Introduction to a Capacitor

A capacitor is a device that stores electrical energy in the form of an electric field. It has two conducting plates separated by an insulating material called a dielectric (air, ceramic, mica, paper, or electrolyte).

q = CV

U = \[\frac {1}{2}\]CV² (also U = Q²/2C or \[\frac {1}{2}\]QV)

1. The battery pulls electrons off one plate (making it positive) and pushes them onto the other plate (making it negative).
2. This charge separation creates an electric field between the plates.
3. Charging continues until the capacitor's voltage equals the battery's voltage, then the current stops.

Analogy: Charging a capacitor is like a pump filling two connected water tanks; one gains water, the other loses it. As the level difference grows, the pump has to work harder, just like charging current drops as voltage across the capacitor rises.

The dielectric prevents the separated charges from recombining, so the capacitor keeps its energy stored in the electric field even without the battery. This stored energy can be released almost instantly — useful in camera flashes, defibrillators, and pulsed-power devices.

Important: Capacitance C is a fixed property of the capacitor's geometry and dielectric — it does not depend on the charge (q) or voltage (V) applied.

• Plate area (A): Larger area → more room for charge → higher capacitance.
• Plate separation (d): Smaller separation → stronger interaction between plates → higher capacitance.
• Dielectric constant (K): Higher K → higher capacitance.

Bringing an uncharged conductor near a charged one induces an opposite charge on its near face (electrostatic induction), lowering the effective potential of the first conductor and letting it hold more charge at the same voltage. Grounding the second plate increases capacitance further, since the ground acts as an unlimited charge reservoir.

• Energy storage & quick discharge: camera flashes, defibrillators, pulsed-power devices
• Voltage regulation: smoothing DC in power supplies, inverters, rectifiers
• Signal filtering: high-pass/low-pass/band-pass filters in audio/RF circuits
• Coupling/decoupling: passing AC while blocking DC, reducing noise
• Computer memory: DRAM stores bits using capacitors
• Wireless communication: resonant circuits in transmitters, antennas, oscillators
• Supercapacitors: high-capacity storage for EVs and renewable energy

• Capacitor stores energy as an electric field; net charge on it is always zero.
• C = q/V depends only on geometry and dielectric, never on q or V.
• Capacitance increases with larger plate area, smaller separation, and higher K.
• Charging stops when capacitor voltage equals source voltage; U = \[\frac {1}{2}\]CV².
• A nearby (especially grounded) conductor increases capacitance via induction.
• Capacitors are vital for energy storage, filtering, coupling, memory, and wireless tech.