Transformers

Starter

• Sort the cards into the correct order to explain how a transformer works.

Secondary Coil

a.c. Out

Primary Coil

a.c. In

1. A potential difference in the primary circuit causes a current to flow in the primary circuit

2. This alternating current in the primary coil of wire induces a magnetic field in the iron core

3. The iron core ‘traps’ this changing magnetic field channelling it to the secondary coil

4. There is now a changing magnetic field inside the secondary coil

5. This changing magnetic field now induces a p.d in the wire in the secondary coil

6. The size of the p.d is directly proportional to the number of turns of wire

7. If the second circuit is complete, the p.d causes a current to flow

The following are untrue – why is this?• Connect the primary coil to a battery• The more coils of wire in the

secondary circuit, the greater the p.d• The iron core conducts this current to

the secondary coil

Transformers

Recall how a simple

transformer works

Compare the efficiency of real

transformers

Explain how efficiency can be improved

Main

• Using the worksheet to help you, set up the equipment safely

• Use an ammeter and voltmeter to measure the power in the primary and secondary circuits.

Main

• Why is the power out less than the power in?• Can you explain why this is in terms of energy?• Why does the core heat up and vibrate?

Main• Why does the non-magnetic copper tube slow the

falling magnet down?The changing magnetic field induces a current in

the copper, these currents have an associated magnetic field that oppose the thing that causes them.• These currents are called Eddy currents. They can be

very large as they flow through the large area of copper (remember resistance decreases with a larger area of wire)

• When you have a large current you get a large heating effect, so the transformer core heats up

• How can we reduce these eddy currents?

A rotating metal disc spinning in between the poles of a magnet will soon slow down. This is due to the large eddy currents produced.

Cutting slits in the metal reduces the size of the eddy currents that can flow. There is less of a braking effect.

If somehow we could cut up the core of a transformer…

Main

• Transformers are made more efficient by cutting up the iron core and laminating it with an insulating layer between the layers – this reduces the size of the eddy currents and the heating effect.

• The iron core is magnetically ‘soft’ so it magnetises easily and quickly. This makes it more efficient.

• The core is as large as possible to ‘trap’ as many lines of the magnetic field as possible.

• The coils are made of a good electrical conductor to reduce their resistance.

Draw a quick sketch diagram of a transformer. Write a sentence for each of the labels above using key words from the list below:Laminate Eddy Currents Soft Iron Heating Magnetic Field Low Resistance