P2 Topic 2 - Using Electricity Foundation

Transcript

Name: _______________________________________________ P2 Topic 2 - Using electricity foundation Date: Time: 28 minutes Total marks available: 28 Total marks achieved: ______ Questions Q1. Using electrical heating Victor rides a motorbike. He buys a kit to heat its handlebars in winter. The kit has two handle bar grips. Each grip contains an electrical heating coil. (a) Complete the sentence by putting a cross ( ) in the box next to your answer. When the coils are switched on, they get hot because (1) A current is lost in the coils B energy is transferred in the coils C resistance is transferred in the coils D atoms flow through the coils (b) The kit also has a three position switch and a 2.4 ohm resistor. The diagram shows how Victor connects the parts. Victor wants to check the voltage of the battery. Add a meter to the diagram to show how to measure the battery voltage. (2) (c) Victor moves the switch to the ‘high heat’ position. The 12 V battery produces a current of 2.5 A in the circuit. (i) Calculate the electrical power produced. (2) electrical power = ...................................... W (ii) R = V / I Calculate the total resistance of the circuit. (2) total resistance = ...................................... Ω (d) Victor moves the switch to the ‘low heat’ position. The grips do not get as hot. Explain why the grips are not as hot. (2) .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. (e) The 2.4 ohm resistor must be fixed somewhere on the motorbike frame where it cannot be touched by the rider. Suggest a reason why. (1) .............................................................................................................................................. .............................................................................................................................................. Q2. A student sets up an experiment to measure the potential difference (voltage) across a filament lamp. She changes the current through the lamp. The diagram shows the circuit she used. (c) Complete the sentence by putting a cross ( (i) The component X in the circuit diagram is a ) in the box next to your answer. (1) A diode B fixed resistor C thermistor D variable resistor (ii) The meter that measures potential difference is (1) A in parallel with the power supply B in parallel with the lamp C in series with the lamp D in series with the component X (iii) Describe how the student should increase the current in the lamp. (2) .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. (b) The student recorded these readings. current / A 0.00 0.20 0.31 0.37 0.42 0.44 Four points are plotted on the graph. potential difference / V 0.0 2.0 4.0 6.0 8.0 10.0 (i) Plot the points for 2.0 V and 8.0 V. (1) (ii) Draw the line of best fit. (1) (c) Calculate the resistance of the lamp when the current is 0.44 A and the potential difference is 10.0 V. (2) resistance = . . . . . . . . . . . . . . . . . . . . . . Ω (d) Describe the energy transfer that takes place in the lamp. (2) .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. (Total for Question is 10 marks) Q3. The students want to measure both the current in the lamp and the potential difference (voltage) across the lamp. (i) Complete the circuit below with an ammeter and a voltmeter correctly connected. The power supply, variable resistor and lamp symbols are already drawn for you. (3) (ii) The current in a lamp is 0.5 A. Its resistance is 8 Ω. Calculate the potential difference (voltage) across the lamp. (2) .............................................................................................................................................. Q4. An inventor is designing a battery-powered torch. She wants the torch to have a brightness control. She builds this circuit to test the lamp in the torch. (i) Add a voltmeter to the circuit which will measure the potential difference (voltage) across the lamp. (1) She sets the control at the "bright" position. The current is 0.26 A and the potential difference (voltage) across the lamp is 6.0 V. Calculate the resistance of the lamp. (2) .............................................................................................................................................. Q1. No Examiner's Report available for this question Q2. (a)(iii) This question showed that candidates do have an understanding of how the current in a circuit can be controlled. Increasing the voltage or decreasing the resistance were both equally popular answers. Results Plus: Examiner Comments This answer was awarded 2 marks for 'higher voltage'. Either 'change the battery' or 'change the variable resistor to a fixed resistor' would qualify for 1 mark. Results Plus: Examiner Tip When stating as an answer that a quantity will change, remember to explain what the change will be, ie bigger or smaller, increase or decrease. (b) Most candidates could score at least 1 mark in their answers to this question. Marks were lost by candidates not reading the question carefully and so not realising that not only have points to be plotted but the line of best fit also has to be drawn. Results Plus: Examiner Comments The point (8.0,0.42) has been plotted incorrectly and the point at the origin has been ignored in attempting to draw the line of best fit. No marks were awarded. Results Plus: Examiner Tip Check the graph scale before plotting points. Consider all the points when drawing a line of best fit. Results Plus: Examiner Comments An incorrect point has been plotted but there is a reasonable line of best fit although it does not go through the top point. 1 mark was awarded. Results Plus: Examiner Tip Remember that the line of best fit may be a curve. Results Plus: Examiner Comments This response scored 2 marks. The points are plotted correctly. The line goes through the origin and has a reasonable distribution of points on either side. (c) This question was generally answered well but some candidates were unable to correctly insert values into an equation that they were given and others are unable to complete the division correctly. Results Plus: Examiner Comments This response includes the substitution and correctly takes the answer to two significant figures (although significant figures were ignored for this calculation). 2 marks were awarded. Results Plus: Examiner Comments This response scored 1 mark for showing the substitution, although the final answer is incorrect. Results Plus: Examiner Tip Always show the examiner what you are doing with numerical values, do not just give the answer. Results Plus: Examiner Comments This candidate could correctly substitute but either did not understand the meaning of the division sign or was using the calculator incorrectly. 