60 mins Mains electricity exam Qs B+ with answers

8/19/2019 60 mins Mains electricity exam Qs B+ with answers

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Madeley High School

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B+ Exam Qs Mains electricity Name:_______________________________________

Q1. (a) Describe the difference between an alternating current (a.c.) and a direct current(d.c.).

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(b) The diagram shows the information plate on the bottom of an electric wallpapersteamer.

(i) Use the equation in the box to calculate the current used by the steamer.

power = current × potential difference

Show clearly how you work out your answer. Give the unit

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Current ..............................(3)

(ii) Which one of the following fuses should be used inside the plug of thesteamer?

Draw a ring around your answer.

1 A 3 A 5 A 10 A 13 A(1)

(Total 6 marks)


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Q2. (a) The picture shows a person using a set of electronic ‘Body Fat Scales’. When theperson stands on the scales, a small, harmless, electric current passes through the person’s body.The scales then calculate the resistance of the person’s body and convert the resistance into a

prediction of body fat content.

(i) The scales contain two 3 V cells joined in series.

Calculate the resistance of a person’s body, if when he stands on the scales, acurrent of 0.12 mA passes through his body.

1000 mA = 1 A

Use the correct equation from the Physics Equations Sheet.

Show clearly how you work out your answer and give the unit.

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Resistance = .............................................................(3)

(ii) The scales can only produce a prediction of body fat content and not anaccurate measurement.

Suggest why.

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


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(iii) It is recommended that the scales are not used immediately after a personhas drunk a large amount of water.

Suggest why.

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(b) The diagram shows how someone could get an electric shock from accidentallycutting into an electric cable. If this happens, and a Residual Current Circuit Breaker(RCCB) is being used, the circuit will switch off automatically.

(i) A faulty appliance or circuit can be switched off by a RCCB or a fuse.

Compare the action of a RCCB with the action of a fuse.

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(ii) The graph shows how the severity of an electric shock depends on the size ofthe current and the time that the current flows through the body.

Using the RCCB helps prevent an electric shock seriously injuring the personusing the hedge trimmers.

Using information from both the diagram and the graph explain how.

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(Total 10 marks)


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Q3. In the UK mains electricity is a 230 volt a.c. supply.

(a) What is the frequency of the a.c. mains electricity in the UK?

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(b) (i) What is an electric current?

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(ii) Explain the difference between an a.c. (alternating current) electricity supplyand a d.c. (direct current) electricity supply.

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(c) A householder has a 10.8 kW electric shower installed in the bathroom.

(i) Calculate the current drawn from the mains electricity supply by the shower.

Write down the equation you use, and then show clearly how you work outyour answer.

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Current = .................................... A(2)


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(ii) The table gives the maximum current that can safely pass through electriccables of different cross-sectional area.

Cross-sectionalarea

in mm 2

Maximum safecurrentin amps

1.0 11.5

2.5 20.0

4.0 27.0

6.0 34.0

10.0 46.0

16.0 62.0

The existing power sockets in the house are wired to the mains electricitysupply using 2.5 mm 2 cable.

Use the data in the table to explain why the shower must not be connected tothe mains electricity supply using 2.5 mm 2 cable.

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(iii) The circuit connecting the shower to the mains electricity supply must includea residual current circuit breaker (RCCB) and not a fuse.

Give two advantages of using a RCCB to protect a circuit rather than a fuse.

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

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(Total 10 marks)


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Q4. Carefully read the following extract from a safety leaflet. Then answer the questions.

An RCD adaptor is an automatic safety switch. It should be used when there is aparticular risk of electric shock. For example, it is recommended that it is usedwith an electric lawnmower.

Inside one make of RCD is an electromagnet that holds the switch closed so thatthe RCD is switched on. An electronic circuit in the RCD monitors the differencebetween the current in the live wire and the current in the neutral wire.

If something goes wrong and this difference is greater than 30 milliamps then theRCD will trip (= switch off) within 40 milliseconds.

(a) Suggest two reasons why there is a particular risk of an electric shock when using

an electric lawnmower.1. ................................................................................................................................

