Capacitors are very interesting and very useful circuit
elements. They can store charge, discharge it rapidly (or more slowly if
used in combination with resistors), and they can essentially block direct
They can be used in timing circuits, filtering circuits ... all
kinds of things. Rapid chargers in electric cars employ capacitors to store
charge rapidly and introduce it to the batteries more slowly later.
Form of a capacitor
capacitor is really pretty simple. ACapacitor is atwo parallel plates, separated by air or some other
plates must be pretty close together, often just the with of a thin plastic
film.. Each plate is connected to a conductor which connects into a circuit.
Unit of capacitance
unit of capacitance is the Farad (F), named after Michael Faraday. The base units of
the Farad are, 1 F = s4·A2·m-2·Kg-1.
That's not often too helpful. The most useful conversion we can make (and there
are a lot of them) is that 1 F = 1 C/V; One Farad is one coulomb of
charge per volt of potential.
practice, one Farad (1F) is a very large amount of capacitance.
Charge a 1 F
capacitor up and touch it and you'll get the shock of your life (maybe your
last). In electronics applications, units of microfarads (1 μF =
10-6 F), nanofarads (1 nF = 10-9 F) or picofarads
(1 pF = 10-12 F) are much more common. Pictured above is a 68
μF capacitor that can be held at a potential of up to 100 V before the
insulator between the plates breaks down and allows current to flow.
depends on geometry and insulation
amount of capacitance (capacity to hold charge) of such a device is dependent
on three things:
area of overlap of the two parallel plates, A
distance between the plates, d
nature of the material in between the plates, which cannot be a conductor.
The two geometric properties, A
& d, aren't difficult to figure out. The quality of
insulation between the plates is ranked with a number, εr,
called the relative static permeability. εr =
1 for air and can range from 1 for a vacuum (we actually define εr
= 1 for a vacuum) to over 12,000 for an inorganic compound called calcium
Mathematically, capacitance scales
linearly with overlap area and εr, and
inversely with the distance between plates:
easy to see that as we increase either the area of overlap of the plates or the
quality of the insulation material, and as we decrease the the distance between
plates, we increase the capacitance of the device. The constant εo
in the capacitance formula is called the dielectric constant, εo
= 8.754 x 10-12 F/m, and it's there, like any
proportionality constant in physics, to get the units right.
In capacitors, we'd like to have
of overlap between parallel plates, and
material with a very high permittivity value*.
*Note that the word
"permittivity" here is the opposite of what we really mean.
High-permittivity substances really do not "permit" the flow of
current very well. It's an old term we're kind of stuck with
Uses of Capacitors
Uses of Capacitors
Solving Capacitor and capacitance
1.A capacitor that has air between its
plates is connected across a potential difference of 12.0V and stores 48.0mC of
charge. What is the capacitance of the capacitor? Give your answer in μF
2.We have a parallel plate capacitor
constructed of two parallel plates, each with area separated by a
distance of 1.00mm. What is the capacitance of this parallel plate capacitor?
3.What happens to the capacitance of the
parallel plate capacitor if increase the distance between plated by the factor
4.A parallel plate capacitor with air
between the plates has a capacitance of 8pF. What will be the capacitance if
the distance between the plates is reduced by half, and the space between them
is filled with a dielectric (dielectric constant=6)?
5.A parallel - plate capacitor has a plate
separation of 1.00mm and a capacitance of 1.00F. Find the area of the plates.
6.Capacitor C1 consists of square parallel
plates 10cm on side, and separated by a distance 4mm, as shown in the figure.
Capacitor C2 has square parallel plates 5cm on a side, and separated by a
distance of 1mm. What is a ratio between the capacitances of two parallel-plate
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What is a capacitor? How does it made up?
Unit of capacitance The unit of capacitance is the
Capacitance depends on geometry and insulation
F/m , and it's there, like any proportionality constant in physics, to get the units right