# Transformers 19 2021 What are transformers and how

• Slides: 13

Transformers 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 Step Up Step Down • This makes the PD in the secondary coil bigger than the primary coil. • This makes the PD in the secondary coil smaller than the primary coil. • Secondary coil has more turns. • 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 I p = V s X I s

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 50 KHz (50’ 000 Hz) to 20 KHz. 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