## Faradays Law of Electromagnetic Induction

Another way of looking at electromotive forces is by Faraday's law of electromagnetic induction. The amount of magnetic flux, Om, is equal to the strength of the magnetic field times the area:

where 9 is the angle between B and A. The electromotive force is then equal to the negative change in magnetic flux with time:

In generators and motors, the magnetic field and area can be kept constant, and the angle between the two changed by rotating a loop of wire. This gives an alternating voltage and current, which vary like a sine wave.

Induction is where you have two coils, where the changing magnetic flux in one coil causes a changing current in the next coil. A transformer works by induction. If the load is pure resistance, then the voltage is in phase (0 phase angle) with the current. For a capacitor the voltage lags the current by 90°, and for an inductor the voltage leads the current by 90° (Figure 7.4). In the figure, all the voltages are set with an angle of zero and the current is then shown in relation to the voltage (starting at a different angle for the sine curve). Check the links for the relation between voltage and current.