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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Identify the given values: N = 50, A = 0.1 m 2 , B = 0.2 T, ω = 60 rad/s.
- S2
Use the formula for induced emf in a rotating coil: ε = B × A × N × ω × sin(ωt).
- S3
The maximum value of sin(ωt) is 1, so the maximum induced emf occurs when sin(ωt) = 1.
- S4
Substitute the values into the formula: ε max = 0.2 T × 0.1 m 2 × 50 × 60 rad/s × 1.
- S5
Calculate the result: ε max = 60 V.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Identify the given values: N = 50, A = 0.1 m 2 , B = 0.2 T, ω = 60 rad/s.
- S2
Use the formula for induced emf in a rotating coil: ε = B × A × N × ω × sin(ωt).
- S3
The maximum value of sin(ωt) is 1, so the maximum induced emf occurs when sin(ωt) = 1.
- S4
Substitute the values into the formula: ε max = 0.2 T × 0.1 m 2 × 50 × 60 rad/s × 1.
- S5
Calculate the result: ε max = 60 V.
Final answer: 60 V
Common mistakes
- Forgetting to include the negative sign in Faraday’s law. — 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). — 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. — 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. — 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). — 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. — 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. — Remember that the maximum induced emf in a rotating coil is given by ε max = B × A × N × ω.
Where the marks go
- Full worked solution (all marking points)4 marks