Electromagnetic Effects (Cambridge (CIE) IGCSE Physics)

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Fig. 10.1 is a simplified top view of a flat coil. There is an alternating current (a.c.) in the coil.

Describe the magnetic effect of this alternating current.

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Fig. 10.2 shows a pan placed above the coil. The base of the pan is made of steel.

State what quantity is induced in the base of the pan.

The pan contains water.

State and explain the effect of the quantity induced in part (b) on the temperature of the water in the pan.

Fig. 11.1 shows in each of the diagrams a current-carrying conductor and a magnetic field pattern.

State the diagram which correctly shows the magnetic field around a current-carrying conductor.

Fig. 11.2 shows three pieces of equipment.

(i) Describe how to generate and detect an electromotive force (e.m.f.) using the equipment in Fig. 11.2. You may draw a diagram.

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(ii) Describe two changes that will generate a larger e.m.f. using similar equipment to that in Fig. 11.2.

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A student connects a lamp and centre-zero galvanometer in series with a generator, as shown in Fig. 11.3.

The student observes the galvanometer needle moving from side to side repeatedly.

Explain why the needle moves in this way.

Fig. 9.1 shows a simple direct current (d.c.) electric motor. The coil rotates about the axis when there is a current in the coil. The coil is connected to the rest of the circuit by the brushes.

(i) On Fig. 9.1, draw a pair of arrows to show which way the coil rotates. Explain the direction you have chosen.

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(ii) On Fig. 9.1, draw an arrow to show the direction in which electrons flow through the coil.

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(iii) Explain why the electrons flow in the direction you have shown in (a)(ii).

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State any difference each of the following changes makes to the rotation of the coil in Fig. 9.1:

(i) changing the polarity of the power supply to that shown in Fig. 9.2

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(ii) changing the coil to the new coil shown in Fig. 9.3

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(iii) using a stronger magnetic field.

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A student has a model electric railway. The model railway uses a step-down transformer.

  The input voltage is 230 V. The transformer has 1710 turns on the input coil and 90 turns on the output coil.

  Calculate the output voltage of the transformer.

output voltage = ....................................................... V 

A step-up transformer is used to increase voltage.

Step-up transformers and step-down transformers have different coil arrangements.

Describe the differences in the coil arrangement for the two types of transformer.

Explain the advantage of transmitting electricity at high voltages, rather than at low voltages.

A transformer consists of two coils of wire wound on a metal core. Fig. 10.1 represents the transformer.

State the name of the metal from which the core is made.

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The primary coil of the transformer is connected to the output voltage of an a.c. generator which supplies an alternating current.

(i) Explain why there is a voltage between the two terminals of the secondary coil.

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(ii) There are 560 turns on the primary coil and 910 turns on the secondary coil of the transformer. The voltage between the two terminals of the secondary coil is 78 V.

Calculate the voltage supplied by the a.c. generator.

 generator voltage = ............................................................[2]

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Transformers are used to increase the voltage when electrical energy is transmitted in cables across long distances.

Explain why power losses in the cables are lower when the voltage is high.

A teacher demonstrates electromagnetic induction using the apparatus shown in Fig.10.1.

The teacher pulls the magnet down until one end is inside the coil. The teacher then releases the magnet. The magnet moves up and down repeatedly. As it moves, one end of the magnet enters and leaves the coil.

  Describe and explain the readings on the centre-zero millivoltmeter as the magnet enters and leaves the coil.

Fig.10.2 shows a system for transmitting electricity from a power station.

State the name of the device used in X and the name of the device used in Y.

A student is experimenting with electromagnetic effects.

Describe an experiment, using any standard laboratory equipment, to demonstrate electromagnetic induction. You may draw a diagram.

Fig. 11.1 shows a transformer connected to an input voltage of 12 V a.c.

(i) State the name of a suitable material for the core of the transformer.

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(ii) Explain how the diagram in Fig. 11.1 shows a step-up transformer.