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

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(b) Why will there be a difference between the current in the live wire and the current in the neutralwire if something goes wrong?

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(c) (i) Use the words charge, current and time to write an equation which shows therelationship between them.

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(ii) Calculate how much charge flows when a current of 30 milliamps passesfor 40 milliseconds.

Clearly show how you get to your answer and give the unit.

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Charge = .....................................................(3)

(Total 7 marks)


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Q5. The diagram shows a type of electric immersion heater in a hot water tank. These hotwater tanks are normally found in airing cupboards.

Information on the immersion heater states:

230 V10 A

a) (i) What is the equation which shows the relationship betweenpower, current and voltage?

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(1)

(ii) Calculate the power of the heater. Show clearly how you get to your answerand give the units.

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Power = ......................................................(2)

(b) (i) What rating of fuse should be in the immersion heater circuit?

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(ii) There are three wires in the cable to the immersion heater. Two of the wiresare connected to the immersion heater. The third wire is connected to thecopper tank.

Explain the function of this third wire and the fuse in the circuit.

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(c) (i) What is the equation which shows the relationship between resistance, current

and voltage?

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(ii) Calculate the resistance of the heater. Show clearly how you get to youranswer and give the units.

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Resistance = ....................................................(2)

(Total 10 marks)


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Q6. The diagram shows two oscilloscope traces, A and B.

Trace A shows how the potential difference between the live and neutral terminals of anelectricity supply changes with time.

(a) Describe how the potential of the live terminal varies with respect to the neutralterminal of the electricity supply.

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(b) What does trace B show?

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(c) Each horizontal division on the oscilloscope represents 0.005 s.

(i) What is the period of this electricity supply?

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Period = .................................. seconds(1)

(ii) Calculate the frequency of the supply.

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Frequency = .................................. hertz(1)

(Total 5 marks)


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Q7. (a) The diagram shows an electric fence, designed to keep horses in a field.

When a horse touches the wire the horse receives a mild electric shock. Explainhow.

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(b) The diagram shows how a person could receive an electric shock from a faultyelectrical appliance. Using a residual circuit breaker (RCB) can help to protect theperson against receiving a serious shock.

Currentto earth

(i) Compare the action of an RCB to that of a fuse.

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(ii) The graph illustrates how the severity of an electric shock depends upon boththe size of the current and the time for which the current flows through thebody.

Within how long must the RCB cut off the current if the person using thelawnmower is to be in no danger of serious injury?

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Time = ................................... milliseconds(2)

(Total 6 marks)


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Q8. The drawing shows an experiment using a low voltage supply, a joulemeter, a smallimmersion heater and a container filled with water.

The voltage was set at 6 V d.c. The reading on the joulemeter at the start of theexperiment was 78 882 and 5 minutes later it was 80 142.

(i) Use the equation:

voltage =

to work out the total charge which flowed through the immersion heater in fiveminutes. Clearly show how you get to your answer and give the unit.

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Charge = ............................................(3)

(ii) Calculate the current through the immersion heater during the 5 minutes. Write theequation you are going to use, show clearly how you get to your answer and givethe unit.

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Current = .........................................(3)

(Total 6 marks)


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M1. (a) d.c. flows in (only) one direction1

a.c. changes direction (twice every cycle)accept a.c. constantly changing direction

ignore references to frequencyaccept answers presented as a clear diagrame.g.

ac:

1

(b) (i) 10 Aallow 1 mark for correct transformation and substitution i.e.

an answer 0.01 gains 1 mark A only – do NOT allow a

3

(ii) 13 Ae.c.f.accept the fuse size that is the next listed value greater thananswer (b)(i)

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M2. (a) (i) 50 000allow 1 mark for correct substitution, ie6 = 0.00012 × Ro r 6 = 0.12 × Ro r answers of 25 000 or 50 gain 1 marko r allow 1 mark for an incorrect answer caused by one erroronly ie using 3V or an incorrect conversion of current

2

ohm / Ω an answer 50kΩ gains 3 marks

1

(ii) (body) resistance changes

or

body fat/resistance affected by (many) factorsaccept named factor, eg age, gender, height, fitness, bonestructure, muscle, drinking water related to body fat /resistance

