Capacitors
Capacitors are devices that store electric charge (and electric potential energy). They
consist of two conductors, separated by an insulator. Each of the conductors is called a plate and
are normally charged with equal and opposite charge by connecting each plate to one terminal of
a battery or power supply so that a voltage difference is applied across the capacitor plates.
[demo of charge storage on C; examples of capacitors]
The amount of electric charge on the plates is found to be proportional to the voltage
difference applied across the plates so that Q = C ∆V, where C, the proportionality constant, is
known as the capacitance. We’ll see later that C depends only on the geometry of the plates and
on the properties of the insulator, but not on the applied voltage or any other electrical parameter
of the situation. Units for C are given by 1 C/V, from the above equation, and this is given a
special name, 1 farad (F) = 1 C/V. The farad is a very large unit of capacitance, since for a 1 V
applied potential, 1 C is an extremely large amount of charge. More typical values for C are pF
(= 10-12 F) or µF (=10-6 F).
The electrical symbol for a capacitor used in circuit diagrams is
similar to a battery or power supply but with both lines of equal length.
Keep clear in your mind that while a battery is an active device that has its own source of energy
(chemical or electrical or even perhaps solar), a capacitor is a passive device made up simply of
conductors and insulators. The origin of this symbol is the fact that two parallel conducting
plates with a small gap of air or another insulator is a common capacitor, known as a parallelplate capacitor. Recall an example in which we calculated the E field everywhere for two
infinite parallel plates and found it to be zero outside and constant between the plates. We will
study these later in some detail along with the energy storage properties of capacitors. For now,
we need to get ready for the lab by studying simple RC circuits.