Why do capacitors in parallel add directly?

They all have the same potential difference across them and their stored charges add, so the total charge per volt is the sum of the individual capacitances.

Why do capacitors in series combine as reciprocals?

They all carry the same charge while the potential differences across them add, so the total volts per coulomb is the sum of the individual reciprocals.

Why is the combination rule for capacitors the opposite way round from that for resistors?

Resistance is potential difference per unit current and capacitance is charge per unit potential difference, so the quantity that adds in series is reversed.

Calculate the total capacitance of 100 μF and 220 μF in parallel.

C = 100 + 220, which is 320 μF.

CCEA A-Level Physics 1210

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Capacitors

Capacitors in series and in parallel

3 things to know
Unit A2 2

Overview

Recall questions on the two combination rules and the reason for each in terms of shared charge or shared potential difference, why the rules are the opposite way round from resistors, worked calculations for both arrangements, which stores more charge at a given supply voltage, and finding the potential difference across each of a series pair.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

A 200 μF and a 300 μF capacitor are in series across 100 V. Calculate the charge stored.

Key term

Give the equation for capacitors connected in parallel.

Key term

Give the equation for capacitors connected in series.

Questions from this subtopic

Each one with the model answer underneath.

1
Give the equation for capacitors connected in parallel.
Model answer
The total capacitance is the sum of the individual capacitances, C = C₁ + C₂ + C₃ and so on.
2
Give the equation for capacitors connected in series.
Model answer
One divided by the total capacitance is the sum of the reciprocals, 1/C = 1/C₁ + 1/C₂ + 1/C₃ and so on.
3
Why do capacitors in parallel add directly?
Model answer
They all have the same potential difference across them and their stored charges add, so the total charge per volt is the sum of the individual capacitances.
4
Why do capacitors in series combine as reciprocals?
Model answer
They all carry the same charge while the potential differences across them add, so the total volts per coulomb is the sum of the individual reciprocals.
5
Why is the combination rule for capacitors the opposite way round from that for resistors?
Model answer
Resistance is potential difference per unit current and capacitance is charge per unit potential difference, so the quantity that adds in series is reversed.
6
Calculate the total capacitance of 100 μF and 220 μF in parallel.
Model answer
C = 100 + 220, which is 320 μF.

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