BACK EMF When the coil L is connected in series with the cell V it produces an increasing magnetic field as the current through the coil rises. This induces a “back emf” in the reverse direction to the emf produced by the cell. The magnetic field stores energy transferred from V When S is moved so that L is in series with R only, the back emf drives a current through R dissipating the energy stored.

Transformers An alternating current through a coil around a soft iron core sets up an alternating magnetic flux. North Pole South Pole South Pole North Pole North Pole South Pole

The simple transformer A.C. Supply Primary Coil Secondary Coil The changing magnetic flux in the core produced by the primary coil, induces an alternating emf in the secondary coil.

A step down transformer Fewer turns on the secondary coil, compared to the primary steps down the voltage Higher voltage in Lower voltage out Iron core 2:1 results in halving the voltage 3:1 gives 1/3 of the voltage etc

A step up transformer Here more turns on the secondary than the primary steps up the output voltage. 1:2 doubles the voltage 1:3 gives 3x the voltage etc.

Explaining the effect The flux linkage through the secondary coil at any point in time :is Φs A.C. Supply Primary Coil Secondary Coil The e.m.f produce in the secondary coil is equal to the rate of change of flux linkage by faraday’s law The changing flux is also producing a back e.m.f. in the primary coil. This is effectively equal to the voltage applied to the primary coil

The transformer formula Now Φ =AB A is the coil cross sectional area B is the flux density These have the same value for both the primary and secondary coils so

This relationship is often written in terms of turns ratio. There is little resistance in the coils so that the emf generated in the secondary coil is effectively the same as the voltage it supplies to the secondary circuit. Secondary voltage = secondary turns Primary voltage primary turns