A simple iron-core transformer is shown below:
Suggest why thermal energy is generated in the core when the transformer is in use.
Consider a section of the transformer core which is coloured blue as shown below:
As the magnetic field produced by the current in the primary coil increases, the magnetic flux linkage of the section of the coil will increase.
By Faraday’s Law, there will be an induced e.m.f. produced in the section of the core. By Lenz’s Law, there will be a current flowing in the section of the core which will produce a magnetic effect to oppose the change.
By the Right Hand Grip Rule, this means that a circular current must be flowing in the section of the core in order to produce the magnetic field. (This circular current is called the eddy current.)
Since an induced current is flowing in the core, by P = I 2 R, there will be power dissipated in the core and thermal energy is generated in the core of the transfomer.