When two coils are placed side by side and the current in one coil is switched on and off or changed, a voltage is induced in the neighboring coil. The effect is called mutual induction.
When an alternating current exists through the primary coil wrapped around one side of iron core as in the diagram, the alternating voltage causes an alternating magnetic field to be set up inside the iron core. This in turn induces an alternating voltage in a secondary coil wrapped around the opposite side of the core. In the way the secondary coil gives out an alternating current.
The size of the voltage in the secondary coil is depend on the ratio of the number of turns wrapped around the sides of the iron core, according to the formula
If the input voltage is greater than the output voltage, the transformer is called a step-down transformer, while if the output voltage is greater than the input voltage, the transformer is a step-up transformer.
Step up transformers:
Vs >Vp, ns >np and Is< Ip
Step down transformers:
Vp >Vs, np >ns and Ip< Is
Figure (A) is the circuit symbol for step-down transformer and figure (B) is the circuit symbol for step-up transformer.
Transformers are very useful devices. They are found wherever the voltages of alternating power supplies are to be modified.
Energy in transformer
If the PD is step-up in a transformer, the current is step-down in the same proportion. In an ideal transformer the energy is conserved and the transformer is 100% efficient. That means that the power of the transformer remains same in primary and secondary coils.
power in primary coil = power in secondary coil