P1 Revision Booklet

Electromagnetic Induction

Hard (Grade B-A*)

Q1.          (a)     A laboratory technician sets up a demonstration.

A flexible wire is suspended between the ends of a horseshoe magnet. The flexible wire hangs from a cotton thread. When the switch is closed, the wire kicks forward.
Identify the effect which is being demonstrated.
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(1)

(b)     A teacher makes some changes to the set-up of the demonstration.
What effect, if any, will each of the following changes have?
(i)      more powerful horseshoe magnet is used.
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(1)

(ii)     The connections to the power supply are reversed.
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(1)
(Total 3 marks)

 Q2.          (a)     In the National Grid, very large step-up transformers link power stations to the transmission cables.
A transformer used for this purpose has 800 turns on its primary coil and 12 800 turns on its secondary coil. The p.d. (potential difference) across its primary coil is 25 kV.
Use the equation in the box to calculate the p.d. across its secondary coil.

Show clearly how you work out your answer and give the unit.
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                 p.d. across secondary coil = .......................................................
(3)

(b)     The primary and secondary coils of a transformer are made of insulated wire.
Why is this insulation necessary?
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(1)

(c)     Describe what happens when an alternating potential difference is applied across the primary coil of a transformer.
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(3)
(Total 7 marks)

 

Q3.          The diagram shows the National Grid system.

Transformers X and Y are an essential part of the National Grid system.
Explain why.
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(Total 4 marks)
 
Q4.          The diagram shows the basic structure of a transformer.

(a)     Explain how a transformer works.
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(5)

(b)     A transformer is used to change the 230 volt mains electricity supply to the 12 volts needed to operate a low voltage halogen lamp. The current through the halogen lamp is 
4 amps.
Calculate the current drawn by the transformer from the mains electricity supply.
Assume that the transformer is 100 % efficient.
Write down the equation you use, and then show clearly how you work out your answer.
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Current = ........................................... amps
(2)
(Total 7 marks)
 Q5.          A transformer is used to reduce the 230 V a.c. mains to the 12 V supply required for the lighting system. The transformer has 1150 turns on its primary coil.
(i)      Write down the equation which links the number of turns of each transformer coil to the voltage across each transformer coil.
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(1)

(ii)      Calculate the number of turns on the secondary coil of the transformer. Show clearly how you work out your answer.
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number of turns on the secondary coil = .........................
(2)
(Total 3 marks)
 Q6.          The diagram shows apparatus used to demonstrate the electric motor effect. When he switch is closed the wire moves.

(i)      Draw an arrow on the diagram to show the direction the wire moves.
(1)

(ii)      Explain why the wire moves.
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(2)
(Total 3 marks)
 Q7.          

          Transformer A produces a very high voltage to transmit the electrical energy through the National Grid.
Explain why electrical energy is transmitted at a very high voltage.
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(Total 3 marks)
 Q8.          The drawing shows a food processor. It has an electric motor. Inside is a blade which spins round and cuts up the food.

          The food processor is designed to transfer electrical energy to kinetic energy. However some of the energy is wasted as heat and sound.
          The power input to the food processor is 1150 W. The power of the spinning blade is 900 W.

(i)      Calculate how much energy is wasted when the food processor is used for two minutes.
          Show clearly how you get to your answer and give the unit.
 
power = 
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Energy = .........................................................
(3)

(ii)      Why does the food processor produce sound when it is switched on?
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(1)
(Total 4 marks)
 Electromagnetic Induction-Mark scheme

M1.          (a)     motor (effect)
1

(b)     (i)      wire kicks further (forward)
accept moves for kicks
accept moves more
accept ‘force (on the wire) increased’
1

(ii)     wire kicks back(wards) / into (the space in) the (horseshoe) magnet
accept moves for kicks
accept ‘direction of force reversed’
1
[3]
 M2.          (a)     400 000
allow 1 mark for correct substitution ie

or

2

volt(s) / V
an answer 400 gains 2 marks
an answer 400 kilovolts / kV gains 3 marks
although the unit mark is independent to gain 3 marks it must be consistent with the numerical value
1

(b)     any one from:
do not accept any response in terms of heat insulation, safety or electric shock
•        (so that there is) no short circuit
•        (so that the) current goes round the coil
do not accept electricity for current
•        (so that the) current does not enter the core
1

(c)     (the alternating p.d. in the primary causes) an (alternating)
current in the primary
reference to the current in the core negates this mark
1

(causes an) alternating / changing (magnetic) field in the (iron) core
1

induces (alternating) p.d. across the secondary (coil)
accept in / through or similar for across
accept current for p.d.
accept output (coil) for secondary (coil)
to gain 3 marks the sequence must be correct
1
[7]
 M3.          transformer X reduces the current through the transmission cables
accept increases p.d. across the cables
1
this reduces the energy loss from the cables
1
which increases the efficiency of the distribution system
1
transformer Y is essential as it reduces the p.d. to a safe
working value for consumers
1
[4]
 M4.          (a)     an alternating input / current to primary (coil)
1
which produces an alternating magnetic field
accept changing magnetic field for alternating magnetic field
if first mark point scores then ‘alternating’ not required here
1
in the (iron) core
this magnetic field links with the secondary coil
1
which induces an (alternating) voltage / p.d. across the
secondary (coil)
1

(b)     0.21
accept 0.2 or any answer that rounds to 0.21
allow 1 mark for correct equation
ie power input = power output
or
allow 1 mark for substitution into correct equation
ie 230 × Ip = 12 × 4
2
[7]

M5.          (i)      
 
accept input for primary
accept output for secondary
accept  or correct transposition
1
(ii)      60
allow 1 mark for correct transformation
2
[3]
M6.          (i)      away from magnet
arrow should be perpendicular to field lines and current as judged by eye
1

(ii)      current in wire creates magnetic field around wire
1
          two fields interact or combine giving a resultant force (on the wire)
1
[3]

M7.          the higher the voltage the smaller the current
small current gives small energy loss
in the form of heat
(or efficiency greater, or energy/heat losses low – gets 1)
for 1 mark each
[3]

 

M8.          (i)      EITHER
30000 (2) joules/J (1)
or 30 kilojoules/kj
3
          OR
power × time = energy
1
time = 120 (seconds)
1
(ii)      vibration (of the food processor / some part of the food processor / the food)
1
[4]