Currents in electrical circuits

Transcription

1 Currents in electrical circuits P2 73 minutes 73 marks Page of 3

2 Q. (a) The diagram shows the inside of a three-pin plug. (i) What name is given to the wire labelled S? Draw a ring around the correct answer. earth live neutral () What is the colour of the insulation around the wire labelled T? Draw a ring around the correct answer. blue brown green and yellow () (b) The plug contains a 3 amp fuse. (i) Which one of the following is the correct circuit symbol for a fuse? Draw a ring around the correct answer. () The diagram shows the parts of the fuse. What would happen if a current of 20 amps passed through the 3 amp fuse? () Page 2 of 3

3 (c) Not all electrical appliances are earthed. (i) Which one of the following appliances must be earthed? Tick ( ) one box. Table lamp Toaster Hair straighteners Give a reason for your answer. (2) Earthing an appliance helps to protect a person against a possible risk. What is the risk? () (d) The diagrams show how two lengths of mains electrical cable were joined. The individual wires have been twisted together and covered with insulating tape. This is not a safe way to join the cables. What is the possible risk from joining the two lengths of mains electrical cable in this way? () Page 3 of 3

4 (e) The diagram below shows a connecting box being used to join two lengths of electrical cable. This is a safe way to join the cables. The cable grips are important parts of the connecting box. Explain why (2) (Total 0 marks) Q2. (a) Electrical circuits often contain resistors. The diagram shows two resistors joined in series. Calculate the total resistance of the two resistors.... Total resistance =... Ω () Page 4 of 3

5 (b) A circuit was set up as shown in the diagram. The three resistors are identical. (i) Calculate the reading on the voltmeter. Reading on voltmeter =... V (2) The same circuit has now been set up with two ammeters. Draw a ring around the correct answer in the box to complete the sentence. smaller than The reading on ammeter A 2 will be equal to the reading on ammeter A. greater than () (Total 4 marks) Page 5 of 3

6 Q3. (a) The diagram shows a negatively charged plastic rod held near to a thin stream of water. The water is attracted towards the rod. Which one of the following statements explains what is happening to the charge in the water? Tick ( ) one box. The positive and the negative charges in the water are attracted to the rod. The positive and the negative charges in the water are repelled by the rod. The negative charge in the water is repelled by the rod and the positive charge is attracted to the rod. The negative charge in the water is attracted to the rod and the positive charge is repelled by the rod. () Page 6 of 3

7 (b) A company that produces bottles of mouthwash found a problem with the automatic filling system. As the bottles go towards the filler, the bottles move around on the conveyor belt and become electrostatically charged. This causes the stream of mouthwash to move sideways, missing the open top of the bottle. The company came up with an answer to the problem. Before the bottles reach the dfiller, the bottles pass through a stream of ionised air. The ions in the air neutralise the charge on the bottles. (i) Explain why the plastic bottles became charged. (2) What happens to the structure of an atom to change the atom into an ion? () (iii) Earthing the conveyor belt with a conducting wire would not have solved this problem. Give a reason why. () (Total 5 marks) Page 7 of 3

8 Q4. (a) The diagram shows the circuit used to investigate the resistance of a sample of a material. The diagram is not complete; the ammeter and voltmeter are missing. (i) Draw the symbols for the ammeter and voltmeter on the diagram in the correct places. How can the current through the material be changed? (2) () Page 8 of 3

9 (b) The material, called conducting putty, is rolled into cylinders of different lengths but with equal thickness. Graph shows how the resistance changes with length. Length in centimetres (i) The current through a 25 cm length of conducting putty was 0.5 A. Use Graph to find the resistance of a 25 cm length of conducting putty. Resistance =... ohms () Use your answer to (b) (i) and the equation in the box to calculate the potential difference across a 25 cm length of conducting putty. potential difference = current x resistance Show clearly how you work out your answer. Potential difference =... volts (2) Page 9 of 3

10 (c) A second set of data was obtained using thicker pieces of conducting putty. Both sets of results are shown in Graph 2. Length in centimetres (i) What is the relationship between the resistance and the thickness of the conducting putty? () Name one error that may have reduced the accuracy of the results. () (iii) How could the reliability of the data have been improved? () (Total 9 marks) Page 0 of 3

