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Physics (Year 12) - Electro-magnetism

Dev Lohar

AC generator

Generators can be considered as the opposite of motors. Motors use electricity to produce torque and rotation. Generators use torque and rotation to ‘generate’ electricity. Since motors and generators are so similar, their parts and the principle behind their workings is also similar. Below is a basic diagram of an AC generator.

An AC generator works because of the following:

  • A coil (or multiple coils) is placed between 2 magnets; a north and south facing each other

  • A force is applied to the coil which makes it rotate

  • The rotating coil cuts through magnetic field lines and hence experiences change in magnetic flux

  • As a result, the coil will induce and EMF (according to Faraday’s law) in order to oppose the change in magnetic flux (according to Lenz’s law)

  • This induced emf will produce a current if the coil is connected to an external closed circuit. The direction of the current will change every time the coil reaches a point perpendicular to the magnetic field; hence every half cycle

DC generator

The only difference between an AC generator and a DC generator is the type of current they produce; AC (alternating current; direction of current periodically changes) or DC (direct current; direction of current is constant). The only part which has an impact on whether a generator is AC or DC, is the usage of a slip ring or the usage of a split ring commutator.

AC generators use a slip ring which allow the coil to rotate without tangling the wires. It is a continuous ring.

On the other hand, DC generators use split ring commutators which are the same as slip rings, but with breaks. Due to these breaks, the direction of the output current is changed every half turn so that the output current is always in the same direction. If you remember the diagram of a motor, you’ll notice that on a fundamental level, a DC motor and a DC generator are identical.

Calculating the maximum emf generated

The equation which calculates the maximum emf generated by an AC generator is:

It is important to note that in some questions, instead of being given the frequency of rotation of the loop, you might be given how long it took for the loop to rotate through one cycle. The latter value is called the period of rotation and can be used to easily calculate the frequency through the relationship: f= 1/T.

The equation above calculated the peak emf. The equation to calculate the average emf is:

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