A2 Notes; Capacitors

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www.fb.com/sabrs Capacitors (A2-Physics)
A capacitor is an electronic component that can store electrical charge and then release it.
It is made of two conducting plates separated by an insulator.
The charge that is stored by the capacitor is due to the potential difference across. We can write this as:
Q V or Q = kV
k is a constant specific to the capacitor, this is called the capacitance and is represented by the symbol C
Q = CV
Capacitance is measured in Farads, F
Charge is measured in Coulombs, C
We can rearrange the equation into C = Q / V and from this we can see that capacitance is a measure of the charge
stored per volt of potential difference. 1 Farad means 1 Coulomb of charge is stored per Volt.
Water Analogy
We can think of the charge stored by a capacitor as the volume of water in a
The cross-sectional area of the bucket represents the capacitance of the capacitor.
We can see that the capacitance of capacitor 1 is higher than the capacitance of
capacitor 2.
The height of the water represents the potential difference across the capacitor.
We can see that the potential difference across capacitor 2 is higher than the p.d.
across capacitor 1. The charge stored by both capacitors is the same.
A capacitor with a lower capacitance can store more charge if the p.d. across it is increased.
Charging and Discharging
When a capacitor is connected to a battery is sends out electrons to one of the plates, this becomes negatively
charged. The same amount of electrons move from the second plate and enter the battery, leaving the plate
positively charged. The capacitor is now storing a charge or is `charged'.
If the charged capacitor is disconnected from the battery and connected to a lamp it will give out the stored charge
or will `discharge'. The electrons on the negative plate move through the circuit and onto the positive plate. The
plates now have no charge on them. The energy stored by the capacitor is transferred to the bulb whilst the
electrons move (whilst a current flows).
Energy Stored by a Capacitor
The top equation shows us that the charge of a capacitor increases with the potential difference
across it. If we plotted p.d. against charge we get a graph that looks like this
We can derive an equation to find the energy that a capacitor stores by considering the energy
transferred during the shaded section on the lower graph.
In this section the charge changes from q to q+q when an average p.d. of v is applied
across it.
Using E = VQ (see AS Unit 1) the energy stored is E = v q.
The total energy is equal to the total of all the little rectangular sections and is given
by E = ½ QV. This is also equal to the area under the graph.
We can use the top equation to derive two more equations for the energy stored by a
1 1 1 Q2
E = QV E = CV 2 E=
2 2 2 C
Energy is measured in Joules, J

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Capacitors (A2-Physics)
Charging and Discharging
In the diagram to the right a capacitor can be charged by the battery if
the switch is moved to position A. It can then be discharged through a
resistor by moving the switch to position B.
Charging a Capacitor
When the switch is moved to A the battery sends electrons to the lower plate and takes them from the upper plate.
This leaves the lower plate negatively charged and the upper plate positively charged.…read more

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Capacitors (A2-Physics)
Exponential Decay
Finding from Graphs
The time constant of a discharging capacitor can be found from a graph of either charge, current or potential
difference against time. After one time constant the value will have dropped to 0.37 of the initial value.
In this case the time constant is 4 seconds.
Quantitative Treatment
We could use the graph above to find the charge on the capacitor after a time, t.…read more



really helpful 10/10 thank u :)

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