Capacitors and capacitance
Capacitance (symbol C) is a measure of a capacitor’s ability to store
charge. A large capacitance means that more charge can be stored.
Capacitance is measured in farads, (symbol F). However 1F is very
large, so prefixes (multipliers) are used to show the smaller values:
μ (micro) means 10-6 (millionth), so 1000000μF = 1F.
n (nano) means 10-9 (thousand-millionth), so 1000nF = 1μF.
p (pico) means 10-12 (million-millionth), so 1000pF = 1nF.
In a way, a capacitor is a little like a battery. Although they work in
completely different ways, capacitors and batteries both store
electrical energy, inside the battery; chemical reactions produce
electrons on one terminal and absorb electrons at the other terminal.
A capacitor is a much simpler device, and it cannot produce new
electrons – it only stores them.
Like a battery, a capacitor has 2 terminals. Inside the capacitor, the
terminals connect to 2 metal plates separated by a dielectric. The
dielectric can be air, paper, plastic or anything else that does not
conduct electricity and keeps the plates from touching each other.
The plate on the capacitor that attaches to the negative
terminal of the battery accepts electrons that the battery is
The plate on the capacitor that attaches to the positive
terminal of the battery loses electrons to the battery.
Once it’s charged, the capacitor has the same voltage as the battery
(1.5 volts on the battery means 1.5 volts on the capacitor). For a
small capacitor, the capacity is small. But large capacitors can hold
quite a bit of charge.
Here you have a battery, a light bulb and a capacitor. If the capacitor
is pretty big, what you would notice is that, when you connected the
battery, the light bulb would light up as current flows from the battery
to the capacitor to charge it up. The bulb would get progressively
dimmer and finally go out once the capacitor reached its capacity.
Then you could remove the battery and replace it with a wire.
Current would flow from one plate of the capacitor to the other. The
light bulb would light and then get dimmer and dimmer; finally going
out once the capacitor had completely discharged (the same number
of electrons on both plates).
The unit of capacitance is a farad (symbol F).
A 1-farad capacitor can store one coulomb (Q) of charge at 1 volt (V).
A 1-farad capacitor would typically be pretty big. So you typically see
capacitors measured in microfarads (millionths of a farad).
These sub units are:
farads 1microfarad( F)10 F also 10 F 1nano Farad
1 6 9
microfarads picofarad pF F 12 1 ( )10
There is a direct relationship between the Voltage (V) placed across
the plates of a capacitor and the charge (Q) held by them. If the
voltage is doubled the charge is doubled, if the charge is halved then
the voltage is halved etc. This tells us that the ratio of charge to
voltage is constant and this is known as the capacitance (C) of the