2.5 Required practical: I-V characteristics
9 exam questions worth 20 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Give two reasons why a variable resistor is included in the circuit.
Mark scheme
to change the potential difference across the component / to change the current (1); to obtain a range of readings / several sets of data (1)
Model answer
It changes the potential difference across the component and the current through it, so that a range of readings can be taken.
The student reverses the connections to the power supply and takes more readings. Explain why.
Mark scheme
to get negative values of potential difference and current (1); to see how the component behaves when the current flows the other way (e.g. a diode does not conduct) (1)
Model answer
Reversing the supply gives negative values, so the student can see how the component behaves in both directions. For example, the diode lets almost no current through in the reverse direction.
A protective resistor is placed in series with the diode. Explain why.
Mark scheme
a diode has a very low resistance in the forward direction (1); so the current could be large enough to damage it, and the resistor limits the current (1)
Model answer
The diode has a very low resistance in the forward direction, so a large current could damage it. The protective resistor limits the current.
A student leaves the circuit switched on for several minutes while investigating a fixed resistor. Explain how this could affect the results.
Mark scheme
the resistor heats up / its temperature rises (1); so its resistance changes and it is no longer a fair test / the results are not valid (1)
Model answer
The resistor heats up. Its resistance changes with temperature, so the results are no longer valid.
Describe a method to find the I-V characteristic of a filament lamp. You may draw a circuit diagram.
Mark scheme
lamp connected in series with an ammeter, variable resistor and power supply (1); voltmeter in parallel across the lamp only (1); change the variable resistor to change the potential difference, record the potential difference and current for at least six settings (1); reverse the supply to get negative readings, then plot current against potential difference (1)
Model answer
Connect the lamp in series with a power supply, an ammeter and a variable resistor. Connect a voltmeter across the lamp only. Adjust the variable resistor to change the potential difference, and record the potential difference and current for at least six settings. Reverse the connections to the power supply to get negative values, then plot a graph of current against potential difference.
In this experiment, which is the independent variable?
Choose one answer.
Correct Not quite: the answer is The potential difference across the component
Mark scheme
B: the potential difference across the component
Model answer
The potential difference across the component (B), because it is the quantity the student changes.
In an experiment with a filament lamp, a student measures a p.d. of 4.0 V and a current of 1.25 A. Later, she measures 2.0 V and 0.85 A. Calculate the resistance each time and describe the change.
Mark scheme
R = 4.0 ÷ 1.25 = 3.2 Ω (1); R = 2.0 ÷ 0.85 = 2.4 Ω (1); the resistance increases as the p.d. increases (as the filament gets hotter) (1)
Model answer
At 4.0 V: R = 3.2 Ω. At 2.0 V: R = 2.4 Ω. The resistance is greater at the higher p.d., because the filament is hotter.
When testing a diode, a student takes more readings between 0.5 V and 0.8 V than between 0.0 V and 0.4 V. Explain why.
Mark scheme
the current changes rapidly in this range (1); so more readings are needed to show the shape of the curve (the current is almost zero below 0.4 V) (1)
Model answer
The current changes very rapidly between 0.5 V and 0.8 V, so more readings are needed to show the shape of the curve. It is almost zero at lower values.
A student notices that one point on her graph is far from the others. Describe what she should do.
Mark scheme
check the readings and repeat the measurement (1); if it was a mistake, ignore the result (but if it is real, keep it) (1)
Model answer
She should repeat the measurement. If the first result was a mistake, she can ignore it, but if it is the same, it must be kept.
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