A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Electric Current and Potential Difference — mark scheme explained
The short answer
In AQA A-Level Physics, understanding the concepts of electric current and potential difference is crucial for grasping how circuits work. This section covers the definitions of these quantities, their relationships, and how to apply them in practical scenarios.
The question
A charge of 5 coulombs flows through a wire in 10 seconds. Calculate the current.
[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, marks are typically awarded for correct substitution of values into the formula, showing working, and providing the final answer with units. For conceptual questions, marks are given for clear and accurate explanations.
What the command words demand
- Calculate
- Perform a numerical calculation using the given data and appropriate formula.
- Determine
- Find the value of a quantity by applying relevant principles or formulas.
- Explain
- Provide a clear and concise explanation, often including reasons or causes.
- Define
- State the meaning of a term or concept in a precise and accurate manner.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate about 2-3 minutes per mark. For a 6-mark question, spend approximately 12-18 minutes.
- Identify the given values: Q = 5 C, t = 10 s.0 marks
- Use the formula for electric current: I = Q / t.1 mark
- Substitute the values into the formula: I = 5 C / 10 s.1 mark
- Calculate the result: I = 0.5 A.1 mark
Final answer: 0.5 A
Work through every step correctly and you earn all 3 marks.
Another worked example
A battery does 20 joules of work to move 4 coulombs of charge between its terminals. Calculate the potential difference.
- Identify the given values: W = 20 J, Q = 4 C.0 marks
- Use the formula for potential difference: V = W / Q.1 mark
- Substitute the values into the formula: V = 20 J / 4 C.1 mark
- Calculate the result: V = 5 V.1 mark
Final answer: 5 V
Work through every step correctly and you earn all 3 marks.
Common mistakes
Confusing current (I) with charge (Q).
Why it happens: Students sometimes mix up the symbols and units for current and charge, leading to incorrect calculations.
Fix: Always double-check the units: I is in amperes (A), and Q is in coulombs (C).
Using the wrong formula for potential difference.
Why it happens: Students might use V = IR instead of V = W / Q when calculating potential difference from work done and charge.
Fix: Ensure you use the correct formula based on the given information: V = W / Q.
Forgetting to convert units before calculations.
Why it happens: Students often forget to ensure all quantities are in consistent units, leading to incorrect results.
Fix: Always check and convert units to their standard forms (e.g., seconds for time, coulombs for charge) before performing calculations.
Incorrectly connecting ammeters and voltmeters in circuits.
Why it happens: Students might connect an ammeter in parallel or a voltmeter in series, leading to incorrect measurements.
Fix: Remember that ammeters should be connected in series and voltmeters in parallel.
Using the wrong formula for resistance.
Why it happens: Students might use R = V / Q instead of R = V / I when calculating resistance.
Fix: Ensure you use the correct formula: R = V / I.
Misinterpreting Ohm's law in practical scenarios.
Why it happens: Students might apply Ohm's law incorrectly, especially when dealing with non-ohmic components or varying temperatures.
Fix: Understand that Ohm's law (V = IR) applies to ohmic conductors under constant temperature conditions.
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 the charge and time to work out the electric current in the wire. | 3 |
| Calculate Potential Difference | Use V = W/Q to find the potential difference across a battery's terminals. | 3 |
| Calculate Resistance | Use Ohm's law to find resistance from a given potential difference and current. | 3 |
| Calculate Potential Difference | Use Ohm's law to find voltage from a given current and resistance. | 3 |
| Calculate Resistance | Use Ohm's law to find resistance from a given voltage and current. | 3 |
| Total across these question types | 15 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.