Name: _______________________________________________

     

 

Electric motors and EM Induction 

 Date:

        Time:   Total marks available:   Total marks achieved: ______                                          

 

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Questions

Q1.The photographs show how an electric toothbrush fits on its charger.

The charger and the toothbrush each have a coil of wire inside them.

The diagram shows how the two coils are linked by a U-shaped core.

This arrangement of core and coils acts as a transformer that reduces voltage.

(a)  (i)  Name the type of transformer that reduces voltage.

(1).............................................................................................................................................

(ii)  Explain why the core is made of a soft magnetic material, such as iron.(2)

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(b)  (i)  State the equation linking the input (primary) and output (secondary) voltages and the turns ratio of a transformer.

(1)

(ii)  The transformer has 520 primary turns and 30 secondary turns.The input voltage to the transformer is 44 V.Calculate the output voltage.

(2)

(c)(i) The alternating current in the transformer has a frequency of 27 000 Hz.

The toothbrush vibrates at the same frequency when it is being charged.Explain why these vibrations cannot be heard.

(2)..................................................................................................................................................................

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(ii)  A circuit in the toothbrush delivers regular pulses of direct current.There is a pulse every 1.5 ms.Calculate the frequency of the pulses.

(2)

frequency = .......................................... Hz

Q2.The photograph shows a halogen lamp.

(a)  The halogen lamp has a power of 50 W when operating at its normal voltage.

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Calculate the amount of electrical energy transferred to the halogen lamp in 40 hours.(3)

electrical energy transferred = ........................................................... J

(b)  A student notices that in addition to producing light, the lamp also gets hot.

She concludes that the lamp cannot be 100% efficient.Explain whether the student's conclusion is correct.

(2) .............................................................................................................................................

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(c)  The lamp must not be connected directly to mains voltage.

A step-down transformer must be used.Describe the structure of a step-down transformer.You may draw a diagram to help your answer.

(3)

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(d)  A step-down transformer reduces voltage from 230 V to 12 V.The secondary current is 4.2 A.

(i)  State the equation linking input power and output power for a transformer.

[assume that the transformer is 100% efficient](1)

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(ii)  Calculate the primary current.(3)

 

 

Q3.The diagram shows a simple generator.

The coil of wire rotates at a constant rate.When the coil of wire rotates, a voltage can be measured across the contacts.

Use the grid to sketch how the measured voltage varies as the coil of wire rotates.

(4)

 

(Total for question = 4 marks)

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 Q4.A student investigates the magnetic fields produced by bar magnets.

(a)  Describe how the student could investigate the shape and direction of the magnetic field around a single bar magnet.

You may draw a diagram to help your answer.(3)

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(b)  The student puts opposite poles of two strong bar magnets near each other.

The diagram shows part of the magnetic field produced.Only a small part of each magnet is shown.

Explain why the diagram shows that the magnetic field is uniform.(2)

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(c)  The student connects the ends of a piece of wire to a sensitive ammeter.

He then moves the piece of wire through the magnetic field between the magnets.(i)  Explain why the ammeter displays a small current when the wire is moved between the magnets.

(2) .............................................................................................................................................

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(ii)  State two ways that the student could increase the current using the same wire.

(2)1 ..........................................................................................................................................

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Q5.(a)  This apparatus can be used to investigate electromagnetic induction.

When the magnet is moved into the coil of wire, the voltmeter shows a negative reading.State two separate changes, each of which would make the voltmeter show a positive reading.

(2) 1 ..........................................................................................................................................

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2 ..........................................................................................................................................

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(b)  The diagram shows a simple electrical generator connected to a lamp.

When the coil is turned, a voltage is induced.

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(i)  Explain why a voltage is induced when the coil is turned.(2)

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(ii)  State two ways that this induced voltage can be increased.

(2) 1 ..........................................................................................................................................

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(iii)  When the lamp is connected to the generator, the coil is hard to turn.When the lamp is disconnected from the generator, the coil is easy to turn.Suggest, in terms of energy, why it is harder to turn the coil when the lamp is connected.

