A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Electromagnetic Induction and Faraday's Law — mark scheme explained
The short answer
In this section, we will explore the fundamental principles of electromagnetic induction, focusing on Faraday’s and Lenz’s laws. We will also delve into the mathematical expressions for induced electromotive force (emf) in various scenarios, such as a straight conductor moving in a magnetic field and a coil rotating uniformly in a magnetic field.
The question
A coil with 50 turns and an area of 0.1 m 2 rotates in a magnetic field of 0.2 T at an angular frequency of 60 rad/s. Calculate the maximum induced emf.
[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 steps, and providing the final answer with units. For conceptual questions, marks are given for accurate explanations that include relevant principles and equations.
What the command words demand
- Calculate
- Perform a numerical calculation to find the answer.
- Determine
- Find the value of a quantity or solve a problem using given data.
- Explain
- Provide a clear and concise explanation, including relevant principles and equations.
- Describe
- Give a detailed account of a process or phenomenon.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to read, understand, and solve each problem accurately.
- Identify the given values: N = 50, A = 0.1 m 2 , B = 0.2 T, ω = 60 rad/s.0 marks
- Use the formula for induced emf in a rotating coil: ε = B × A × N × ω × sin(ωt).1 mark
- The maximum value of sin(ωt) is 1, so the maximum induced emf occurs when sin(ωt) = 1.1 mark
- Substitute the values into the formula: ε max = 0.2 T × 0.1 m 2 × 50 × 60 rad/s × 1.1 mark
- Calculate the result: ε max = 60 V.1 mark
Final answer: 60 V
Work through every step correctly and you earn all 4 marks.
Another worked example
A straight conductor of length 0.5 m moves with a velocity of 2 m/s in a magnetic field of 0.3 T. Calculate the induced emf.
- Identify the given values: L = 0.5 m, v = 2 m/s, B = 0.3 T.0 marks
- Use the formula for induced emf in a straight conductor: ε = B × L × v.1 mark
- Substitute the values into the formula: ε = 0.3 T × 0.5 m × 2 m/s.1 mark
- Calculate the result: ε = 0.3 V.1 mark
Final answer: 0.3 V
Work through every step correctly and you earn all 3 marks.
Common mistakes
Forgetting to include the negative sign in Faraday’s law.
Why it happens: Students often overlook the importance of the negative sign, which indicates that the induced emf opposes the change in magnetic flux.
Fix: Always include the negative sign when applying Faraday’s law and remember its significance according to Lenz’s law.
Confusing the units of magnetic flux (Wb) with those of magnetic field strength (T).
Why it happens: Students may mix up the units, leading to incorrect calculations and answers.
Fix: Remember that magnetic flux is measured in weber (Wb) and magnetic field strength is measured in tesla (T).
Using the wrong formula for induced emf in a rotating coil.
Why it happens: Students might use the formula for a straight conductor moving in a magnetic field instead of the correct one for a rotating coil.
Fix: Use the correct formula: ε = B × A × N × ω × sin(ωt) for a rotating coil.
Forgetting to consider the direction of the induced emf according to Lenz’s law.
Why it happens: Students may focus only on the magnitude of the induced emf and neglect its direction, which is crucial for understanding the behavior of circuits.
Fix: Always apply Lenz’s law to determine the direction of the induced emf, ensuring it opposes the change in magnetic flux.
Incorrectly calculating the rate of change of magnetic flux (ΔΦ / Δt).
Why it happens: Students may make arithmetic errors or misinterpret the given values, leading to incorrect results.
Fix: Double-check the calculation of ΔΦ and Δt, ensuring they are correctly substituted into Faraday’s law.
Using the wrong value for the angular frequency (ω) in the formula for a rotating coil.
Why it happens: Students might confuse angular frequency with other quantities, such as linear velocity or time period.
Fix: Ensure that the correct value of ω is used, which is typically given in radians per second (rad/s).
Failing to recognize when the induced emf reaches its maximum value in a rotating coil.
Why it happens: Students may not understand that the maximum emf occurs when sin(ωt) = 1, leading to incorrect answers.
Fix: Remember that the maximum induced emf in a rotating coil is given by ε max = B × A × N × ω.
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 Maximum EMF | Find the peak induced emf of a coil rotating in a magnetic field. | 4 |
| Calculate Induced EMF | Find the emf induced in a straight conductor moving through a magnetic field. | 3 |
| Calculate Induced EMF | Apply Faraday's law to find the emf from a changing magnetic field through a coil. | 4 |
| Calculate Induced EMF | Use the rotating coil emf equation to find the induced emf at a given time. | 5 |
| Total across these question types | 16 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.