Relay An electrically operated switch, for example a 9V battery circuit connected to the coil can switch an AC mains circuit. The rectangle represents the coil. NO = Normally Open, COM = Common, NC = Normally Closed.
Earphone A transducer which converts electrical energy to sound.
Zener diode A zener diode can be used to maintain a fixed voltage.
Electricity can flow only if there is a complete circuit from the battery through wires to components and back to the battery again.