11 Q5. The diagram shows the structure of a cable. The cable is part of an undersoil heating circuit inside a large greenhouse. (a) The cable is connected to the mains electricity supply through a residual current circuit breaker (RCCB). If the cable is accidentally cut the RCCB automatically switches the circuit off. (i) What is the frequency of the mains electricity supply in the UK? () What happens, as the cable is cut, to cause the RCCB to switch the circuit off? (2) (iii) A circuit can also be switched off by the action of a fuse. Give one advantage of using a RCCB to switch off a circuit rather than a fuse. () (b) The 230 volt mains electricity supply causes a current of amps to flow through the cable. (i) Calculate the amount of charge that flows through the cable when the cable is switched on for 2 hours and give the unit. Use the correct equation from the Physics Equations Sheet. Charge =... (3) Page of 3

12 Calculate the energy transferred from the cable to the soil in 2 hours. Use the correct equation from the Physics Equations Sheet. Energy transferred =... J (2) (c) The heating circuit includes a thermistor. The thermistor is buried in the soil and acts as a thermostat to control the increase in the temperature of the soil. Describe how an increase in the temperature of the soil affects the thermistor (2) (Total marks) Q6. (a) The resistance of a 24 W, 2 V filament lamp depends on the current flowing through the lamp. For currents up to 0.8 A, the resistance has a constant value of 2.5 Ω. (i) Use the equation in the box to calculate the potential difference across the lamp when a current of 0.8 A flows through the lamp. potential difference = current resistance Show clearly how you work out your answer. Potential difference =... V (2) Page 2 of 3

13 When the potential difference across the lamp is 2 V, the current through the lamp is 2 A. On the axes below, draw a current potential difference graph for the filament lamp over the range of potential difference from 0 to 2 volts. (2) (iii) Why does the resistance of the lamp change when the current through the lamp exceeds 0.8 A? () Page 3 of 3

14 (b) The lamp is now included in a circuit. The circuit is switched on for 2 minutes. During this time, 72 coulombs of charge pass through the lamp. Use the equation in the box to calculate the energy transformed by the lamp while the circuit is switched on. energy transformed = potential difference charge Show clearly how you work out your answer Energy transformed =... J (2) (Total 7 marks) Q7. A student has designed the circuit shown in the diagram to measure the water level in a beaker. The student is going to use the ammeter to indicate the water level. (a) Use words from the box to label the three components, J, K and L, on the diagram. battery cell fuse lamp resistor switch (3) Page 4 of 3

15 (b) The resistance of the variable resistor changes as the cork moves up and down inside the beaker. This changes the electric current flowing in the circuit. (i) What is meant by the term electric current? () When a voltmeter was mistakenly added to the circuit in series, the reading on the ammeter was zero. Suggest why. () (c) The table gives the variable resistor value and the ammeter readings for different water levels. The table is not complete. Water level Variable resistor value in ohms Ammeter reading in amps Full Half full Empty 8 Calculate the reading on the ammeter, in amps, when the beaker is empty. Assume the ammeter, battery, fuse and wires have zero resistance Ammeter reading =... A (3) Page 5 of 3

16 (d) A second student copied the design using a 2A fuse, but he forgot to include the 6Ω resistor. What will happen to the fuse in this circuit as the water reaches the top of the beaker? Explain your answer (5) (Total 3 marks) Q8. Electrical circuits have resistance. (a) Draw a ring around the correct answer to complete the sentence. When the resistance of a circuit increases, the current in the circuit decreases. increases. stays the same. () (b) Use the correct answer from the box to complete each sentence. a filament bulb an LED an LDR An electrical component which has a resistance that increases as the temperature increases is.... An electrical component which emits light only when a current flows through it in the forward direction is.... (2) Page 6 of 3

17 (c) When some metals are heated the resistance of the metal changes. The equipment for investigating how the resistance of a metal changes when it is heated is shown in the diagram. In this question you will be assessed on using good English, organising information clearly and using specialist terms where appropriate. Describe an investigation a student could do to find how the resistance of a metal sample varies with temperature. The student uses the equipment shown. Include in your answer: how the student should use the equipment the measurements the student should make how the student should use these measurements to determine the resistance how to make sure the results are valid Page 7 of 3

