Transformers

This question is about transformers.

Describe the structure of a step‑up transformer.

You may draw a diagram to help your answer.

A step‑up transformer has an input voltage of 230 V and an input current of 4.5 A.

The output current of the transformer is 0.21 A.

State the formula linking input power and output power for a transformer that is 100% efficient.

Calculate the output voltage of the transformer.

This question is about transformers.

The diagram shows a step‑down transformer.

The table gives some data for the step‑down transformer.

State the formula linking input (primary) voltage, output (secondary) voltage and the turns ratio for a transformer.

Use data from the table to calculate the output (secondary) voltage of the transformer.

This transformer is not 100% efficient.

The efficiency of a transformer can be calculated using the formula

Calculate the efficiency of the transformer.

Explain how transformers are used in the efficient, large‑scale transmission of electrical energy.

You may draw a diagram to help your answer.

The photograph shows transmission cables used for long-distance transmission of electricity.

The diagram shows a power station and a school.

Add to the diagram by drawing the structures needed to efficiently transfer energy from the power station to the school using electricity.

Power station
School

Explain how the amount of current in the transmission cables increases the efficiency of the transmission of electricity.

The diagram shows a step-down transformer.

The input power to the transformer is 16 W.

The transformer is used for 2.5 hours.

Calculate the energy transferred to the transformer during this time.

Explain how a transformer works.

In your answer, include reasons for using

• two coils

• the iron core

• an a.c. power supply

State how the primary coil of the transformer can be changed to increase the output voltage.

The National Grid is used in the United Kingdom for the large-scale transmission of electricity. The photograph shows solar panels connected to the National Grid.

Solar panels provide direct current to a device that outputs an alternating current so that the energy from the solar panels can be supplied to the National Grid.

Explain why a step-up transformer is used to supply the National Grid.

This is the label on the step-up transformer.

State the formula linking input voltage, output voltage and turns ratio for a transformer.

Calculate the number of turns on the secondary coil of the step‑up transformer.

Solar panels produce constant direct current (d.c.).

Explain why a transformer will not work when connected to constant direct current.

A transformer is used to charge a mobile phone.

The diagram shows a label on the transformer.

Explain how the information in the label shows that the transformer is a step‑down transformer.

Show that the transformer is approximately 100% efficient.

State the formula linking input voltage, output voltage and turns ratio for a transformer.

The primary coil has 1500 turns.

Calculate the number of turns on the secondary coil.

The diagram shows part of the National Grid.

There is a step-up transformer at the power station end of a transmission line and a step-down transformer at a distant town.

Explain why step-up transformers and step-down transformers are used in this way.

State the formula linking the input voltage, the output voltage and the turns ratio for a transformer.

The primary coil on a step-up transformer has 3300 turns.

Calculate the number of turns required on the transformer’s secondary coil to step up the voltage from 15 kV to 340 kV.

Transformers in the National Grid have efficiencies less than 100%.

Suggest which energy store of the transformer increases as a result of the efficiency being less than 100%.

The iron core of a transformer is an electrical conductor.

When the transformer is in use, the primary coil causes a changing magnetic field in the iron core. This causes a small current to be induced in the core.

Explain how a current is induced in the core of the transformer.

This question is about electromagnetism.

Diagram 1 shows the construction of an electromagnet.

Name a suitable metal for the core.

State what must be done to the coil of wire to produce a magnetic field.

Electromagnets are used in transformers to change the size of a voltage.

Diagram 2 shows the construction of a simple transformer.

primary coil (1500 turns) secondary coil (280 turns)

Diagram 2

State the formula linking the input (primary) voltage, output (secondary) voltage and the turns ratio for a transformer.

The input voltage to the transformer is 115 V.

Show that the output voltage is approximately 20 V.

The input current to the transformer is 1.2 A.

Calculate the output current.

Assume that the transformer is 100% efficient.

output current =_........

State two factors that would increase the output current of the transformer.

Transformers are useful in the transmission of electricity.

The diagram shows a step‑down transformer.

Give the name of a suitable metal that could be used in the core of the transformer.

State the formula linking input power and output power for a transformer.

The step‑down transformer has an input voltage of 275 kV and an output voltage of 230 V.

The transformer has an output current of 95 A.

Calculate the input current to the transformer.

Assume the transformer is 100% efficient.

Explain how transformers are useful in the large‑scale transmission of electricity.

You may draw a diagram to support your answer.

The diagram shows a step‑down transformer.

Explain the operation of a step‑down transformer.

State the formula linking input voltage, output voltage and the turns ratio of a transformer.

Calculate the output voltage for the transformer.

Welding is a process where pieces of metal are melted and then joined together.

The diagram shows apparatus used for welding.

The welding apparatus uses the transformer to decrease the voltage from 230 V.

State the name of this type of transformer.

State the formula linking the turns ratio, the input voltage and the output voltage for a transformer.

The table gives some information about the transformer.

Use information from the table to calculate the output current of the transformer.

Assume that the transformer is 100% efficient.

A fuse is installed in the plug which supplies the input current.

Which fuse should be used in this plug?

The diagram shows the first stages in a method of transferring energy electrically through long distances.

The table shows data about the step-up transformer.

State the formula linking input voltage, output voltage, number of primary turns and number of secondary turns for a transformer.

Calculate the number of turns on the secondary coil.

Explain why step-up and step-down transformers are used in the large-scale transmission of electricity.

You may draw a diagram to support your answer.

A student investigates a transformer.

This is the student’s method.

● use a primary coil with 10 turns

● connect the primary coil to a constant maximum input voltage

● measure the output voltage across the secondary coil

● repeat using an increasing number of turns on the primary coil

The table shows the student’s results.

Plot a graph of the student’s results on the grid.

Draw a curve of best fit.

Describe the relationship between the output voltage and the number of turns on the primary coil.

State the formula linking input and output voltages and the turns ratio for the transformer.

The input voltage of the transformer is 6.8 V.

Calculate the number of turns on the secondary coil.

A student investigates transformers.

The diagram shows a typical transformer.

State the name given to this type of transformer.

The student investigates the effect of changing the number of turns in the secondary coil.

This is his method.

● apply a constant maximum voltage to a primary coil with 1200 turns

● use a secondary coil with 100 turns

● measure the output voltage of the transformer

● replace the secondary coil with one that has 200 turns

● measure the output voltage again

The student repeats this method using different numbers of turns in the secondary coil.

Suggest how the student could improve the reliability of his investigation.

These are the student’s results.

number of turns = 100, number of turns = 400, output voltage = 1.3 V output voltage = 5.0 V number of turns = 200, number of turns = 500, output voltage = 2.5 V output voltage = 6.3 V number of turns = 300, number of turns = 600, output voltage = 3.8 V output voltage = 7.5 V

Draw a table of these results.

Suggest how the student could improve the precision of his voltage measurement.

The student plots this graph to show the results of his investigation.

The student plots one of his results incorrectly.

Draw a circle around the incorrectly plotted result on the graph.

Draw the line of best fit.

Describe the relationship shown by the graph.

The primary coil has 1200 turns.

Using the student’s data, calculate the primary voltage used in his investigation.