What is capacitor? How capacitor stores charge? How big is Farad?

How Does a Capacitor Store Energy? Working, Formula & Applications

A capacitor, also known as a condenser, is one of the most widely used passive components in electrical and electronic circuits. It is primarily used to store electrical energy in the form of an electrostatic field. Capacitors are available in a wide range of sizes—from picofarads (pF) in electronic circuits to kilovolt-ampere reactive (kVAR) ratings in industrial applications.

They have vast applications, including:

• Starting of single-phase induction motors

• Improving power factor in industries (reducing electricity bills and generator burden)

• Oscillation and tuning circuits

• Smoothing rectified AC outputs

• Radio receivers

• Time-delay circuits

• Electrical filters

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Unit of Capacitance

The unit of capacitance is the Farad (F), symbolized by “F”.

Definition:
A capacitor has a capacitance of one Farad if a potential difference of 1 Volt produces a charge of 1 Coulomb on its plates.

$$C = \frac{Q}{V}$$

where:

• $C$ = Capacitance (Farads)

• $Q$ = Charge (Coulombs)

• $V$ = Voltage (Volts)

Since 1 Farad is extremely large, in practical circuits we use:

• Microfarad (µF = 10⁻⁶ F)

• Nanofarad (nF = 10⁻⁹ F)

• Picofarad (pF = 10⁻¹² F)

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How Large is One Farad?

Let’s visualize how big 1 Farad actually is.
Using the capacitance formula:

$$C = \varepsilon \cdot \frac{A}{d}$$

where:

• $\varepsilon$ = Permittivity of dielectric medium (for air = 8.85 × 10⁻¹² F/m)

• $A$ = Area of plates (m²)

• $d$ = Distance between plates (m)

If we assume:

• Plate separation = 1 mm (0.001 m)

• Dielectric medium = air (ε = 8.85 × 10⁻¹² F/m)

• Capacitance = 1 F

Then,

$$A = \frac{C \cdot d}{\varepsilon} = \frac{1 \times 0.001}{8.85 \times 10^{-12}} \approx 113,000,000 \, m^2$$

This is equivalent to a plate of 10 km × 10 km (over 6 miles square!) — practically impossible.
👉 That’s why real capacitors use special dielectric materials to achieve high capacitance in a small size.

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How Does a Capacitor Store Charge?

A basic capacitor consists of two parallel plates separated by a dielectric medium.

• When connected to a DC supply:

  • One plate accumulates positive charge.

  • The other plate accumulates negative charge.

• The dielectric prevents direct current flow and instead stores energy in the form of an electrostatic field.

If air alone is used as the dielectric, excessive voltage may cause a spark discharge. Hence, practical capacitors use solid dielectric materials to increase charge storage and prevent breakdown.

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Common Dielectric Materials Used in Capacitors

1. Paper

2. Plastic film

3. Mica

4. Glass

5. Ceramic

6. Air

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Capacitor Equations

1. Charge Equation:

$$Q = C \cdot V$$

2. Energy Stored:

$$W = \frac{1}{2} C V^2$$

3. Capacitor Current Equation (Displacement Current):

$$I = C \cdot \frac{dV}{dt}$$

⚡ Note: This current is called displacement current. It is not conduction current but results from changing electric fields across the dielectric.

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Types of Capacitors

The types depend on the dielectric used. Some common ones include:

• Ceramic Capacitors

• Electrolytic Capacitors

• Mica Capacitors

• Paper Capacitors

• Film Capacitors

👉 Detailed Types of Capacitors – Read Here

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Capacitors in Series and Parallel

• In series, the overall capacitance decreases.

• In parallel, the overall capacitance increases.

👉 Read More: Capacitors in Series and Parallel

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✅ Final Note: Capacitors are the backbone of both electrical power systems (power factor correction, motor starting) and electronic circuits (filtering, tuning, timing). Their ability to store and release energy instantly makes them indispensable in modern technology.

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