18 (6) (d) The table shows some data for samples of four metals P, Q, R and S. The metal samples all had the same cross-sectional area and were the same length. Metal sample Resistance at 0 C in ohms Resistance at 00 C in ohms P Q R S A graph of the results for one of the metal samples is shown. Temperature in C Page 8 of 3

19 (i) (iii) (iv) Which metal sample, P, Q, R or S, has the data shown in the graph? One of the results is anomalous. Circle this result on the graph. Suggest a reason for the anomalous result... The same equipment used in the investigation could be used as a thermometer known as a resistance thermometer. () () () Suggest two disadvantages of using this equipment as a thermometer compared to a liquid-in-glass thermometer (2) (Total 4 marks) Page 9 of 3

20 M. (a) (i) neutral brown (b) (i) centre symbol ringed accept any correct indication fuse (wire) melts accept fuse blows accept fuse breaks / snaps accept burns out appliance doesn t work is insufficient overheat is insufficient do not accept blows up (c) (i) Toaster (only one with) a metal case / outside reason only scores if toaster chosen accept it is metal accept outside / case / metal conducts (electricity) do not accept it conducts electricity electric shock / electrocution do not accept explosion (d) overheating of cables or fire it refers to the electric cable accept short circuit / sparking overheating of tape is insufficient do not accept electrocution / shock (e) stop cable(s) being pulled loose / free accept wire(s) for cable(s) accept to hold cables in place and so breaking the circuit or and shorting (one wire to another) or and causing a fire risk / sparking accept appliance / plug would not work [0] Page 20 of 3

21 M2. (a) 25(Ω) (b) (i) 2(V) allow mark for showing a correct method, ie 6 / 3 2 equal to [4] M3. (a) 3 rd box The negative charge in the water is repelled by the rod and the positive charge is attracted to the rod. (b) (i) friction between bottles and conveyor belt / (plastic) guides accept bottles rub against conveyor belt / (plastic) guides charge transfers between bottles and conveyor belt / (plastic) guides accept specific reference eg electrons move onto / off the bottles reference to positive electrons / protons negates this mark (the atom) loses or gains one (or more) electrons (iii) charge will not (easily) flow off the conveyor belt / bottles accept the conveyor belt / bottles is an insulator / not a conductor accept conveyor belt is rubber [5] M4. (a) (i) ammeter symbol correct and drawn in series accept do not accept lower case a voltmeter symbol correct and drawn in parallel with the material do not accept Page 2 of 3

22 adjust / use the variable resistor accept change the resistance or change the number of cells accept battery for cell accept change the pd / accept change the voltage accept increase / decrease for change (b) (i) 37.5 (Ω) accept answer between 36 and 39 inclusive 5.6(25) or their (b)(i) 0.5 allow mark for correct substitution ie 37.5 or their (b)(i) 0.5 provided no subsequent step shown 2 (c) (i) the thicker the putty the lower the resistance answer must be comparative accept the converse any one from: measuring length incorrectly accept may be different length measuring current incorrectly do not accept different currents measuring voltage incorrectly do not accept different voltage ammeter / voltmeter incorrectly calibrated thickness of putty not uniform do not accept pieces of putty not the same unless qualified meter has a zero error do not accept systematic / random error accept any sensible source of error eg putty at different temperatures do not accept human error without an explanation do not accept amount of putty not same (iii) repeat readings and take a mean accept check results again accept do experiment again accept do it again and take mean(s) accept compare own results with other groups do not accept take more readings [9] Page 22 of 3

23 M5. (a) (i) 50(Hz) ignore any unit given any two from: (some) current flows to Earth accept ground for Earth current flows through copper braid accept current flows through the earth wire accept electricity for current in either the first or second marking point but not both RCCB detects difference between current in live and neutral wire 2 (iii) can be reset accept does not need replacing or faster acting accept switches circuit off faster (b) (i) allow mark for correct substitution, ie = an answer 22 gains mark coulombs / C do not accept c accept for 2 marks 8 26 kj or 8.26 MJ or (c) 230 their (b)(i) correctly calculated allow mark for correct substitution, ie 230 their (b)(i) or allow mark for power calculated as 2530(W) increases temperature of thermistor 2 changes resistance (of thermistor) do not accept increases resistance (of thermistor) an answer decreases resistance (of thermistor) gains 2 marks [] Page 23 of 3

