing in one direction.    Alternating current    Electricity from the mains is continuously changing direction, this is alternating current: a.c.                                                                                 The wave shape in the oscilloscope                                       shows the a.c is continuously changing direction.    Electricity from the mains flows back and forth 50 times in one second.       Frequency of mains electricity is 50 Hz.