Energy stored in a capacitor
In a nutshell
The work done depositing the charge onto the plates of the capacitor is the same as the energy stored by the capacitor. The area under a V against Q graph gives the energy stored by a capacitor.
Equations
Description | Equation |
Capacitance equation | C=VQ |
Energy stored by a capacitor | W=21QVW=21V2CW=21CQ2 |
Variable definitions
Quantity Name | Symbol | Derived Unit | SI BASE Units |
capacitance | | | kg−1m−2s4A2 |
| | | |
potential difference | | | kgm2s−3A−1 |
enery stored by capacitor | | | kgm2s−2 |
Energy stored by capacitors
When a capacitor is charged, one of the plates becomes negatively charged and the opposite plate becomes positively charged.
The work done depositing electrons on the negative plate and removing the electrons from the positive plate, is supplied by the power supply. This work is then stored as the electric potential between the plates.
The equation for charge stored on a capacitor is given as:
Q=CV
The charge stored by a capacitor is directly proportional to the potential difference across it. This means that the graph will be a straight line which passes through the origin:
| A | Potential difference | B | Charge | |
The work done on the capacitor plate, or the energy stored, is the product of the charge and the average potential difference.
The average potential difference can be calculated as:
Vav=2Vi+VfVav=20+VVav=2V
Therefore the energy stored by the capacitor is:
W=Q×2VW=21QV
It is also the area under a V against Q graph.
| A | Potential difference | B | Charge | 1 | Energy stored | |
Equation variations
Using variations of the capacitance equation, two other derivations for the energy stored by a capacitor can be derived:
Using Q=CV to eliminate Q:
W=21(CV)VW=21V2C
Using V=CQ to eliminate V:
W=21Q(CQ)W=21CQ2
Example
A 48μF capacitor is charged using a 12V battery. Calculate the amount of energy stored in the capacitor.
Firstly, write down the known values:
C=48μF=48×10−6FV=12V
Next, write down the equations needed and rearrange if necessary:
W=21V2C
Then, substitute the values into the equation:
W=21×122×48×10−6W=3.456×10−3
Make sure to include units and round to the lowest number of significant figures of the values given by the question:
energy stored by the capacitor, W=3.5×10−3J
The energy stored by the capacitor is 3.5×10−3J.