What are transformers and how do they work? 19 + 20/21 What are transformers and how do they work? Starter : Sketch or make a set of bullet points showing the steps of getting electricity from a power station to a house.

Glossary Transformer – this is used to change the size of an alternating potential difference.

Transformers

Primary coil is connected to an AC supply which produces a magnetic field in the iron core. P.D. is induced in a secondary coil because the iron core has a changing magnetic field. Alternating potential difference is induced in coil 2 and therefore an AC in the second circuit

Step up and step down transformers This makes the PD in the secondary coil bigger than the primary coil. Secondary coil has more turns. This makes the PD in the secondary coil smaller than the primary coil. Secondary coil has fewer turns.

Step up transformer

Step down transformer

Transformer Equations The higher the pd the greater the efficiency of transferring electrical power through the grid. A lower pd would need a higher current and energy would be lost as heat. pd across primary Vp = no. turns on primary, np pd across secondary Vs no. turns on secondary ns

Efficiency of Transformers A transformer is 100% efficient if 100% of the electrical power given to the primary coil is the same as that given to the appliance. Power supplied to transformer = power delivered by transformer Primary pd x primary current = secondary pd x secondary current. Vp X Ip = Vs X Is

Key point Transformers only work with AC not DC. With DC there is no changing magnetic field.

“Switch Mode” transformers These are suitable for appliances like phone chargers. They work at much higher frequencies 50KHz (50’000 Hz) to 20KHz. Benefits: Smaller and lighter than traditional transformer because they contain a ferrite core not iron. Use little power when the device is not connected.

 Now complete the questions