2. Transformers  A transformer is used to change alternating p.d.s. from a lower to a higher voltage or vice versa. A simple transformer consists of two separate coils wound on a laminated soft iron core.

3. Principle of the Transformer  When the primary coil is connected to an alternating supply voltage the current sets up an alternating magnetic flux. This threads through the coil links and induces an alternating e.m.f. In the secondary coil. The ratio of the voltages is the same as the ratio of the number of turns of the coils. The secondary voltage can be deduced from the relationship. 

4. Diagram of the Transformer

5. Efficiency of Transformers  Practical transformers are never 100% efficient. This is because:  (1) heat is produced in both coils.  (2) eddy currents are induced in the core, and  (3) all the magnetic flux produced by the primary coil does not reach the secondary coil.  These problems are reduced by having low resistance coils, laminating the core and having a good design for the core  A properly designed transformer has a high degree of efficiency. (95-99%)

6. An ideal transformer  In an ideal transformer there will be no loss of energy and the power output will be equal to the power input. If I p and I s are primary and secondary currents then: