P2 Topic 3 - Using electricity Higher

Transcription

1 Name: P2 Topic 3 - Using electricity Higher Date: Time: 32 minutes Total marks available: 32 Total marks achieved:

2 Questions Q1. (a) The diagram shows an electric circuit with two resistors, R and S. (i) R has a resistance of 11 ohms. Calculate the potential difference across R. (2) (ii) Use information from the diagram to calculate the current in S. (iii) Complete the sentence by putting a cross ( ) in the box next to your answer. A student wants to measure the battery voltage with a voltmeter. The voltmeter should be placed A in series with the battery B in parallel with the battery C in parallel with the ammeter D in series with either resistor R or S (b) Explain why the temperature of a resistor increases when a current passes through it. *(c) A resistor is a circuit component. Two other circuit components are a light dependent resistor (LDR) and a thermistor. Explain how LDRs and thermistors can be used to control the current in a circuit. (2)

3 (6) (Total for Question is 12 marks) Q2. Using electric current (a) The photograph shows a device used to measure the amount of light. The circuit diagram shows how the components inside this device are connected. (i) Complete the sentence by putting a cross ( The meter used in this circuit is A a voltmeter connected in series with the LDR B a voltmeter connected in parallel with the LDR C an ammeter connected in series with the LDR ) in the box next to your answer. D an ammeter connected in parallel with the LDR (ii) The amount of light entering the light-dependent resistor (LDR) increases. Which row of the table correctly describes the change in the resistance of the LDR and the change in the current in the circuit?

4 Put a cross ( ) in the box next to your answer. %%% resistance of the LDR current in circuit A increases decreases B increases increases C decreases decreases D decreases increases (iii) The device is used as a light meter. It has a 9 V battery. Calculate the current when the resistance of the LDR is 600 Ω. (3) current =... A (iv) When the light meter has been used for a long time, the meter reading becomes incorrect. Suggest what has happened. (2) (b) A different light meter has a diode in its circuit. (i) Explain what happens when the battery is reversed. (ii) Suggest why the manufacturer of the light meter has included a diode. (2)

5 Q3. (a) A designer is going to use a thermistor in a temperature gauge. He connects the thermistor into this circuit. He heats the thermistor and measures the current at different temperatures. Here are some of the results plotted on a graph. At 47 C the current was A. (i) Plot this value on the graph. (ii) Draw the curve of best fit through the points.

6 (iii) The supply voltage is 12 V. At 20 C the current is A. Calculate the resistance of the thermistor at this temperature. (3) (iv) Use this graph of current against temperature to explain the relationship between resistance and temperature for this thermistor. (b) (i) When there is an electric current in a resistor, the resistor gets hot. Explain why the resistor gets hot. (ii) Suggest why the thermistor in a temperature gauge might indicate a temperature slightly higher than the actual temperature of its surroundings. (2) (2) (Total for Question = 10 marks) Examiner's Report Q1. (a)(i) This question involved the straightforward use of the equation V = IR for which the vast majority of candidates scored 2 marks. The most common error for those who did not was to use 0.06 A as the current in R.

7 Results Plus: Examiner Comments The correct current is used and the working is clearly shown. 2 marks were awarded. (ii) This question tested the idea that at a junction in a circuit, current is conserved and most candidates answered it correctly. (b) Responses in terms of collisions between electrons and ions in the lattice or even atoms in the metal gained full credit.

8 Results Plus: Examiner Comments In this answer 'atom' was accepted instead of 'ion'. This response scored both marks. (c) Candidates were asked to explain how LDRs and thermistors can be used to control the current in a circuit. A Level 1 response would typically say that the light falling on an LDR and the temperature of a thermistor would affect the resistance or current. A Level 2 response typically gave the correct relationship between light intensity and resistance for an LDR and temperature and resistance for a thermistor. A Level 3 response correctly linked light intensity, resistance and current for an LDR and temperature, resistance and current for a thermistor. Most candidates knew that the resistances of these components depended on light and temperature and an encouraging number knew the correct relationship. Fewer candidates went on to say how this affected the current. Errors occurred where candidates did not express their answer as a logical series of linked statements.

9 Results Plus: Examiner Comments This response is presented in a logical, well-ordered way. It deals first with an LDR and then with a thermistor. It is a Level 3 answer, which scored 6 marks. The scientific terminology is used correctly and there are only a few spelling, punctuation and grammatical errors. Results Plus: Examiner Tip In a question such as this, plan your answer so that it is in logical steps. This will also help you to exclude information that is not necessary for the answer.

10 Results Plus: Examiner Comments This candidate has the correct relationship for resistance in both cases but does not go on to say how this affects the current. This makes it a Level 2 answer, which scored 4 marks.

11 Q2. No Examiner's Report available for this question Q3. (a) (iii) This was the second of the calculations and involved the transformation of an equation of general form X = Y x Z. The equation to be used was V = I x R. The three marks were for transposition, substitution and evaluation. Results Plus: Examiner Comments The candidate here carried out the three tasks in that order and scored 3 (full) marks for that even though the number of significant figures shown was somewhat excessive. The marks for transposition and substitution can be in either order. Results Plus: Examiner Comments Here, 1 mark was scored because although the equation was transposed incorrectly, the candidate has correctly linked the numerical values to the appropriate symbol. This

12 mark was only awarded because the working was shown. Results Plus: Examiner Tip Show your working in all calculations. Some marks are then possible even if you arrive at the wrong answer. The same wrong answer without working scores zero. Results Plus: Examiner Comments The writing of the triangle is not the same as writing the equation. Clearly, this candidate does not know how to find the equation from the triangle. Although some candidates find the triangle a useful tool for transposition, the result of that use needs to be shown. Results Plus: Examiner Tip If you want to use a triangle, make sure that you write the equation down in symbols after using the triangle. (a) (i)-(ii) The ability to plot a point on a graph and draw the curve of best fit were tested here.

13 Results Plus: Examiner Comments

14 Many candidates failed to plot the point correctly, erring in use of one or both scales. This response scored 0 for both parts. Results Plus: Examiner Comments

15 Some leeway was allowed in the curve drawing. Both accuracy of point plotting and curve drawing were sufficient here. This scored both marks. Results Plus: Examiner Comments The point was plotted correctly in many cases but the curve of best fit was often poor.

16 This scored 1 for part (ai) and 0 for (aii). (a) (iv) Some students found difficulty in following through the steps in an argument. This item tested this skill. 'Explain...' items usually involve a minimum of two steps although the outcome can be stated in one. Results Plus: Examiner Comments This response omits any explanation and simply states the relationship. It scores 1 of the 2 marks available. The graph only shows a relationship between current and temperature. Results Plus: Examiner Comments The argument continues that an increased current occurs when the resistance decreases. This response scored 2 marks. Results Plus: Examiner Tip If asked to use data, then the examiner expects you to use it.

17 Results Plus: Examiner Comments To make this response, it is likely that the candidate has assumed that the graph was one of resistance against time. (b) (i) This item was based on the specification statement: 2.13 Explain the energy transfer (in 2.12 above) as the result of collisions between electrons and the ions in the lattice. Results Plus: Examiner Comments Clearly this response scores 2 marks for the mention of electrons (as charge carriers) and for collisions either with ions or with other electrons. Results Plus: Examiner Comments One mark only here for the mention of electrons but no mark for the collision process.

18 There were varied incorrect ideas for this including a brief statement about 'friction'. Results Plus: Examiner Comments The idea of a traffic jam or a black hole was insufficient to score any marks, not least because the reference was to current rather than electrons. (b) (ii) The influence of the act of measurement on the quantity being measured was tested in this item. The majority of candidates recognised a common problem of measurement. Results Plus: Examiner Comments This candidate realised that the current needed to operate the device itself heated the thermistor to a temperature above the surroundings and thus influenced the measurement. It scored 1 mark.

19 Results Plus: Examiner Comments This illustrates the most common incorrect answer - involving safety. Important though safety is, this scores 0. Mark Scheme Q1. (a)(i) (a)(ii) Answer 11x 0.4 (substitution) 4.4 (V) (A) Acceptable answers Full marks for correct answer with no calculation 0.2 or 1/5 (A) Mark (a)(iii) B (b) An explanation linking: electrons colliding with / bumping into ions in the lattice /atoms in the metal (2) colliding with other electrons If no other marks scored, allow for 1 mark for "electrical energy transferred/changed into thermal/heat energy" do not allow energy being created or produced Indicative Content Mark QWC *(c) A explanation including some of the following points Light dependent resistors (LDR) Resistance changes with light intensity Bright light, low resistance No light (dark), high resistance Low resistance gives high current.(ra) (6) (2)

20 Thermistor Resistance changes with temperature Negative temperature coefficient High temperature, low resistance Low temperature, high resistance Low resistance gives high current (RA) Level 0 No rewardable content a limited explanation linking light affecting LDR AND heat affecting thermistor OR a correct relationship for one device, e.g. thermistors change resistance when the temperature changes and LDRs change resistance when it gets dark OR the resistance decreases/ current increases of a LDR when the light gets brighter the answer communicates ideas using simple language and uses limited scientific terminology spelling, punctuation and grammar are used with limited accuracy a simple explanation correctly linking the temperature and light with resistance or current for both devices OR a correct relationship for one device with a link to the way this affects the current and resistance. e.g. the resistance of a LDR increases when the light gets dimmer and when the temperature lowers the resistance of a thermistor increases OR the resistance of a LDR decreases when the light gets brighter and this increases the current the answer communicates ideas showing some evidence of clarity and organisation and uses scientific terminology appropriately spelling, punctuation and grammar are used with some accuracy a detailed explanation including the qualitative relationships for both devices and a link to the way resistance change affects the current in BOTH of them, e.g. the resistance of a LDR is less when the light gets brighter which increases the current. When the temperature lowers the resistance

21 Q2. of a thermistor increases. This means that the current will decrease as the thermistor cools down. the answer communicates ideas clearly and coherently uses a range of scientific terminology accurately spelling, punctuation and grammar are used with few errors Total for question = 12 marks Question Answer Acceptable answers Mark Number (a)(i) C Question Answer Acceptable answers Mark Number (a)(ii) D Question Answer Acceptable answers Mark Number (a)(iii) substitution 9 = I 600 transposition I = 9/600 evaluation = (A) OR=15 ma allow substitution and transposition in either order give full marks for correct answer, no working (3) Question Answer Acceptable answers Mark Number (a)(iv) Question Number (b)(i) Question Number (b)(ii) An explanation linking the following battery voltage has decreased (so) less current (than expected) (2) Answer Acceptable answers Mark An explanation linking the following points stops working (because) diode will only conduct in one direction (2) Answer Acceptable answers Mark so that the meter connections are the correct way round / eq will prevent damage if battery is (accidentally) connected the wrong way round Q3. (a)(i) (a)(ii) Answer Correctly plotted point Smooth line through most (at least 5) Acceptable answers +/- ½ a small square Do not accept clearly dot-to-dot or Mark

22 (a) (iii) (a) (iv) (b)(i) (b) (ii) crosses / points Substitution: 12 = R Transposition: R = 12/0.047 Evaluation: R = 260 An explanation linking current increases with temperature with (so) resistance decreases or the voltage is constant with (so) resistance decreases (with temperature increase) An explanation linking electrons / negative charges collide with ions/lattice/electrons A suggestion including energy transfer in the thermistor/ any component part of the electrical circuit causes a rise in temperature of thermistor (above surroundings) excessive tramlining Ignore any part of line after 45 transposition and substitution in either order substitution mark can be scored when incorrectly transposed word/symbol equation is given 255.3, 255 give full marks for correct answer no working power of 10 errors with no working score max 1 mark (3) (for this first MP) ignore faster/slower (charge/current) ignore references to heat, current flows more can score both marks by quoting two suitable pairs of values from graph For full marks, there must be a reference to change of either I or R with temperature (2) atoms / nuclei allow for 1 mark,electrical energy transferred to thermal/heat energy if no other scored thermistor/resistor gets hot/is heated (2)