1

(iii) gives misleading / wrong/inaccurate valuedo not credit if specifically linked to a change in mass /weight

1

(because) high water content changes body resistanceaccept a specific change to resistance

water changes body mass is insufficient1

(b) (i) RCCB – detects difference between current in live and neutral (wires)accept RCCB can be reset

1

fuse – (overheats and) meltsaccept blows for melts

1


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(ii) switches the circuit / hedge trimmers off within 60 millisecondsallow for 1 mark the RCCB / it is (very) fast.do not accept the bigger the current the faster the RCCBswitches off

2

[10]

M3. (a) 50 hertz1

(b) (i) a flow of charge / electrons1

(ii) a.c. is constantly changing direction1

whilst d.c. always flows in the same direction1

(c) (i) 46.9accept 47.0allow 1 mark for correct transformation and substitution

ie2

(ii) current (46.9 A) exceeds maximum safe current for2.5 mm 2 cable

accept cable needs to be 16.0 mm 21

therefore if a 2.5 mm 2 cable were used it would overheat / meltcable needs to be 10.0 mm 2 limits maximum credit to 1 mark

1


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Madeley High School

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(iii) can be reset1

disconnects circuit faster (than a fuse)1

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M4. (a) any two from

(risk of) cutting (through the) cable

accept cutting the wire

grass may be weto r it may rain

wires may be loose (because cable experiences a lot of movement)accept cable may be loose

(risk of) touching exposed part(s)2

(b) some current will go through (the rest of) the lawnmower / the user / to earthdo not credit any reference to the electromagnet

1

(c) (i) charge = current × timeo r any transposed versionaccept Q = I × to r any transposed versionaccept C = A × so r coulombs = amperes × secondso r any transposed version

o r

but only if subsequently used correctly1


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(ii) EITHER1200 microcoulombs / μC

o r 1.2 millicoulombs / mCo r 0.0012 coulombs /C

3

ORcorrect arithmetic

eitherconverting milliamps to ampsand milliseconds to secondsor correct multiplication

unit given as coulombs /Cor millicoulombs / mCor microcoulombs / μC

example : charge = 30 × 40 = 1200 millicoulombs should becredited with 2 marks

1[7]

M5. (a) (i) P = V × 1

or equivalentcredit a triangle if part (ii) correctly uses the relationshipcredit power = volts × amps o r watts V × Ado not accept C for current

1

(ii) (P = 230 × 10 =) 2300credit 2.3

1

W or J/skW

1

(b) (i) 15 Acredit 13 A o r amps

1

(ii) any three from

earth

any short (to the metal tank) causes fuse to blow

fuse is in the live wire

to prevent damage to the heater credit to stop the current3


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(c) (i) V = I × R

or equivalentcredit a triangle if part (ii) correctly uses the relationship

1

(ii) (230 = 10 × R =) 23

ohms or Ω 2

[10]

M6. (a) alternatesaccept switchesaccept (constantly) changesaccept goes up and down

1

between positive and negative1

(b) potential difference between the neutral and earth (terminal)accept voltage for p.d

or potential of the neutral terminal with respect to earth1

(c) (i) 0.025 (s)1

(ii) 40 (Hz)accept 1 ÷ their (a)(i)

1

[5]

M7. (a) horse completes circuit between wire and earth

or horse earths the wire

1

charge or electrons or current or electricity flows through the horse1

(b) two from:

• RCB breaks circuit when it detects a difference betweencurrents in live and neutral wires

• fuse breaks circuit only when fuse rating exceeded or when it melts

• RCB is resettable


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(ii) 500 (ms)leakage current = 0.02A 1 mark only

1

[6]

M8. (i) either210 C3

or1260 (joules of energy transferred)

unit not required1

2101

coulomb(s) Callow J/V

1

(ii) either0.7 A

3

orcharge = current × time

o r 210* = current × 3001

or Q = Ito r * same as candidate’s answer to

part (a)(I) provided correct unit given in part (a)(i)o r * as follows from above

1

current = 0.7*amp(s) or A

1

[6]

60 mins Mains electricity exam Qs B+ with answers

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