Meaning and mechanism
A capacitor is a component that stores electrical charge between two separated conductors. The capacitance, denoted as C, is the ratio of the amount of charge stored to the potential difference, expressed as Q/V.
Increasing the area of a parallel plate, or decreasing the distance between them, allows for the storage of more charge at the same voltage. Dielectrics can alter the relationship of electric fields, thereby increasing capacitance.
Relationships and conditions
If the edge effect is ignored, it results in C=ε₀εrA/d. With a complete connection, the voltage (V) is constant, and without leakage after separation, the charge (Q) is constant; therefore, when the shape changes, the change in energy also changes.
The stored energy is ½CV² = Q² / 2C). In reality, when considering the energy balance of a real process, we must also include the work done by external forces, such as moving the electrodes.
What to observe in the simulation
Connected to the battery, V is constant; after disconnection, stored Q is constant.
Edge fields, breakdown and leakage are omitted. Each run uses a fixed reference geometry for disconnection. Experiment on screen; do not reproduce this with real power supplies, outlets or high voltage.
Key relationships and symbols
C = ε₀εrA/d · Q = CV · U = Q²/(2C)
| Symbol | Meaning | Unit |
|---|---|---|
| C | Capacitance | F |
| A, d | Plate area and separation | m², m |
| εr | Relative permittivity | Dimensionless |
Worked example
With the battery disconnected and charge fixed, what happens to stored energy if spacing doubles?
2U
Common misconceptions
- Increasing the capacity does not mean that voltage and charge simultaneously increase in all connection conditions.