Practise Resistors. 19 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy6 marks
A student investigates how the current in a filament lamp depends on the potential difference across it, using a lamp, a variable resistor, a battery, an ammeter and a voltmeter.
(a) Write a hypothesis for this investigation.[1]
(b) Identify the independent variable and the dependent variable, and state one variable that must be controlled.[3]
(c) Give one hazard in this investigation and a suitable control measure.[2]
Show the answer and mark scheme
(a)Answer: As the potential difference across the lamp increases, the current increases, but not in direct proportion, because the resistance of the filament increases as it gets hotter.
as potential difference increases, current increases, but resistance also increases (so current is not directly proportional to potential difference), because the filament gets hotter
(b)Answer: Independent: the potential difference across the lamp (set using the variable resistor). Dependent: the current in the lamp. Control: the same lamp must be used throughout.
independent variable: the potential difference across the lamp (set by the variable resistor)
dependent variable: the current in the lamp
a control variable, e.g. using the same lamp throughout, or allowing the lamp to return to the same starting temperature before each reading
(c)Answer: A high current can make the lamp and connecting wires hot enough to burn skin; switch off between readings and avoid touching the lamp while it is lit or straight after.
a hazard, e.g. the lamp or wires become hot and could burn skin, or a short circuit could damage the power supply
a control measure that matches the hazard, e.g. switch off between readings, avoid touching the lamp while lit, or check connections before switching on
Question 2Medium4 marks
A student investigates the current–potential difference characteristic of a diode, using a variable power supply, an ammeter, a voltmeter and a protective resistor connected in series with the diode.
(a) Explain why a protective resistor is included in series with the diode.[2]
(b) Describe how the student obtains readings for negative values of potential difference across the diode.[2]
Show the answer and mark scheme
(a)Answer: Once a diode conducts, its resistance is very low, so without a protective resistor a small increase in potential difference could cause a very large, damaging current; the protective resistor limits the current to a safe value.
once the diode conducts (in its forward direction) its resistance is very low (and changes rapidly), so a small increase in potential difference could cause a very large current
the protective resistor limits the current to a safe value, so the diode is not damaged
(b)Answer: Reverse the connections to the power supply (swap the two connecting leads), so the potential difference across the diode is in the opposite direction, and take current and potential difference readings again.
reverse the connections to the power supply / swap the two connecting leads to the diode
take current and potential difference readings again, with the potential difference now in the opposite (reverse) direction
Question 3Hard6 marks
A student wants to plot a current–potential difference graph for a filament lamp, obtaining data for both positive and negative values of potential difference.
Describe a method the student could use, including how both positive and negative values of potential difference and current are obtained.[6]
Show the answer and mark scheme
Answer: Connect the lamp in series with a variable resistor (or variable power supply), an ammeter, with a voltmeter across the lamp; take readings over a range of positive potential differences; reverse the connections to obtain negative values; repeat and plot a graph.
connect the lamp in series with a variable resistor (or use a variable power supply) and an ammeter, with a voltmeter connected in parallel across the lamp
adjust the variable resistor (or power supply) to obtain a range of potential difference values, e.g. 0 V up to the maximum safe value, recording the current at each
repeat each reading and calculate a mean, to check the results are repeatable
reverse the connections to the power supply (swap the two leads) so that current now flows through the lamp in the opposite direction
take current and potential difference readings again with the connections reversed, to obtain negative values
take enough readings to plot a smooth curve, including some closely spaced near 0 V
plot a graph of current (y-axis) against potential difference (x-axis), with axes through the origin, and draw a smooth curve through the points
Marked with levels of response: the full level descriptors are in the app.