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Summary Capacitance Physics A-Level
Capacitance Physics A-Level
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CapacitanceSpecification
(a) understand that a simple parallel plate capacitor consists of a pair of equal parallel metal
plates separated by vacuum or air,
(b) understand that the capacitor stores energy by transferring charge from one plate to the
other, so that the plates carry equal but opposite charges (the net charge being zero),
(c) define capacitance as for a parallel plate capacitor, with no dielectric,
(e) know that a dielectric increases the capacitance of a vacuum-spaced capacitor;
(f) recall that the E field within a parallel plate capacitor is uniform and of value V/d,
(g) use the equation for the energy stored in a capacitor,
(h) use formulae for capacitors in series and in parallel,
(i) understand the process by which a capacitor discharges through a resistor,
(j) use the equation where RC is the time constant.
Capacitors and charge
What is a capacitor?
A capacitor is a device that stores charge and therefore electrical potential energy.
They are made of two parallel metal plates separated by an insulator called a dielectric
(although for calculations assume separated by air or vacuum).
Dielectric
Dielectric = insulator between the plates of a capacitor which makes the capacitance larger
than if there were just empty space.
How a dielectric increases capacitance:
There are two types of dielectrics. Polar dielectrics are made up of polar molecules.
Non-polar dielectrics are made of non-polar molecules.
, Under an applied electric field, the molecules in the dielectric align with the electric
field as shown. NB the molecules in non-polar dielectrics are first polarised and then
align with the electric field whereas the molecules in polar dielectrics are already
polarised.
Between layers of molecules, charges cancel except at the top and bottom surface
(where there is a layer of positive and negative charge).
The aligned molecules then generate an electric field that is opposite to the applied
field. This decreases the effective (not the actual electric field strength) electric
field strength between the plates.
This lowers the potential of each plate because the strength of the pull of the plates on
charge decreases. This allows more charge to transfer onto the plate to maintain the
same potential difference.
The potential difference stays constant and so C ∝ Q. Thus, capacitance increases.
Charging up a capacitor
Theory:
When a capacitor is connected to a power supply, electrons flow from the negative
terminal of the battery to the neutral plate as there is a potential difference between
them. At the same time, electrons flow from the other neutral plate of the capacitor to
the positive terminal of the battery as there is a potential difference between them.
As the potential difference between the terminals and plates decrease, the current
decreases.
When the potential difference between the terminals and the plates is 0, the current is
0. This occurs when the potential difference across the source and capacitor is the
same.
Conservation of charge states that net charge in a circuit is zero. Therefore throughout
the whole process of charging, the plates carry equal and opposite charge.
Graphical analysis:
NB the I-t graph is exponential decay but the Q-t and V-t graphs are not exponential
growth.
Relationship between charge and p.d.:
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