(2) .............................................................................................................................................

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Q6.Photograph E shows a rechargeable torch.

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(a) When a student shakes the torch, the magnet moves through the coil and back again.This induces a voltage across the ends of the coil.The voltage is used to provide current to recharge the battery.

(i) Explain why a voltage is induced.(2)

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      .............................................................................................................................................. (ii) State one way to increase this voltage.

(1)      ..............................................................................................................................................      ..............................................................................................................................................       (b) Photograph F shows the components inside the torch.

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The torch uses a light-emitting diode (LED) to provide light.(i) When the LED is on, it shows that

(1)

   A    the current is alternating

   B    the torch is switched off

   C    there is a current in the circuit

   D    there is a fault in the circuit

(ii) The manufacturer of the torch states, "An LED is a more efficient source of light than a filament lamp."

Explain this statement in terms of energy transfer.(2)

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      ..............................................................................................................................................Q7.Many food shops have devices that attract and kill flying insects.

The devices consist of

  an ultraviolet lamp   a plastic mesh on the outside   an electrified wire grid below the plastic mesh   a transformer that is connected to the wire grid

(a)  The ultraviolet lamp attracts many flying insects towards the device.

Ultraviolet is an electromagnetic wave.(i)  State two properties of electromagnetic waves.

(2)1 ..........................................................................................................................................

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(ii)  Which of these electromagnetic radiations has a frequency greater than ultraviolet?

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(1)   A    infrared   B    gamma rays   C    radio waves   D    visible light

(b)  The transformer supplies an output voltage of 2000 V a.c. to the wire grid.

The input voltage to the transformer is 230 V a.c.(i)  Give the name of this type of transformer.

(1) .............................................................................................................................................

(ii)  State the relationship between input (primary) voltage, output (secondary) voltage, primary turns and secondary turns.

(1)

(iii)  There are 110 turns on the primary coil.Calculate the number of turns on the secondary coil.

(3)

number of turns ...........................................................

(iv)  Suggest a reason why there is a plastic mesh on the outside of the device.(1)

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(Total for question = 9 marks) 

Q8.A wind turbine generates electricity for the National Grid.

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(a) The useful energy transfer in the wind turbine is(1)

   A    chemical energy to electrical energy

   B    gravitational potential energy to electrical energy

   C    kinetic energy to electrical energy

   D    sound energy to electrical energy

(b) The generator in the wind turbine transfers 39 MJ of energy in 1 minute.The generator current is 490 A.(i) Calculate the output voltage of the generator.

(3)

(ii) The generator output voltage is then increased to 132 kV for transmission.

Explain why electrical energy is transmitted using very high voltages.(4)

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      ..............................................................................................................................................(c) The generator provides a direct current (d.c.).This d.c. is changed to an alternating current (a.c.).

The frequency of the alternating current is 50 Hz.(i) Explain the meaning of 50 Hz alternating current.

(2)      ..............................................................................................................................................

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      ..............................................................................................................................................(ii) Explain why the d.c. from the generator must be changed to a.c. before it is transmitted.      ..............................................................................................................................................

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      ..............................................................................................................................................Q9. A transformer is used to convert the 25 kV output from the power station to 115 kV.

(i) State the equation linking power, voltage and current.(1)

(ii) Compare the input current and the output current of the transformer.Assume there are no energy losses in the transformer.

(3)

(iii) State one advantage of transmitting electricity at high voltages.(1)

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      ..............................................................................................................................................Q10.Explain how the transmission of electrical power is made more efficient by using step-up and step-down transformers.

(5).............................................................................................................................................

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Q11.A student investigates how to produce a voltage.

He hangs a magnet from a spring, above a coil that is connected to a data logger.

(a) The student pulls the magnet through the coil to X and then releases it.

The magnet moves up and down through the coil.The data logger produces this graph of voltage against time.

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(i) Explain why the data logger records a varying voltage.(2)

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(ii)   Which feature of the graph shows that the voltage is alternating?(1)

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(iii)   Suggest why the voltage changes as shown by the graph.(2)

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(b)The student repeats the experiment using two magnets taped together.

