A2 OCR B Capacitors

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  • Created on: 20-02-16 16:57
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ADVANCING PHYSICS
CAPACITANCE
How much charge a capacitor can keep separated at a given Potential Difference.
The amount of charge stored per volt
Q
C=V
C = Capacitance, Q = Charge, V = Potential Difference
Capacitance is measured in Farads or F
The simplest form of capacitor consists of two insulated conducting plates that face each other.
As the electrons flow from one plate to the other, one becomes negatively charged as it gains the
electrons and the other becomes positively charged due to the loss of electrons. This means that the
plates gain equal and opposite amounts of charge and thus a potential difference.
Initially the current is high.
However, as the charge builds up on the plates, it becomes harder for more electrons to be deposited due
to electrostatic repulsion.
When the potential difference across the battery is equal to the potential difference across the capacitor,
the current falls to 0.
I = Q
t
I = Current, Q = Charge, t = Time

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E= 1
2 QV
E = Energy on a capacitor, Q = Charge, V = potential difference
2
E=1
2 CV
E = Energy on a capacitor, C = Capacitance, V = potential difference
When the capacitor is connected around a resistor, the current flows in the opposite direction from the
charging current. The capacitor starts to discharge.
The capacitor is fully discharged when the potential difference and the current are both zero.
The amount of charge left on the plates decreases exponentially with time.…read more

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Q = Q0 eRC
Q = Charge left on plates, Q0 = charge of capacitor when fully charged, t = Time, RC = time constant
The graphs for potential difference or current against time for charging and discharging are also
exponential.…read more

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