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Madeley High School B+ Exam Qs Mains electricity Q1. Name:_______________________________________ (a) Describe the difference between an alternating current (a.c.) and a direct current (d.c.). ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... (2) (b) The diagram shows the information plate on the bottom of an electric wallpaper steamer. (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 ........................................................................................................................... ........................................................................................................................... Current .............................. (3) (ii) Which one of the following fuses should be used inside the plug of the steamer? Draw a ring around your answer. 1A 3A 5A 10 A 13 A (1) (Total 6 marks) Page 1 Madeley High School Q2. (a) The picture shows a person using a set of electronic ‘Body Fat Scales’. When the person 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, a current 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. ............................................................................................................... ............................................................................................................... ............................................................................................................... Resistance = ............................................................. (3) (ii) The scales can only produce a prediction of body fat content and not an accurate measurement. Suggest why. ............................................................................................................... ............................................................................................................... ............................................................................................................... (1) Page 2 Madeley High School (iii) It is recommended that the scales are not used immediately after a person has drunk a large amount of water. Suggest why. ............................................................................................................... ............................................................................................................... ............................................................................................................... ............................................................................................................... ............................................................................................................... (2) (b) The diagram shows how someone could get an electric shock from accidentally cutting 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. ............................................................................................................... ............................................................................................................... ............................................................................................................... ............................................................................................................... ............................................................................................................... (2) Page 3 Madeley High School (ii) The graph shows how the severity of an electric shock depends on the size of the current and the time that the current flows through the body. Using the RCCB helps prevent an electric shock seriously injuring the person using the hedge trimmers. Using information from both the diagram and the graph explain how. ............................................................................................................... ............................................................................................................... ............................................................................................................... ............................................................................................................... ............................................................................................................... (2) (Total 10 marks) Page 4 Madeley High School 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? ........................................................................................................................ (1) (b) (i) What is an electric current? ............................................................................................................... (1) (ii) Explain the difference between an a.c. (alternating current) electricity supply and a d.c. (direct current) electricity supply. ............................................................................................................... ............................................................................................................... ............................................................................................................... ............................................................................................................... (2) (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 out your answer. ............................................................................................................... ............................................................................................................... ............................................................................................................... ............................................................................................................... Current = .................................... A (2) Page 5 Madeley High School (ii) The table gives the maximum current that can safely pass through electric cables of different cross-sectional area. Cross-sectional area in mm 2 Maximum safe current in 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 electricity supply using 2.5 mm2 cable. Use the data in the table to explain why the shower must not be connected to the mains electricity supply using 2.5 mm2 cable. ............................................................................................................... ............................................................................................................... ............................................................................................................... ............................................................................................................... (2) (iii) The circuit connecting the shower to the mains electricity supply must include a residual current circuit breaker (RCCB) and not a fuse. Give two advantages of using a RCCB to protect a circuit rather than a fuse. 1 ............................................................................................................ ............................................................................................................... 2 ............................................................................................................ ............................................................................................................... (2) (Total 10 marks) Page 6 Madeley High School 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 a particular risk of electric shock. For example, it is recommended that it is used with an electric lawnmower. Inside one make of RCD is an electromagnet that holds the switch closed so that the RCD is switched on. An electronic circuit in the RCD monitors the difference between 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 the RCD 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. ................................................................................................................................ ..................................................................................................................................... 2. ................................................................................................................................. ..................................................................................................................................... (2) (b) Why will there be a difference between the current in the live wire and the current in the neutral wire if something goes wrong? ..................................................................................................................................... ..................................................................................................................................... (1) (c) (i) Use the words charge, current and time to write an equation which shows the relationship between them. .......................................................................................................................... (1) (ii) Calculate how much charge flows when a current of 30 milliamps passes for 40 milliseconds. Clearly show how you get to your answer and give the unit. .......................................................................................................................... .......................................................................................................................... Charge = ..................................................... (3) (Total 7 marks) Page 7 Madeley High School Q5. The diagram shows a type of electric immersion heater in a hot water tank. These hot water tanks are normally found in airing cupboards. Information on the immersion heater states: 230 V 10 A a) (i) What is the equation which shows the relationship between power, current and voltage? .......................................................................................................................... (1) (ii) Calculate