Capacitor summary

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Electric fields in circuits
Electrons move round a circuit due to the force exerted on them by an electric field that is
set up within the conduction material.
However when there is a gap in the circuit, the electrons may be able to feel the force of
the electric field, but the force is not strong enough to let them move across the gap.
If you create a circuit with two metal plates separated by a gap the charge will build up,
with one side being ve and the other
+ve. An electric field will form between
them due to the attraction between the
two plates this field will increase until the
potential difference is equal to that of the
power supply.
A pair of plates with an insulator between
them is known as a capacitor.
At the stage where the pd across the
plates is equal to the power supply , the
capacitor is said to be fully
charged. The amount of charge a capacitor can hold per volt applied is known
as it's capacitance ,C, and is measured in Farads(F).
The capacitance depends on the size of the plates, the insulator and
the distance between them.
Energy stored on a charged capacitor
A charged capacitor can be used to store electrical potential energy.
Normally E=QV however with a capacitor E=1/2QV.
from this we can get E=½ Q2/C
Growth and decay curves
The graph for the discharge of a capacitor is a gradual decay graph
This is because as the capacitor
loses charge the pd across the capacitor
becomes less and this means that the
electrical field is weaker which leads to
less force pushing the electrons away.
Hence a decay graph is formed.
The rate of discharge can be found
from the time constant

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The time constant tells you how long it takes
for the current to fall to 37% that of it's starting
Charging the capacitor gives similar curves.
Discharging capacitor maths
The graphs above have a constantly changing
gradient which in itself does not change at a constant rate. They are known as
exponential curves. These are produced by y=ex.…read more


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