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(iii) Using the information in Fig. 11.1, calculate the output voltage of the transformer.

output voltage = ...................................................... V [3]

Fig. 10.1 shows a straight wire AB placed in the magnetic field between the poles of a magnet.

The ends of AB are connected to a galvanometer.

When AB is moved vertically, the needle of the galvanometer shows a deflection.

State three factors that affect the size of the deflection.

Fig. 10.2 shows a transformer.

(i) The primary coil P has 8000 turns and an input of 240 V. The secondary coil S has an output of 12 V.

Calculate the number of turns in the secondary coil.

number = ......................................................... [2]

(ii) A circuit containing a resistor is connected to the terminals A and B.

  A direct current (d.c.) is required in this resistor.

  On Fig. 10.1, draw this circuit.

[1]

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A student makes a transformer that uses an alternating current (a.c.) supply with an electromotive force (e.m.f.) of 12.0 V to induce an output potential difference (p.d.) of 2.0 V.

The student is provided with two lengths of insulated wire and the U-shaped piece of iron shown in Fig. 7.1.

(i) Complete and label Fig. 7.1 to show the transformer connected to the supply and the output from the transformer.

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(ii) Explain the function of the piece of iron in the transformer.

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(iii) The output of the transformer is connected to a lamp. The current in the lamp is 100 mA.

The transformer is 100% efficient.

Calculate the input current to the transformer.

current = ......................................................... [2]

Another transformer is used in a school laboratory to step down a mains supply with a p.d. of 110 V to 12 V. This transformer is mounted in a metal case.

State and explain an essential safety feature required for this arrangement.

Fig. 11.1 shows a vertical conductor passing through a horizontal piece of card.

(i) On Fig. 11.1, draw a cell and a switch in series with the conductor to form a complete circuit.

 Use the correct circuit symbols.

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(ii) A student sprinkles iron filings onto the card and closes the switch. There is a current in the conductor. Describe the pattern of the magnetic field seen.

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(iii) The student reverses the direction of the current in the conductor. State the effect, if any, on the pattern he sees.

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Describe an experiment to show that a force acts on a current-carrying conductor in a magnetic field. Show how to arrange the equipment. Include a diagram in your answer.

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Fig. 9.1 shows a coil supplied with current using a split-ring commutator.

(i) State and explain any motion of the coil.

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(ii) The coil in Fig. 9.1 consists of three turns of wire. The magnetic field strength of the magnet is M. With a current of 2.0 A in the coil, the coil experiences a turning effect, T.

 The first row of Table 9.1 shows this data.

Table 9.1

Complete Table 9.1 to give the turning effect for the changes made to the arrangement shown in Fig. 9.1.

Choose your answers from the box.

[3]

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Explain why the voltage of the supply to the primary coil of a transformer must be alternating.

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Fig. 10.1 shows a transformer.

There are 8000 turns in the primary coil of the transformer. The primary coil is connected to a 240 V mains supply. A 6.0 V lamp connected to the secondary coil operates at full brightness.

(i) Calculate the number of turns in the secondary coil

number of turns = ...........................................................[2]

(ii) The current in the lamp is 2.0A. The transformer operates with 100% efficiency.

Calculate the current in the primary circuit.

current = ...........................................................[2]

(iii) The primary circuit contains a 6.0 A fuse.

Calculate the maximum number of lamps, identical to the lamp in (ii), that can be connected in parallel in the secondary circuit without blowing the fuse.

 number of lamps = ...........................................................[1]

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Fig. 9.1 shows a coil ABCD with two turns. The coil is in a magnetic field.

When there is a current in the coil, the coil experiences a turning effect.

(i) Explain why there is a turning effect.

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(ii) The value of the current is 3 A.

 Place one tick in each column of the table to indicate how the turning effect changes with the change described.

 [3]

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Fig. 9.2 shows a magnet held just below a vertical coil connected to a galvanometer.

The magnet is released.

(i) State any effect on the galvanometer.

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(ii) State any effect on the magnetic field produced by the coil.

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