How Capacitors Work
A capacitor stores electrical energy by separating positive and negative electric charges on two conductive plates. The plates are separated by an insulating material called the dielectric, which prevents current from flowing directly between them. When connected to a voltage source, a capacitor charges. When connected to a load, it releases its stored energy by discharging.
Charging a Capacitor
When a capacitor is connected to a DC voltage source, electrons move from one plate to the other through the external circuit.
- The plate connected to the negative terminal gains electrons.
- The opposite plate loses electrons and becomes positively charged.
- An electric field develops between the two plates.
- The stored energy increases as the voltage across the capacitor rises.
As charging continues, the capacitor voltage gradually approaches the supply voltage.
Fully Charged Capacitor
Once the capacitor reaches the supply voltage, charging current falls to almost zero.
In an ideal DC circuit, a fully charged capacitor behaves like an open circuit, preventing further current flow.
In practice, a very small leakage current may still exist due to imperfections in the dielectric.
Discharging a Capacitor
If the voltage source is removed and a load is connected across the capacitor, the stored energy is released.
- Electrons flow through the external circuit.
- The capacitor voltage decreases.
- The stored energy is converted into useful work or heat.
- Eventually the capacitor becomes fully discharged.
Behaviour in DC Circuits
| Condition | Capacitor Behaviour |
|---|---|
| Immediately after connection | Acts almost like a short circuit and draws charging current. |
| While charging | Current gradually decreases. |
| Fully charged | Acts as an open circuit. |
Behaviour in AC Circuits
Alternating current continuously changes direction, so the capacitor is constantly charging and discharging.
Because of this continual exchange of energy, capacitors allow AC signals to pass while blocking steady DC voltages.
| Signal | Behaviour |
|---|---|
| DC | Blocks current after charging. |
| AC | Allows current to flow through continuous charging and discharging. |
Energy Storage
The energy stored in a capacitor depends on both its capacitance and the voltage across it.
E = Ā½ C VĀ²
| Symbol | Meaning |
|---|---|
| E | Stored energy (joules) |
| C | Capacitance (farads) |
| V | Voltage across the capacitor (volts) |
Factors Affecting Capacitance
The capacitance of a capacitor depends on several physical factors.
- Plate surface area.
- Distance between the plates.
- Dielectric material.
- Dielectric constant (relative permittivity).
Larger plates, smaller spacing and higher-permittivity dielectric materials produce greater capacitance.
Typical Applications
- Power supply smoothing.
- Noise filtering.
- Signal coupling.
- Power supply decoupling.
- Timing circuits.
- Oscillators.
- Motor starting.
- Audio crossover networks.
- Energy storage.
- Backup power systems.
Common Misconceptions
| Misconception | Correct Explanation |
|---|---|
| A capacitor stores current. | A capacitor stores electrical energy and charge, not current. |
| Current flows through the dielectric. | The dielectric acts as an insulator; current flows only in the external circuit. |
| Capacitors only work with AC. | Capacitors operate in both AC and DC circuits but behave differently in each. |
| A charged capacitor is always safe. | Large capacitors can retain dangerous voltages even after power is removed. |
Safety
- Disconnect power before handling capacitors.
- Discharge large capacitors safely using an appropriate resistor.
- Never short large capacitors directly with a screwdriver or wire.
- Observe polarity when installing electrolytic capacitors.
- Use capacitors with suitable voltage ratings.
Key Points
- A capacitor stores energy in an electric field.
- It charges when connected to a voltage source.
- It discharges when connected to a load.
- It blocks steady DC after charging but allows AC signals to pass.
- Capacitors are fundamental components in power supplies, amplifiers, filters and timing circuits.