Electronics
How does a capacitor work?
A capacitor is two metal plates with a thin insulator between them. Connect it to a battery and charge is pushed onto one plate while the same amount is pulled off the other. The plates end up at the battery's voltage, and the capacitor holds energy in the electric field across the gap.
Charge per volt
How much charge the plates hold for each volt across them is the capacitance, C, measured in farads (F):
Q is the charge on each plate, in coulombs. One farad is enormous, so real capacitors are measured in µF, nF and pF: 1 µF = 1000 nF, and 1 nF = 1000 pF. A 100 µF capacitor charged to 9 V holds 0.9 mC on each plate.
Bigger plates and a thinner gap give more capacitance. Capacitors in parallel add: two 470 µF side by side make 940 µF.
Live experiment
Fill a capacitor
A 9 V battery charges two plates through 3.3 kΩ. Close the switch and watch charge pile onto one plate as it leaves the other, while the plot draws the voltage climbing to 9 V. Then try a bigger capacitor, and open the switch part-way through.
“Capacitors” is lesson 1 of 5 in Capacitor Caves, Electronics World 3. Free with an account. Lessons unlock in order, so you’ll do the 15 before it first.
Full means no flow
While a capacitor fills, current flows round the loop. As its voltage climbs towards the supply's, there's less push left to drive charge in, so the flow slows. Once it's full, the current stops: a capacitor blocks steady DC.
That filling takes time, set by the resistance it fills through and its capacitance. A bigger capacitor fills more slowly but holds more charge at the same voltage. That's the RC time constant.
Stored energy
The energy in a charged capacitor, in joules, is
Take 1000 µF charged to 12 V. It holds Q = 0.001 F × 12 V = 12 mC, and E = ½ × 0.001 × 12² = 0.072 J. The voltage is squared, so doubling it stores four times the energy. That's why a camera flash charges its 120 µF capacitor to about 330 V, for about 6.5 J.
A capacitor isn't a small battery, though. An AA cell holds around 10,000 J, and a capacitor's voltage falls as it empties.
The right way round
Large capacitors are usually electrolytic, and they're polarised: on an aluminium electrolytic, the leg marked with a stripe goes to the more negative side, and the + leg is longer. Each has a voltage rating, like 16 V. Fitted backwards or run above that rating, an electrolytic can burst.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Capacitors” is lesson 1 of 5 in Capacitor Caves, Electronics World 3. Free with an account. Lessons unlock in order, so you’ll do the 15 before it first.
Or start from the very first lesson, no account neededWork it out
- RC time constant calculatorτ, charge time, cutoff frequency and the voltage at any moment.
- Capacitor energy calculatorJoules and charge in a capacitor, and how much your circuit can use.
- Capacitor practice problemsCharge, capacitance, series and parallel, and stored energy, with the working.
- Electronics unit prefixes and conversionsp, n, µ, m, k, M and G, conversion tables, and what 4k7 or 2n2 means.
- Capacitor code chartTurn 104, 473J or 2A into a value, a tolerance and a voltage rating.
Practice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: quadruples. Energy goes with V². Twice the voltage gives 2² = 4 times the energy. That's why camera flashes charge their capacitors to hundreds of volts.
- Why not “halves”? Energy goes up with voltage, not down. E = ½ × C × V², so twice the voltage stores four times the energy.
- Why not “stays the same”? At a higher voltage a capacitor holds more charge, and each coulomb carries more energy, so the stored energy rises.
- Why not “doubles”? The charge doubles, since Q = C × V, but each bit of it now sits at twice the voltage too. Energy goes with V², so it's 2² = 4 times.
Show the answer
Answer: doubles. Q = C × V, so twice the capacitance at the same voltage holds twice the charge. It's energy that goes with V².
- Why not “quadruples”? Charge goes up in step with capacitance, Q = C × V. Four times is what happens to the stored energy when you double the voltage.
- Why not “halves”? A bigger capacitance holds more charge at a given voltage, not less. Q = C × V, so twice C gives twice Q.
Show the answer
Answer: 37%. In each τ it closes 63% of the gap to 0 V, so 37% is left.
- Why not “0%”? A capacitor doesn't empty completely in one τ. After 1τ it has lost 63% of its voltage, and it takes about 5τ to get close to 0 V.
- Why not “63%”? 63% is how much it has lost, not what's left. Each τ closes 63% of the gap to 0 V, so 100 − 63 = 37% remains.
“Capacitors” is lesson 1 of 5 in Capacitor Caves, Electronics World 3. Free with an account. Lessons unlock in order, so you’ll do the 15 before it first.
Questions people ask
What does a capacitor do in a circuit?
It stores energy in the electric field between its plates and gives it back quickly. That lets it smooth a power supply, steady the supply pin of a chip, set a delay with a resistor, or pass a changing signal while blocking a steady DC level.
Does a capacitor block DC?
Once it's charged, yes. Current flows while it fills, then stops when its voltage matches the supply, so no steady current gets through. A changing voltage keeps charging and discharging it, which is why a capacitor passes AC.
What is a farad?
The unit of capacitance: one coulomb of charge per volt. A farad is huge, so real parts are measured in microfarads (µF, 10⁻⁶ F), nanofarads (nF, 10⁻⁹ F) and picofarads (pF, 10⁻¹² F).
Can a capacitor shock you after the power is off?
Yes. Large capacitors, like those in camera flashes, TVs and power supplies, can stay charged at high voltage long after they're unplugged. Never open devices that plug into the wall.
Related topics
- RC time constant explainedτ = R × C: how long a capacitor takes to charge through a resistor, and the 63% rule.
- What is a decoupling capacitor?Why every chip has a capacitor beside its power pins, how big, and why it must be close.
- How does an inductor work?A coil that fights changes in its current, the high-voltage spike it makes, and how to tame it.