An electrical generator is an apparatus that converts mechanical energy into electrical energy. In this case the magnetic field is stationary and does not vary with time. It is the circuit that is made to rotate through the magnetic field. Since the area that admits the passage of magnetic field lines changes while the circuit rotates, the flux through the circuit will change, thus inducing a current (Figure 4). Generally, a turbine is used to provide the circuit's rotation. The energy required to move the turbine may come from steam generated by nuclear or fossil fuels, or from the flow of water through a dam. As a result, the mechanical energy of rotation is changed into electric current.
Transformers are devices used to transfer electric energy between circuits. They are used in power lines to convert high voltage electricity into household current. Common consumer electronics such as radios and televisions also use transformers. By making use of mutual inductance, the transformer's primary circuit induces current in its secondary circuit. By varying the physical characteristics of each circuit, the output of the transformer can be designed to meet specific needs.