24 M6. (a) (i) 2 allow mark for correct substitution i.e provided no further step shown straight line drawn from origin to 2, 0.8 or their (a)(i), 0.8 curve from 2, 0.8 to 2,2 or their (a)(i) 0.8 to 2,2 accept curve from 2, 0.9 to 2,2 or their (a)(i) 0.9 to 2,2 convex curve required accept a curve that flattens between 0 and 2V 2 (iii) filament / lamp gets hot accept temperature increases (b) 08 allow mark for correct substitution i.e provided no further step shown 2 [7] M7. (a) J fuse K switch L battery (b) (i) a flow of (electrical) charge because voltmeters have a very high resistance (c) total resistance = 24 Ω Page 24 of 3

25 0.5 (A) correct answer with or without working gains 3 marks an answer of 0.66 or 0.67 gains 2 marks alone gains mark (d) the fuse melts therefore breaking the circuit because without the 6 Ω resistor as the water reaches the top of the beaker the total resistance approaches 0 Ω therefore the current rises above the 2 A fuse rating and the heating effect increases [3] M8. (a) decreases (b) a filament bulb allow bulb an LED Page 25 of 3

26 (c) Marks awarded for this answer will be determined by the Quality of Communication (QoC) as well as the standard of the scientific response. 0 marks No relevant content. Level ( 2 marks) There is a basic description of the method. This is incomplete and would not lead to any useful results. Level 2 (3 4 marks) There is a description of the method which is almost complete with a few minor omissions and would lead to some results. Level 3 (5 6 marks) There is a detailed description of the method which would lead to valid results. To gain full marks an answer including graph, or another appropriate representation of results, must be given. examples of the physics points made in the response: read V and I read temperature apply heat allow hot water to cool read V and I at least one other temperature determine R from V / I range of temperatures above 50 C extra detail: use thermometer to read temperature at regular intervals of temperature remove source of heat and stir before taking readings details of attaining 0 C or 00 C last reading taken while boiling graph of R against T at least 3 different temperatures (d) (i) Q (80, 3.8) 6 (iii) any one from: measurement of V too small measurement of I too big incorrect calculation of R thermometer misread allow misread meter ignore any references to an error that is systematic Page 26 of 3

27 (iv) any two from: not portable allow requires a lot of equipment allow takes time to set up needs an electrical supply cannot be read directly accept it is more difficult to read compared to liquid-in-glass 2 [4] Page 27 of 3

28 E. (a) (i) Half of the students scored this mark. This was better known with a majority of students giving the correct answer. (b) (i) A majority of students were able to identify the circuit symbol for a fuse. A majority of students answered this question correctly, with most of the incorrect responses being in terms of the fuse blowing up or exploding, or statements regarding the shattering of the glass or metal caps. (c) (i) Although most students correctly identified the toaster as the appliance needing an earth wire, the second mark was harder to achieve as a significant number of students thought that the metal casing was present due to heat considerations. Most students answered correctly that an electric shock or electrocution would be the possible risk from the appliance, the most common incorrect response involved overheating and subsequent burns to a person. (d) (e) Few students were aware that the inappropriate connection of the wires would cause sparking and heating of the wires. There were many vague responses solely in terms of the overheating of the tapes or the possibility of electrocution. A majority of the students explained how the use of a connecting box could be a secure way to join two lengths of electrical cable but there were very few correct answers that gained both marks by continuing the answer to explain why this was important. E2. (a) A large proportion of the students provided the correct answer of 25 ohms. The most common incorrect response was 5 ohms, achieved by subtracting the numerical values of the resistors. (b) (i) Almost half of students gained both marks, with very few gaining one mark. Common incorrect answers were achieved by multiplying or dividing the answer to part (a) by 6. Perhaps surprisingly, only just over half of the students knew that the two ammeters would have the same reading. E3. (a) About half of the students gained this mark. (b) (i) About half of the students gained one mark for the idea that the bottles rubbed against the conveyor belt / guides. Few students went on to link this to the transfer of charge. (iii) About half of the students were able to state what happens to change an atom into an ion. Very few students gained this mark even though a simple answer like the conveyor belt is rubber was sufficient. Page 28 of 3