the power of the heater. Show clearly how you get to your answer and give the units. .......................................................................................................................... Power = ...................................................... (2) (b) (i) What rating of fuse should be in the immersion heater circuit? .......................................................................................................................... (1) Page 8 Madeley High School (ii) There are three wires in the cable to the immersion heater. Two of the wires are connected to the immersion heater. The third wire is connected to the copper tank. Explain the function of this third wire and the fuse in the circuit. .......................................................................................................................... .......................................................................................................................... .......................................................................................................................... .......................................................................................................................... (3) (c) (i) What is the equation which shows the relationship between resistance, current and voltage? .......................................................................................................................... (1) (ii) Calculate the resistance of the heater. Show clearly how you get to your answer and give the units. .......................................................................................................................... Resistance = .................................................... (2) (Total 10 marks) Page 9 Madeley High School Q6. The diagram shows two oscilloscope traces, A and B. Trace A shows how the potential difference between the live and neutral terminals of an electricity supply changes with time. (a) Describe how the potential of the live terminal varies with respect to the neutral terminal of the electricity supply. .................................................................................................................................... .................................................................................................................................... (2) (b) What does trace B show? .................................................................................................................................... .................................................................................................................................... (1) (c) Each horizontal division on the oscilloscope represents 0.005 s. (i) What is the period of this electricity supply? .......................................................................................................................... Period = .................................. seconds (1) (ii) Calculate the frequency of the supply. .......................................................................................................................... Frequency = .................................. hertz (1) (Total 5 marks) Page 10 Madeley High School 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. Explain how. .................................................................................................................................... .................................................................................................................................... .................................................................................................................................... (2) (b) The diagram shows how a person could receive an electric shock from a faulty electrical appliance. Using a residual circuit breaker (RCB) can help to protect the person against receiving a serious shock. Current to earth (i) Compare the action of an RCB to that of a fuse. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... (2) Page 11 Madeley High School (ii) The graph illustrates how the severity of an electric shock depends upon both the size of the current and the time for which the current flows through the body. Within how long must the RCB cut off the current if the person using the lawnmower is to be in no danger of serious injury? ........................................................................................................................... ........................................................................................................................... Time = ................................... milliseconds (2) (Total 6 marks) Page 12 Madeley High School Q8. The drawing shows an experiment using a low voltage supply, a joulemeter, a small immersion 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 the experiment 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 five minutes. Clearly show how you get to your answer and give the unit. ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... Charge = ............................................ (3) (ii) Calculate the current through the immersion heater during the 5 minutes. Write the equation you are going to use, show clearly how you get to your answer and give the unit. ..................................................................................................................................... ..................................................................................................................................... Current = ......................................... (3) (Total 6 marks) Page 13 Madeley High School M1. (a) d.c. flows in (only) one direction 1 a.c. changes direction (twice every cycle) accept a.c. constantly changing direction ignore references to frequency accept answers presented as a clear diagram e.g. ac: 1 (b) (i) 10 A allow 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 A e.c.f. accept the fuse size that is the next listed value greater than answer (b)(i) 1 [5] Page 14 Madeley High School M2. (a) (i) 50 000 allow 1 mark for correct substitution, ie 6 = 0.00012 × R or 6 = 0.12 × R or answers of 25 000 or 50 gain 1 mark or allow 1 mark for an incorrect answer caused by one error only 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) factors accept named factor, eg age, gender, height, fitness, bone structure, muscle, drinking water related to body fat / resistance 1 (iii) gives misleading / wrong/inaccurate value do not credit if specifically linked to a change in mass / weight 1 (because) high water content changes body resistance accept a specific change to resistance water changes body mass is insufficient 1 (b) (i) RCCB – detects difference between current in live and neutral (wires) accept RCCB can be reset 1 fuse – (overheats and) melts accept blows for melts 1 Page 15 Madeley High School (ii) switches the circuit / hedge trimmers off within 60 milliseconds allow for 1 mark the RCCB / it is (very) fast. do not accept the bigger the current the faster the RCCB switches off 2 [10] M3. (a) 50 hertz 1 (b) (i) a flow of charge / electrons 1 (ii) a.c. is constantly changing direction 1 whilst d.c. always flows in the same direction 1 (c) (i) 46.9 accept 47.0 allow 1 mark for correct transformation and substitution ie 2 (ii) current (46.9 A) exceeds maximum safe current for 2.5 mm2 cable accept cable needs to be 16.0 mm2 1 therefore if a 2.5 mm2 cable were used it would overheat / melt cable needs to be 10.0 mm2 limits maximum credit to 1 mark 1 Page 16 Madeley High School (iii) can be reset 1 disconnects circuit faster (than a fuse) 1 [10] M4. (a) any two from (risk of) cutting (through the) cable accept cutting the wire grass may be wet or 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 earth do not credit any reference to the electromagnet 1 (c) (i) charge = current × time or any transposed version accept Q = I × t or any transposed version accept C = A × s or coulombs = amperes × seconds or any transposed version or but only if subsequently used correctly 1 Page 17 Madeley High School (ii) EITHER 1200 microcoulombs / μC or 1.2 millicoulombs / mC or 0.0012 coulombs /C 3 OR correct arithmetic either converting milliamps to amps and milliseconds to seconds or correct multiplication unit given as coulombs /C or millicoulombs / mC or microcoulombs / μC example : charge = 30 × 40 = 1200 millicoulombs should be credited with 2 marks 1 [7] M5. (a) (i) P=V×1 or equivalent credit a triangle if part (ii) correctly uses the relationship credit power = volts × amps or watts V × A do not accept C for current 1 (ii) (P = 230 × 10 =) 2300 credit 2.3 1 W or J/s kW 1 (b) (i) 15 A credit 13 A or 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 current 3 Page 18 Madeley High School (c) (i) V=I×R or equivalent credit a triangle if part (ii) correctly uses the relationship 1 (ii) (230 = 10 × R =) 23 ohms or Ω 2 [10] M6. (a) alternates accept switches accept (constantly) changes accept goes up and down 1 between positive and negative 1 (b) potential difference between the neutral and earth (terminal) accept voltage for p.d or potential of the neutral terminal with respect to earth 1 (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 horse 1 (b) two from: • RCB breaks circuit when it detects a difference between currents in live and neutral wires • fuse breaks circuit only when fuse rating exceeded or when it melts • RCB is resettable Page 19 Madeley High School 2 (ii) 500 (ms) leakage current = 0.02A 1 mark only 1 [6] M8. (i) either 210 C 3 or 1260 (joules of energy transferred) unit not required 1 210 1 coulomb(s) C allow J/V 1 (ii) either 0.7 A 3 or charge = current × time or 210* = current × 300 1 or Q = It or* same as candidate’s answer to part (a)(I) provided correct unit given in part (a)(i) or* as follows from above 1 current = 0.7* amp(s) or A 1 [6] Page 20