charge stored per unit potential difference. C = Q ÷ V.

What is the unit of capacitance?

the farad. One farad is one coulomb per volt.

The p.d. across a capacitor is doubled. What happens to the charge it stores?

It doubles. The capacitance stays the same, and Q = CV.

What is 1 µF in farads?

10⁻⁶ F. Micro means 10⁻⁶.

Capacitors and capacitance · Cambridge A Level Physics

In plain words

A capacitor is a pair of metal plates with a gap between them. Connect a battery and charge piles up, positive on one plate and negative on the other, each held in place by the attraction of the other across the gap. The capacitance tells you how much charge is stored for each volt.

Three things to know

Capacitance C = Q ÷ V: the charge stored per unit potential difference. Unit: the farad (F), which is one coulomb per volt.
Real capacitors are measured in microfarads (µF, 10⁻⁶ F), nanofarads (nF, 10⁻⁹ F) or picofarads (pF, 10⁻¹² F).
Q is the charge on one plate. One plate has +Q and the other has −Q.

Worked example

A 470 µF capacitor is charged to a p.d. of 12 V. Find the charge stored.

Q = CV.
= 470 × 10⁻⁶ × 12.
= 5.6 × 10⁻³ C.

Tips and tricks

Change microfarads to farads before you calculate.
A capacitor is charged at a steady current I for a time t: the charge is Q = It.

5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Capacitors and capacitance: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

What is capacitance?
- charge stored per unit potential difference (the answer)
- energy stored per unit charge
- potential difference per unit charge
- charge stored per unit time
C = Q ÷ V.
What is the unit of capacitance?
- the farad (the answer)
- the coulomb
- the tesla
- the henry
One farad is one coulomb per volt.
A 100 µF capacitor has a p.d. of 5.0 V across it. How much charge does it store?
- 2.0 × 10⁻⁵ C
- 5.0 × 10⁻⁴ C (the answer)
- 500 C
- 5.0 × 10⁴ C
Q = CV.
The p.d. across a capacitor is doubled. What happens to the charge it stores?
- It doubles. (the answer)
- It halves.
- It stays the same.
- It becomes four times bigger.
The capacitance stays the same, and Q = CV.
What is 1 µF in farads?
- 10⁻³ F
- 10⁻⁶ F (the answer)
- 10⁻⁹ F
- 10⁻¹² F
Micro means 10⁻⁶.

5 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

3 questions, 7 marks. Write your answers on paper, then check them.

Capacitors and capacitance

Cambridge International AS & A Level Physics 9702 · 7 marks · papermunch.org

Name ______________________________ Date ______________

A capacitor stores a charge of 3.0 × 10⁻⁴ C when the p.d. across it is 6.0 V. Find its capacitance.[2]
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5.0 × 10⁻⁵ F (50 µF).
I got this one
Find the p.d. across a 2.0 µF capacitor that holds a charge of 9.0 µC.[2]
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4.5 V.
I got this one
A capacitor is charged by a steady current of 20 µA for 30 s, and the p.d. across it reaches 3.0 V. Find its capacitance.[3]
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2.0 × 10⁻⁴ F (200 µF). The charge is 20 × 10⁻⁶ × 30 = 6.0 × 10⁻⁴ C.
I got this one

Answers: Capacitors and capacitance

1. 5.0 × 10⁻⁵ F (50 µF).
2. 4.5 V.
3. 2.0 × 10⁻⁴ F (200 µF). The charge is 20 × 10⁻⁶ × 30 = 6.0 × 10⁻⁴ C.