Transformers

Transformers are used to change alternating potential differences.

Figure 11 shows a transformer.

Why is the core of the transformer made from iron?

A transformer has 345 turns on the primary coil and 6000 turns on the secondary coil.

The potential difference across the secondary coil is 400 kV.

Calculate the potential difference across the primary coil.

Use the Physics Equations Sheet.

Give your answer in volts.

There is an alternating current in the primary coil of a transformer.

Explain why there is an alternating potential difference across the ends of the secondary coil of the transformer.

Figure 21 shows a transformer used to power a lamp using the mains electricity supply.

What material is used to make the core of the transformer? Give the reason for using this material.

Determine the current in the secondary coil when the power output of the transformer is 6.9 W. The transformer is 100% efficient. Use the Physics Equations Sheet.

The National Grid uses transformers to change potential difference (pd).

Figure 12 shows a transformer.

Identify the parts of the transformer labelled in Figure 12.

There is an alternating input pd of 230 V.

Determine the output pd.

Use the Physics Equations Sheet.

The input pd causes an alternating current.

Explain why there is an alternating current in the output when the transformer is connected to a circuit.

Figure 13 shows a large cable supported by two wooden poles. The cable is

connected to an electricity supply.

There is a force on the cable due to the Earth’s magnetic field when the current is in the direction A to B.

What is the direction of this force?

Tick (✓) one box.

The cable experiences a force of 0.045 N due to the Earth’s magnetic field.

magnetic flux density = 60 T

current = 50 A

Calculate the length of the cable between A and B.

Use the Physics Equations Sheet.

State one assumption you made in your calculation.

Figure 10 shows a portable power supply.

The portable power supply has an alternator connected to a transformer. The transformer can be adjusted to have different numbers of turns on the secondary coil. Suggest why.

A lamp is connected to the power supply. The lamp requires an input potential difference of 5.0 V. The alternator generates a potential difference of 1.5 V. The primary coil of the transformer has 150 turns. Calculate the number of turns needed on the secondary coil.

Figure 11 shows the inside parts of the alternator.

The handle of the alternator is turned, causing the coil to rotate. Explain why an alternating current is induced in the coil.

Suggest the purpose of the slip rings.

The alternator from the portable power supply is disconnected from the transformer and lamp. Explain why the handle of the alternator becomes much easier to turn.

A student used a simple transformer to investigate how the number of turns on the secondary coil affects the potential difference (p.d.) across the secondary coil. The student kept the p.d. across the primary coil fixed at 2V. Figure 12 shows the results collected by the student.

Figure 12

Figure 12 contains one anomalous result. Suggest one possible reason why this anomalous result occurred.

The transformer changes from being a step-down to a step-up transformer. How can you tell from Figure 12 that this happens?

A spot-welder is a device that uses a transformer to produce a large current to join sheets of metal together. Figure 13 shows a transformer demonstrating how a large current can heat and join two nails together.

Figure 13

How does the amount of infrared radiation emitted by the nails change when the power supply is switched on?

Calculate the current from the power supply needed to provide a power output of 336 W. Use the data in Figure 13. The transformer is 100% efficient.