Capacitance — A-Level Physics Revision
Revise Capacitance for A-Level Physics. Step-by-step explanation, worked examples, common mistakes and exam-style practice aligned to AQA, Edexcel, OCR, WJEC, Eduqas, CCEA, Cambridge International (CIE), SQA, IB, AP.
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Go to Magnetic FieldsWhat is Capacitance?
Capacitance is the ability of a component, called a capacitor, to store electrical energy in an electric field. It is defined as the charge stored per unit potential difference. This topic covers the function of capacitors, the factors affecting their capacitance, and how to calculate the total capacitance for capacitors connected in series and parallel. You will also study the exponential nature of capacitor charging and discharging through a resistor, introducing the concept of the time constant (RC).
Board notes: Capacitance is a major A-Level topic for all exam boards (AQA, Edexcel, OCR). All specifications cover the definition of capacitance, energy storage, and the analysis of charging and discharging curves using the time constant. The rules for combining capacitors in series and parallel are also fundamental across all boards.
Step-by-step explanationWorked example
A 100 µF capacitor is charged to a potential difference of 12 V. To find the energy stored, use E = ½CV². Convert capacitance to Farads: 100 µF = 100 x 10^-6 F. So, E = 0.5 * (100 x 10^-6 F) * (12 V)² = 0.0072 J. The energy stored is 7.2 mJ.
Mini lesson for Capacitance
1. Understand the core idea
Capacitance is the ability of a component, called a capacitor, to store electrical energy in an electric field. It is defined as the charge stored per unit potential difference.
Can you explain Capacitance without copying the notes?
2. Turn it into marks
A 100 µF capacitor is charged to a potential difference of 12 V. To find the energy stored, use E = ½CV².
Underline the method, evidence, or command-word move that would earn credit in A-Level Paper 2 — Thermal, Fields & Nuclear.
3. Fix the likely mark leak
Watch for this mistake: Applying the resistor rules for series and parallel to capacitors. The rules are reversed: for capacitors in series, you add the reciprocals (1/Ct = 1/C1 + 1/C2), and for capacitors in parallel, you add them directly (Ct = C1 + C2).
Write one correction rule before doing another practice question.
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Capacitance practice questions
These are original StudyVector questions for revision practice. They are not official exam-board questions.
Question 1
In one A-Level sentence, explain what Capacitance is testing.
Answer: Capacitance is the ability of a component, called a capacitor, to store electrical energy in an electric field. It is defined as the charge stored per unit potential difference.
Mark focus: Precise definition and topic focus.
Question 2
A Capacitance question uses an unfamiliar context. What should the answer do before adding detail?
Answer: It should name the process, variable, equation, particle model, or evidence being tested, then explain the result using precise scientific vocabulary.
Mark focus: Method selection and command-word control.
Question 3
A student makes this mistake: "Applying the resistor rules for series and parallel to capacitors. The rules are reversed: for capacitors in series, you add the reciprocals (1/Ct = 1/C1 + 1/C2), and for capacitors in parallel, you add them directly (Ct = C1 + C2)." What should their next repair task be?
Answer: Do one Capacitance question and review the mistake type.
Mark focus: Error correction and next-step practice.
Capacitance flashcards
Core idea
What is the main idea in Capacitance?
Capacitance is the ability of a component, called a capacitor, to store electrical energy in an electric field. It is defined as the charge stored per unit potential difference.
Common mistake
What mistake should you avoid in Capacitance?
Applying the resistor rules for series and parallel to capacitors. The rules are reversed: for capacitors in series, you add the reciprocals (1/Ct = 1/C1 + 1/C2), and for capacitors in parallel, you add them directly...
Practice
What is one useful practice task for Capacitance?
Answer one Capacitance question and review the mistake type.
Exam board
How should you use board notes for Capacitance?
Capacitance is a major A-Level topic for all exam boards (AQA, Edexcel, OCR). All specifications cover the definition of capacitance, energy storage, and the analysis of charging and discharging curves using the time...
Common mistakes
- 1Applying the resistor rules for series and parallel to capacitors. The rules are reversed: for capacitors in series, you add the reciprocals (1/Ct = 1/C1 + 1/C2), and for capacitors in parallel, you add them directly (Ct = C1 + C2).
- 2Confusing the equations for energy stored in a capacitor. There are three equivalent forms: E = ½QV, E = ½CV², and E = ½Q²/C. Students must choose the most appropriate one based on the information given in the problem.
- 3Misunderstanding the time constant (τ = RC). The time constant is the time it takes for the charge, current, or voltage to fall to approximately 37% of its initial value during discharge, or to rise to 63% of its final value during charging. It is not the time to fully charge or discharge.
Capacitance exam questions
Exam-style questions for Capacitance with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel, OCR, WJEC, Eduqas, CCEA, Cambridge International (CIE), SQA, IB, AP specifications.
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Step-by-step method
Step-by-step explanation
4 steps · Worked method for Capacitance
Core concept
Capacitance is the ability of a component, called a capacitor, to store electrical energy in an electric field. It is defined as the charge stored per unit potential difference. This topic covers the …
Frequently asked questions
What does a capacitor do in a circuit?
A capacitor stores electrical charge and energy. It can be used in timing circuits, for smoothing out variations in direct current, in filter circuits, and to store energy for a rapid release, such as in a camera flash.
Why can't you add the capacitances directly for capacitors in series?
In a series circuit, each capacitor stores the same amount of charge, but they share the total potential difference. This sharing of voltage leads to a total capacitance that is less than any of the individual capacitances.