Transformers

18* (a) Describe how an experiment can be conducted to determine how the output current of a step‑up transformer depends on the number of turns on the secondary coil.

Explain how the data collected can be analysed to establish the relationship between the output current and the number of turns on the secondary coil.

You are provided with wire and a suitable core on which to wind the wire, as well as any other normal laboratory equipment.

Use the space below to draw a labelled circuit diagram.

State Faraday’s law.

Show that the number of turns on the secondary coil is 20.

At one particular instant, the output voltage from the transformer is 3.4 V.

Calculate the change in magnetic flux experienced by the secondary coil in a short time interval of 1.2 ms and state its unit.

Assume that the output voltage from the transformer remains constant at 3.4 V over this time interval.

The diagram below shows two long vertical current-carrying wires X and Y.

The direction of the current in each wire is the same.

Explain why wire Y experiences a force and deduce the direction of this force.

State Faraday’s law of electromagnetic induction.

The number of turns on the coils of four ideal iron-cored transformers A, B, C and D are shown in the table below.

Each transformer is connected in turn to an alternating 240 V supply.

Which transformer will give the largest output current?

Your answer

Explain the shape of the graph in terms of the magnetic flux linking the coil.

The coil shown in Fig. 5.1 has 150 turns. The maximum induced e.m.f. V₀ across the coil is 1.2 V when the magnet is rotating at 24 revolutions per second.

Calculate the maximum magnetic flux through the coil using the equation

V₀ = 2π × (frequency) × (maximum magnetic flux linkage)

Give a unit with your answer.

* A student is given a transformer with coils X and Y, as shown in Fig. 5.4.

The student is intending to investigate how the maximum induced e.m.f. V₀ in coil Y depends on the frequency f of the alternating current in coil X.

The changing magnetic flux density in coil X induces an e.m.f. in coil Y. Faraday’s law indicates that the maximum induced e.m.f. V₀ should be directly proportional to f.

Describe how you would investigate the suggested relationship between V₀ and f in the laboratory using these coils. In your description include all of the equipment used and how you would analyse the data collected.

Use the space below to draw a suitable diagram.

* A student conducts an experiment to confirm that the uniform magnetic flux density B between the poles of a magnet is 30 mT.

A current-carrying wire of length 5.0 cm is placed perpendicular to the magnetic field.

The current I in the wire is changed and the force F experienced by the wire is measured. Fig. 22.1 shows the graph plotted by the student.

The student’s analysis is shown on the graph of Fig. 22.1 and in the space below.

F = BIL

This is just 2 mT out from the 30 mT value given by the manufacturer, so the experiment is very accurate.

Evaluate the information from Fig. 22.1 and the analysis of the data from the experiment. No further calculations are necessary.

Use Faraday’s law of electromagnetic induction to explain why the filament lamp is lit.

The primary coil has 400 turns and the secondary coil has 20 turns. The potential difference across the lamp is 12 V and it dissipates 24 W. The transformer is 100% efficient.

1. Calculate the current in the primary coil.

2. The alternating voltage supply is replaced by a battery and an open switch in series. The switch is closed. The lamp is lit for a short period of time and then remains off. Explain this observation.