A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Electrical Characteristics and Ohm's Law — mark scheme explained
The short answer
In this section, we will explore the electrical characteristics of different components: ohmic conductors, semiconductor diodes, and filament lamps. We will also delve into Ohm’s law and how it applies to these components under constant physical conditions.
The question
A resistor has a resistance of 10 Ω. If a potential difference of 5 V is applied across it, calculate the current flowing through the resistor.
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For calculation questions, show all steps clearly and use appropriate units. For explanation and description questions, provide clear and concise answers that include relevant physics concepts and principles. Ensure that your answer addresses the specific component or situation described in the question.
What the command words demand
- Calculate
- Perform a numerical calculation using given data and appropriate formulae.
- Explain
- Provide a detailed account of why something happens or how it works, including relevant concepts and principles.
- Describe
- Give a detailed account of the characteristics or behavior of a component or system.
- Determine
- Find out or establish by calculation or measurement.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate about 2-3 minutes per mark for this topic to ensure you have enough time to show all working and provide detailed explanations.
- Identify the given values: R = 10 Ω, V = 5 V.0 marks
- Use Ohm's law to find the current: I = V / R = 5 V / 10 Ω = 0.5 A.3 marks
Final answer: 0.5 A
Work through every step correctly and you earn all 3 marks.
Another worked example
A filament lamp is connected to a variable power supply. The following data was recorded: Voltage (V) = 2 V, Current (I) = 0.1 A; Voltage (V) = 4 V, Current (I) = 0.15 A. Calculate the resistance of the filament lamp at each voltage and explain why the resistance changes.
- Calculate the resistance at 2 V using R = V / I = 2 V / 0.1 A = 20 Ω.2 marks
- Calculate the resistance at 4 V using R = V / I = 4 V / 0.15 A ≈ 26.7 Ω.1 mark
- Explain that the resistance changes because the temperature of the filament increases with voltage, leading to a higher resistance.2 marks
Final answer: 20 Ω at 2 V and 26.7 Ω at 4 V; the resistance increases due to the rise in temperature.
Work through every step correctly and you earn all 5 marks.
Common mistakes
Assuming all components follow Ohm's law
Why it happens: Students often assume that all electrical components, including diodes and filament lamps, follow Ohm's law. However, only ohmic conductors have a linear I-V relationship.
Fix: Understand the specific characteristics of each component: ohmic conductors have a linear I-V relationship, while diodes and filament lamps do not.
Confusing the roles of ammeters and voltmeters
Why it happens: Students sometimes confuse the ideal characteristics of ammeters (zero resistance) and voltmeters (infinite resistance), leading to incorrect circuit analysis.
Fix: Remember that an ideal ammeter has zero resistance and is connected in series, while an ideal voltmeter has infinite resistance and is connected in parallel.
Forgetting the effect of temperature on filament lamps
Why it happens: Students often overlook the fact that the resistance of a filament lamp increases with temperature, leading to a nonlinear I-V characteristic.
Fix: Understand that the resistance of a filament lamp increases with temperature, causing the I-V curve to become less steep (it flattens and bends towards the voltage axis) as the voltage increases.
Misinterpreting the forward and reverse bias regions of diodes
Why it happens: Students sometimes misinterpret the sharp knee in the forward conduction region and the low current in the reverse bias region, leading to incorrect conclusions about diode behavior.
Fix: Recognize that in forward bias, the current increases exponentially with a small increase in voltage, while in reverse bias, the current remains very low until the breakdown voltage is reached.
Incorrectly plotting I-V characteristics
Why it happens: Students may plot the I-V characteristic graphs incorrectly, either by reversing the axes or not accurately representing the nonlinear behavior of components like filament lamps and diodes.
Fix: Ensure that the I-V characteristic graph is plotted correctly with the appropriate axis (I on the horizontal or vertical) and that the shape of the curve reflects the component's behavior.
Failing to consider constant physical conditions in Ohm's law
Why it happens: Students sometimes apply Ohm's law without considering that it only holds true under constant physical conditions, such as temperature.
Fix: Understand that Ohm's law (V = IR) is valid only when the physical conditions (e.g., temperature) remain constant. For components like filament lamps, the resistance changes with temperature.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Calculate Current | Use Ohm's law to find the current from a given voltage and resistance. | 3 |
| Filament Lamp Resistance | Calculate resistance at two voltages and explain why it changes with temperature. | 5 |
| Explain Diode Behaviour | Describe and explain how the diode conducts in forward bias but blocks in reverse bias. | 4 |
| Calculate Resistance | Use Ohm's law with the given voltage and current to find the component's resistance. | 3 |
| Calculate Resistance Change | Find the resistance at each voltage using R=V/I, then calculate the difference. | 5 |
| Total across these question types | 20 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.