Capacitors
Tiny parts that store charge, smooth power and keep time.
The question
How can an LED keep glowing for a moment after you disconnect the battery?
What you’ll discover: what a capacitor stores, how fast it charges and empties, and how to handle the polarized kind safely.
In simple terms
BeginnerA small, fast-filling bucket
A capacitor stores electric charge. Inside are two metal plates (or foils) separated by a thin insulator. Connect it to a battery and charge piles up on the plates; disconnect the battery and the charge stays there, ready to flow back out.
Think of it as a small bucket that fills and empties quickly. A battery is more like a big water tank: it stores far more energy, but releases it slowly through a chemical reaction.
Capacitors are measured in farads (F). One farad is huge, so most are in microfarads (µF, millionths) or nanofarads (nF) and picofarads (pF).
The two kinds you’ll meet
- Ceramic capacitors are small, often yellow or tan discs, and can go either way round. Their value is usually a three-digit code: 104 means 10 followed by four zeros, in picofarads: 100,000 pF = 100 nF = 0.1 µF.
- Electrolytic capacitors are little cylinders that hold much more charge, and they are polarized: they must go the right way round. The longer leg is +, and a stripe with minus signs marks the − side. Each one is also marked with a maximum voltage.
Go deeper
IntermediateHow fast does it charge? The time constant
A capacitor charging through a resistor doesn’t fill at a steady rate: it fills quickly at first, then more and more slowly. The time constant, τ (the Greek letter tau), tells you how fast:
τ = R × C (seconds = ohms × farads)
After one time constant, the capacitor is about 63% charged. After five, it’s over 99% charged, which engineers treat as “full.” Emptying works the same way in reverse.
Example: 1,000 µF (0.001 F) through 330 Ω gives τ = 330 × 0.001 = 0.33 seconds. That’s the heart of the Fading Light build.
Capacitors also smooth power: placed across a supply, they soak up brief spikes and fill in brief dips, which is why you’ll find a small ceramic capacitor right beside most chips on a circuit board.
AdvancedEnergy, filters and AC
The charge stored is Q = C × V, and the energy is E = ½ C V². A 1,000 µF capacitor at 4.5 V holds only about 0.01 joules, which is harmless. A camera flash capacitor charged to around 300 V holds several joules, enough to give a painful, dangerous shock. Energy rises with the square of the voltage, which is why high-voltage capacitors deserve so much respect.
A capacitor blocks steady DC once charged but lets changing signals through. Its opposition to AC, called reactance, is XC = 1 ÷ (2πfC): smaller at higher frequencies. Combined with resistors, that makes filters that pass high or low frequencies. Stage 6 covers them properly.
Try it
Build the Fading Light: press a button, let go, and watch an LED fade out as a capacitor empties.
Quick check
What does a capacitor store?
Electric charge (and so, energy).
What is the value of a ceramic capacitor marked 103?
10 followed by three zeros, in picofarads: 10,000 pF, which is 10 nF.
How do you know which way round an electrolytic capacitor goes?
The longer leg is +, and the stripe with minus signs marks the − leg.
What is the time constant of 470 µF with 1 kΩ?
τ = 1,000 × 0.00047 = 0.47 seconds.
Words to know
- Capacitor
- A part that stores electric charge on two plates separated by an insulator.
- Farad (F)
- The unit of capacitance. Most capacitors are in µF, nF or pF.
- Polarized
- Must be connected the right way round.
- Time constant (τ)
- R × C: the time to charge about 63% of the way.
Learn more
- Science World (Vancouver): Current Electricity: eight hands-on circuit activities
- Build: the Fading Light
- Next: Diodes
Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.