A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Definition of Capacitance: C = Q / V — mark scheme explained
The short answer
Capacitance is a fundamental concept in the study of electrical circuits and fields. It describes how much electric charge a device can store for a given voltage. The definition of capacitance, as specified by AQA A-Level Physics, is: C = Q / V Where: C is the capacitance measured in farads (F).
The question
A capacitor with a capacitance of 20 μF is charged to a voltage of 15 V. Calculate the charge stored on the capacitor. [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: C = 20 μF, V = 15 V
- S2
Convert the capacitance to farads: 20 μF = 20 × 10 -6 F
- S3
Use the formula Q = C × V
- S4
Substitute the values: Q = 20 × 10 -6 F × 15 V
- S5
Calculate the charge: Q = 300 × 10 -6 C = 300 μC
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Identify the given values: C = 20 μF, V = 15 V
- S2
Convert the capacitance to farads: 20 μF = 20 × 10 -6 F
- S3
Use the formula Q = C × V
- S4
Substitute the values: Q = 20 × 10 -6 F × 15 V
- S5
Calculate the charge: Q = 300 × 10 -6 C = 300 μC
Final answer: 300 μC
Common mistakes
- Confusing the units of capacitance, charge, and voltage. — Always double-check the units and convert them to standard units (farads and coulombs) before performing calculations.
- Using the wrong formula for capacitance, charge, or voltage. — Memorize the main formula C = Q / V and derive the other forms as needed. Practice using each form in different contexts.
- Forgetting to convert units before performing calculations. — Always check and convert units before substituting values into the formula. Use consistent units throughout the calculation.
- Misinterpreting the relationship between charge, voltage, and capacitance. — Understand that the relationship is linear: Q = C × V. Practice problems that involve changing one variable while keeping another constant.
- Using the wrong value for capacitance or charge in calculations. — Carefully read the problem statement and identify the correct values to use. Double-check your work to ensure you are using the right values in the formula.
- Failing to check the final answer for reasonableness. — Always check your final answer to ensure it is reasonable and consistent with the given values and units. Use estimation techniques to verify the order of magnitude.
Where the marks go
- Full worked solution (all marking points)4 marks