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Assignment 3
Introduction
Aims
In this assignment you will answer questions based on Section 3. You will:
?? describe practical situations and interpret measurements ?? solve problems in electricity, using the methods taught in Section 3 and the equations on the data sheet.
These are the question types that are used in the A level examination papers.
The assessment objectives for the A level are to: ?? demonstrate knowledge and understanding of scientific ideas,
processes, techniques and procedures
?? apply knowledge and understanding of scientific ideas,
processes, techniques and procedures:
?? in a theoretical context ?? in a practical context ?? when handling qualitative data ?? when handling quantitative data
?? analyse, interpret and evaluate scientific information, ideas and evidence, including in relation to issues, to: ?? make judgements and reach conclusions ?? develop and refine practical design and procedures.
Your tutor will assess the following aspects of your work: ?? your application of appropriate physical principles ?? your use of appropriate equations ?? the accuracy of your calculations ?? your use of appropriate units.
An error in calculation in one step of a question will not normally lead to a loss of marks in subsequent calculations, provided the principles are applied correctly and your answers are consistent with the figures you have used.
Are you ready to do this assignment?
Before you start this assignment, make sure that you have done the quizzes and most of the practice exam questions in each of Chapters 7 to 11 of the textbook.
This assignment contains material from two of the core practicals. If you were able to do these please use your own data. However, sample data are given for your use if you were not able to do this. You should also do the pre-assignment quiz to check your knowledge and understanding of the concepts and principles in Section 3.
In addition to the usual writing materials (or computer) you will need a sharp pencil, ruler and protractor, graph paper and a calculator.
What to do
In calculations, use g = 9.81 m s–2 for the acceleration of free fall unless told otherwise.
1 Three 2-O resistors are connected in parallel to a power supply as shown in Figure 1 and the potential difference is kept constant.
Figure 1
If one of these resistors is removed, choose the correct pattern of behaviour from the options in Table 1.
Table 1
(a) The total resistance rises The current is seen to fall
(b) The total resistance falls The current is seen to fall
(c) The total resistance rises The current is seen to rise
(d) The total resistance falls The current is seen to fall
(1 mark)
2 Two wires made from the same material but of different diameters are joined. This is shown the diagram in Figure 2.
Figure 2
When connected to a power supply, which of the following is not the same in the two wires?
(a) The number of charge carriers per unit volume
(b) The charge on the charge carriers
(c) The drift velocity of the charge carriers
(d) The current
(1 mark)
3 Which of these is equivalent to a Joule?
(a) kg m s
(b) kg m2 s
(c) kg m s–2
(d) kg m2 s–2
(1 mark)
4 Four graphs are shown in Figure 3.
Figure 3
From the graphs A to D in Figure 3, which could be a thermistor?
(1 mark)
5 From the graphs in Figure 3, which graph could be a diode?
(1 mark)
6 What is the SI unit that corresponds to a volt?
(a) kg m2 A–1 s–3
(b) kg m2 A–1 s–2
(c) kg m2 A s–3
(d) kg m2 A s–2
(1 mark)
7 Figure 4 shows three resistors, with meters in place.
Which is the statement that is correct in relation to the resistors in Figure 4?
(a) Potential difference V1 = V2 = V3
(b) The total current is I1 + I2 + I3
(c) The total resistance is R1 + R2 + R3
(d) I1 = I2 + I3
(1 mark)
8 This question concerns Core Practical 2, which measured the resistivity of a wire.
(a) Write a report of the experiment that you conducted to measure the resistivity of a wire. Ensure you mention all the items of equipment that you used, how you arranged the circuit (with a circuit diagram) and what measurements you made. Present all your raw data.
If you were unable to carry out this experiment, please submit a description of the experiment normally conducted
in schools. (4 marks)
Use the raw data that you have obtained, or the raw data in Table 2.
Table 2
Potential Current/A Wire Resistance/ difference/V length/cm O
0.28 0.61 10
0.45 0.49 20
0.55 0.42 30
0.60 0.35 40
0.75 0.35 50
(b) Complete the table of results, choosing an appropriate
number of significant figures. (2 marks)
(c) The diameter of the wire was 0.25 mm. Plot a suitable graph from these data and use it to obtain a value for the resistivity of the wire material. (4 marks)
(d) Why is it important to avoid heating the wire in this
experiment? (2 marks)
9 The circuit illustrated in Figure 5 is assembled. A is a resistor of value 10 O. The variation of current with potential difference for B is shown in the graph in Figure 6.
Figure 5
The reading on the voltmeter is 3 V.
(a) Determine the current in resistor R. (2 marks)
(b) Calculate the power dissipated in resistor R. (2 marks)
(c) B is a filament bulb. Explain what happens to the current if the potential difference is kept constant but the temperature of the filament is raised. Ensure you include a discussion of the drift velocity and lattice vibrations in your explanation.
(4 marks)
(d) If B is removed from the circuit above, what happens to the
reading on the voltmeter? (3 marks)
10 This question concerns Core Practical 3, the measurement of the internal resistance of a cell. As before, if you were unable to do this experiment, please describe the school experiment.
(a) Write a report of the experiment to measure the internal resistance of a cell. Ensure you include the diagram of the circuit, and how you will obtain several sets of data.
(4 marks)
(b) State the equation that links potential difference, current, internal resistance and terminal potential difference.
(1 mark)
Our data are shown in Table 3.
Table 3
Current/A Potential Difference/V
0.61 0.71
0.48 0.92
0.55 0.79
0.38 1.04
0.33 1.17
0.32 1.19
(c) Plot a suitable graph using either the figures in Table 3 or
(d) Use the graph to find the internal resistance of the cell and
the e.m.f. of the cell. (3 marks)
(e) Distinguish between the terms terminal potential difference
and e.m.f. for a cell. (2 marks)
11 By considering the principles that the distribution of current in a circuit is the result of charge conservation, and the distribution of potential differences in a circuit is the result of energy conservation, derive:
(a) the combined resistance of two resistors R1 and R2 connected in series (4 marks)
(b) the combined resistance of two resistors R1 and R2 connected in parallel. (4 marks)
12 The efficiency of a small electric motor is being investigated. It is set up to lift a mass while the current and potential difference are both monitored. The measurements are as follows:
Mass raised = 500 g
Height raised = 1.2 m
Time taken = 12 s
Potential difference = 3.2 V Current = 0.6 A.
(a) Calculate the work done on the mass (2 marks)
(b) Calculate the useful power output of the motor (2 marks)
(c) Calculate the electrical power supplied (2 marks)
(d) Calculate the efficiency of the motor (2 marks)
(e) What happened to the energy that was ‘lost’? (2 marks)
Total for assignment: 62 marks