Compared to one magnet, these two magnets take a longer time to move up and down.The dotted line on the grid shows the original graph for one magnet.On the same grid, sketch the graph that would be produced using two magnets.

(3)

Q12.A soldering iron is a tool used when joining electronic components in a circuit.

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It has an electric heater.

(a) Soldering iron A operates when connected to the mains supply.

Soldering iron A is labelled 230 V, 30 W.(i) What does 30 W tell you about the energy transfer in the soldering iron?

(2)      ..............................................................................................................................................      ..............................................................................................................................................

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      ..............................................................................................................................................(ii) This soldering iron has an earth connection. Explain how an earth connection protects the user.

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      ..............................................................................................................................................(b) Soldering iron B is connected to a low voltage power supply.

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Soldering iron B is labelled 24 V, 70 W.A student says:

(i) Use information from the soldering iron labels to evaluate this statement.(3)

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      ..............................................................................................................................................(ii) There is a step-down transformer in the power supply for soldering iron B.Describe the structure of a step-down transformer. You may draw a labelled diagram to help your answer.

(3)

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      ..............................................................................................................................................Q13.Some students are investigating a simple electric motor.

(a) When the switch is closed the coil spins on the axle.State why.(2)

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      ..............................................................................................................................................(b) State two changes that would make the motor spin faster.

(2)1 ....................................................................................................................................................................      ........................................................................................................

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      ..............................................................................................................................................(c) The students change their circuit, replacing the cell and switch with a lamp.

When they spin the coil by hand, the lamp lights.Explain why the lamp lights.

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      ..............................................................................................................................................Q14.A laptop battery charger contains a step-down transformer.

(a) The number of turns on the primary coil of a step-down transformer is(1)

   A    the same as the number of secondary turns

   B    more than the number of secondary turns

   C    less than the number of secondary turns

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   D    zero

(b) This transformer is designed to reduce the voltage from 230 V to 12 V.The primary current is 0.25 A.

(i) State the equation linking primary voltage, primary current, secondary voltage and secondary current for a transformer.

(1)

(ii) Calculate the secondary current, assuming that the transformer is 100% efficient.(2)

(c) A student notices that the charger becomes warm when it is working.Suggest how this will affect the output of the transformer.

(2)      ..............................................................................................................................................      ..............................................................................................................................................

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      ..............................................................................................................................................Q15.The diagram shows a transformer that is 100% efficient.

(a)  (i)  State the equation linking input power and output power for the transformer.

(ii)  State the equation linking input power and output power for the transformer.(2)

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output current = ........................................................... A

(b)  (i)  State the equation linking input voltage, output voltage and turns ratio for the transformer.

(1)

(ii)  Calculate the number of turns on the secondary coil of the transformer.(2)

number of turns = ...........................................................

(c)  Explain how a transformer works.In your answer, you should include the reasons for using two coils an iron core an alternating supply

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Q16.Electrical energy can be transmitted using a high voltage of 132 kV.

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(a)  A voltage of 132 kV is the same as (1)

       A    132 V

       B    1320 V

       C    132 000 V

       D    132 000 000 V

(b)  Using a high voltage increases the (1)

       A    current in the wires

       B    efficiency of transmission

       C    energy lost as heat

       D    resistance of the wires

(c)  The high voltage can be reduced using a

(1)

       A    generator

       B    magnet

       C    transformer

       D    transmitter

  Q17.The diagram shows a magnet held above a coil. The coil is connected to a voltmeter.

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(a)The magnet is released and falls into the coil.(i) Explain why the voltmeter shows a reading.

(2)      ..............................................................................................................................................

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      ..............................................................................................................................................(ii) The magnet is released from a greater height. How does this affect the voltmeter?Explain your answer.

(2)      ..............................................................................................................................................      ..............................................................................................................................................

      ..............................................................................................................................................(b) State how the voltmeter reading changes when the same magnet

(i) moves more slowly into the coil(1)

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(ii) moves into a coil with more turns(1)

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(iii) is reversed so that the S-pole enters the coil first.(1)

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