Capacitance (AQA A Level Physics)
Revision Note
Capacitance
Capacitors are electrical devices used to store energy in electronic circuits, commonly for a backup release of energy if the power fails
They are in the form of two conductive metal plates connected to a voltage supply (parallel plate capacitor)
There is commonly a dielectric in between the plates, this is to ensure charge does not freely flow between the plates
The capacitor circuit symbol is:
The capacitor circuit symbol is two parallel lines
Capacitors are marked with a value of their capacitance. This is defined as:
The charge stored per unit potential difference (between the plates)
The greater the capacitance, the greater the energy stored in the capacitor
The capacitance of a capacitor is defined by the equation:
Where:
C = capacitance (F)
Q = charge (C)
V = potential difference (V)
A capacitor used in small circuits
Capacitance is measured in the unit Farad (F)
In practice, 1 F is a very large unit
Often it will be quoted in the order of micro Farads (μF), nanofarads (nF) or picofarads (pF)
If the capacitor is made of parallel plates, Q is the charge on the plates and V is the potential difference across the capacitor
The charge Q is not the charge of the capacitor itself, it is the charge stored on the plates
This capacitance equation shows that an object’s capacitance is the ratio of the charge stored by the capacitor to the potential difference between the plates
Worked Example
A parallel plate capacitor has a capacitance of 1 nF and is connected to a voltage supply of 0.3 kV. Calculate the charge on the plates.
Answer:
Step 1: Write down the known quantities
Capacitance, C = 1 nF = 1 × 10-9 F
Potential difference, V = 0.3 kV = 0.3 × 103 V
Step 2: Write out the equation for capacitance
Step 3: Rearrange for charge Q
Q = CV
Step 4: Substitute in values
Q = (1 × 10-9) × (0.3 × 103) = 3 × 10-7 C = 300 nC
Examiner Tips and Tricks
The ‘charge stored’ by a capacitor refers to the magnitude of the charge stored on each plate in a parallel plate capacitor or on the surface of a spherical conductor. The capacitor itself does not store charge. The letter ‘C’ is used both as the symbol for capacitance as well as the unit of charge (coulombs). Take care not to confuse the two!
Polar Molecule in an Electric Field
A dielectric is made up of many polar molecules
These are molecules that have a 'positive' and 'negative' end (poles)
When no charge is applied to the capacitor:
There is no electric field between the parallel plates and the molecules are aligned in random directions
When there is a charge applied:
One of the parallel plates becomes positively charged and the other negatively charged hence an electric field is generated between the plates (from positive to negative)
The negative ends of the polar molecules are attracted to the positive plate and vice versa
This means all the molecules rotate and align themselves parallel to the electric field
Polar molecules align themselves when an electric field is between two parallel plates
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