1 mark was awarded. Results Plus: Examiner Tip Make sure you can divide correctly. Results Plus: Examiner Comments Candidates must know what the symbols used in the equation mean. This answer did not receive any marks. Results Plus: Examiner Tip Use the formulae sheet at the front of the examination paper. This gives the equations in both words and symbols. (d) A large number of candidates did not mention any type of energy in their answer. The simple answer of 'electrical energy is converted to light and heat' was not often given. Results Plus: Examiner Comments This answer mentions light but not as a form of energy and so was not awarded any marks. Results Plus: Examiner Comments This answer does not give the name of the energy that was lost and therefore does not score any marks. Results Plus: Examiner Tip Remember to name energy types when dealing with energy transfer. Q3. (i) Most candidates could draw the correct symbols for at least one ammeter or voltmeter and they usually put ammeter in series. Candidates were less confident in recalling that the voltmeter should be connected in parallel with the bulb. The most common error was placing the voltmeter in series with the bulb and the ammeter followed by having the voltmeter in parallel with the variable resistor, the battery or even random sections of the connecting wires. Most candidates used a pen to draw the wiring diagram and then added the meters so that they had an (incorrect) line drawn through the symbol which was not penalised. Only 15% of candidates failed to score any marks on this question and 23% scored all 3 marks. This response was the most common way that one mark was scored. Results Plus: Examiner Comments One mark for a correct meter symbol. This response scored 3 marks Results Plus: Examiner Comments The error in connecting to the variable resistor was ignored. (ii) Most candidates did the correct substitution and evaluation. As before a significant number divided instead of multiplying and too many failed to show any working. One of the many responses scoring both marks. Results Plus: Examiner Comments Correct substitution and evaluation. One of the candidates who chose to divide 8 by 0.5. Results Plus: Examiner Comments Would the same mistake have been made if the equation was copied out? Q4. i The majority of candidates knew the correct symbol for a voltmeter but most of them placed the voltmeter in series with the other components. A large number seemed to know that it had to be placed in parallel with something but then placed it across the ammeter or the battery. The voltmeter must be connected across the lamp to measure the voltage across the lamp. Results Plus: Examiner Comments This is a correct answer. Results Plus: Examiner Comments This is an incorrect answer. ii Because the formula at the front of the booklet has V as the subject of the equation, candidates were provided with a box containing the re-arranged expression for calculating current. Most candidates were able to substitute the correct values into this expression. The evaluated answer was not a whole number and better candidates were able to round this to a suitable number of significant figures to give an answer of 23. Examiners would, however, accept a value which could be rounded down to 23. Mark Scheme Q1. Question Number (a) Acceptable answers Answer Acceptable answers • voltmeter symbol (1) • connected across battery (1) (b) Question Number (c)(i) Answer substitution (1) 2.5 × 12 evaluation (1) 30 (W) Question Number (c)(ii) Answer substitution (1) 12 ÷ 2.5 evaluation (1) 4.8 (Ω) Question Number Mark (1) B Question Number (d) Answer Answer An explanation linking any two of Mark (2) Acceptable answers give full marks for correct answer, no working Mark (2) Acceptable answers give full marks for correct answer, no working Mark (2) Acceptable answers Mark (2) the following points • greater resistance in the circuit (1) • less current (in the circuit) (1) • less energy transfer in the coils (1) • less voltage across the coils (1) • less power generated in the coils (1) Question Number (e) Answer Acceptable answers Any one from the following points • heat will be generated in the resistor • it will get hot • could burn the rider Mark (1) Q2. Answer (a)(i) (a)(ii) (a)(iii) D variable resistor B in parallel with the lamp A description including  resistance changed (1)  reduced/decreased/l owered (1) OR (b)(i) (b)(ii )  voltage/p.d /EMF (of supply) changed (1)  increased /turned up/higher(1) both points correct (1) curve of best fit judged by eye (1) Acceptable answers Mark (1) (1) (2) remove (variable) resistor /component X (2) number of batteries/number of cells add another cell/battery/battery pack/power pack/power supply (2) allow + / - half square Must pass through zero and two other points. 5th point can be either (8.0,0.42) or (8.0, 0.44) straight line of best fit (1) (1) through origin tolerance between lines A and B shown on the diagram (c) (d)(i) substitution (1) 10/0.44 or 250/11 evaluation (1) 23 (ohms) an explanation linking two of the following points  electric(al )(energy) (1)  (is converted) to heat / thermal (energy) (1)  (is converted) to light (1) 5th point can be either (8.0,0.42) or (8.0, 0.44) give full marks for correct answer, no working 22.7(ohms),22.73 (ohms), 22.72(ohms) Ignore excessive decimal places. electricity (2) (2) Total marks for question =10 Q3. Answer (i) correct symbol for ammeter or voltmeter (seen anywhere) (1) one meter connected in parallel with lamp/variable resistor/supply and one meter in series with lamp(1) both meters correctly connected (ammeter in series and voltmeter in parallel with lamp) (1) Acceptable answers Ignore gaps, lines through symbols and wire connected to side of variable resistor Symbols do not have to be correct for this mark voltmeter connected across both components is same as voltmeter connected across supply Symbols do not have to be correct for this mark any shape, labelled ammeter, in series Mark (3) (ii) substitution 0.5 × 8 evaluation 4 (V) with lamp AND any shape, labelled voltmeter, in parallel with lamp gains marking points 2 and 3 bald 4.0 x 10n gains 1 mark eg bald 40 or 0.4 = 1 mark (BOD for correct substitution) give full marks for correct numerical answer, 4 (V) even if no working (1) (1) (2) Q4. Answer (i) connected in parallel with lamp (1) (ii) Substitution R = 6.0 / 0.26 (1) Evaluation = 23 ) Acceptable answers recognisable symbol such as a box with letter V inside or box with the word voltmeter inside it accept voltmeter across both lamp and ammeter Mark (1) An answer which rounds to 23 Give full marks for correct answer no working (1 (2)