29 E4. (a) (i) This was poorly answered. Just over half of the students could put the ammeter into the circuit correctly but few could add the voltmeter correctly. Common errors included using the wrong symbol and / or placing the voltmeter in series. Just under half of the students gained this mark, usually for adding or subtracting a cell or simply stating change the volts. (b) (i) About two thirds of students were able to extrapolate the graph line to obtain the correct value. Those students scoring a mark in part (b)(i) were usually able to go on to score both marks in this part. (c) (i) About three quarters of students were able to give an acceptable version of the relationship. A significant number of students answered in terms of length rather than thickness as instructed. (iii) Many students considered the error to be one of plotting, very few gave an acceptable answer. A majority of students gained the mark for this part. E5. (a) (i) This was poorly answered with about a 50/50 split between the numbers 50 and 230. (iii) Just over three quarters of the students failed to score any marks often confusing RCCBs saying they switched off the circuit when the (live) current was too high. Others thought that they worked like fuses with something in them melting at too high a current. Of the minority who scored one mark it was usually obtained by correctly stating that the current in the live and neutral wires differed, but then went on to describe the mechanics of how the device worked rather than giving a reason for the currents being different. The very few who achieved both marks obtained these by linking the current flowing to ground through the earth wire/copper braid but omitted the way the RCCB detected the fault. The majority of students scored this mark for the idea of quicker operation or the ability to reset / reuse RCCBs. Some students gave insufficient detail with answers such as automatic operation or safer with little exemplification. (b) (i) Since the equation for this is given it was disappointing how few students managed to use the equation, transform it correctly or substitute values in correctly. Approximately half the students either did not manage to correctly change hours to seconds or did not realise this had to be done. A minority scored both marks on this and very few scored one mark if students found the correct equation they could put the values in and do the arithmetic. Unfortunately too many students chose the incorrect equation trying to use E = P t and using p.d. for power rather than E = V Q. Some students tried to use E = V I t but again incorrectly converted hours to seconds. However it was pleasing to see that most students showed the working they used towards getting their answer even though it was often wrong. (c) Most students did not appreciate that the thermistor increased in temperature but related the answer to heating the ground. The link between the thermistor and a decrease in resistance was well understood. Weaker students did not understand the way in which a thermistor works or the effect of temperature on its resistance. Page 29 of 3

30 E6. (a) (i) The majority of students, over 90%, scored 2 marks. Very few gained mark. (iii) Very few students produced a correct graph to gain 2 marks. A large percentage of students did not appreciate the importance of using the answer to part (a)(i). Common wrong answers included a straight line from the origin to (2,2), a straight line from (2,2) through (2,0.8) and continued back to the y axis, and two straight lines, one from origin to (2,0.8) and then from that point to (2,2). Otherwise, all sorts of curves, including s-shaped and convex all the way from the origin. There were only a minority of correct answers, less that 25%. Many answers referred to how the resistance changed as the current increased but not why. There were also lots of answers given in terms of the resistance changing to allow or stop the current flowing and for safety to keep the fuse from blowing. (b) The majority of students gave correct answers. The most common incorrect response was 44, by multiplying the charge by the time in minutes. An answer scoring one mark was rare and seemingly from students who did not possess a calculator. E8. (a) Nearly all students knew that when the resistance of a circuit increases the current in it decreases. (b) (c) Nearly three-quarters of the students recognised the description of a filament bulb and a LED. The Quality of Communication question was a description of an experiment where the change in resistance of metal with temperature was investigated. Many students wasted time, and used a substantial fraction of the answer lines, describing the electrical circuit provided. Just under half of the students scored four marks out of six for an adequate account that could be repeated to give sufficient data. Students who scored more than four marks often included a graph of resistance against temperature or some detail such as removing the Bunsen burner and stirring the water before taking readings. Those who scored three marks or less often did not state how resistance could be calculated from the meter readings, or did not state that the meters had to be read at all but that resistance had to be recorded at each temperature. (d) (i) Almost all students could relate a range of resistance values in a table to those represented on a graph. (iii) (iv) Almost all students were able to circle an anomalous value on the graph. Surprisingly, less than half the students were able to suggest a reason for the anomalous results such as misreading the thermometer or meters or incorrectly calculating resistance. About a third of students were able to suggest a disadvantage of a resistance thermometer compared to a liquid-in-glass thermometer. About one tenth could suggest two, including the need for an electrical supply and that temperature could not be read directly. Page 30 of 3

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