NEET Physics Notes Alternating Current-AC Generator
AC Generator
AC Generator
An electric generator or dynamo is a device used to produce electrical energy at the expense of mechanical/thermal energy.
It works on the principle of electromagnetic induction when a coil is rotated in a uniform magnetic field, an induced emf is set up between its ends. The induced emf is given by 
The direction of the induced emf is alternating in nature.
Transformer
It is a device which works in AC circuits only and is based on the principle of mutual induction.
Transformer is used to suitably increase or decrease the voltage in an AC circuit. Transformer which transforms strong AC at low voltage into a weaker current at high alternating voltage is called a step up transformer. A step down transformer transforms weak current at a higher alternating voltage into a strong current at a lower alternating voltage.
where, k is known as the transformation ratio.
For a step up transformer, k > 1 but for a step down transformer, k < 1
In a transformer, the input emf and the output emf, differ in phase by 
The efficiency of a transformer is given by
For an ideal transformer, 
However, for practical transformers, 
.
Possible causes of energy loss in transformer are
- Heating due to winding resistance
- Eddy current losses
- Magnetic flux leakage and
- Hysteresis loss.
- To minimise these losses, the transformer core is made up of a laminated soft iron strips.
DC Motor
An electric motor converts electrical energy into mechanical energy.
It is based on the fact that a current carrying coil in a uniform magnetic field experiences a torque.
As the coil rotates in the magnetic field, the flux linked with the rotating coil will change and hence an emf called back emf is developed in the coil. The back emf e is proportional to the angular speed of the coil ω and opposes the impressed voltage E. So, if R is the resistance of motor, armature current
When the motor is first turned on, the coil is at rest and so there is no back emf. The start up current can be quite large because it is limited only by the resistance of the coil.