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Why does a capacitor hold charge when disconnected from the supply ?

A capacitor holds charge when disconnected from the supply due to the fundamental principles of electrostatics and the behavior of electric fields within the capacitor. Understanding this phenomenon requires examining the processes that occur when a capacitor is charged and how it stores energy.

  1. Charging Process:
    • When a capacitor is connected to a voltage source, such as a battery, current flows into the capacitor. This process charges the capacitor by redistributing charges on its plates. Electrons accumulate on one plate, while an equal number of electrons are repelled from the other plate, creating an electric field between them.
  2. Energy Storage in Electric Field:
    • The charging process stores electrical energy in the electric field between the capacitor plates. The electric field represents the potential energy associated with the separated charges. The energy is stored in the form of potential energy, and this process continues until the potential difference across the capacitor matches the applied voltage.
  3. Dielectric Properties:
    • The space between the capacitor plates is filled with a dielectric material, which is an insulator. The dielectric enhances the capacitor’s ability to store charge by increasing its capacitance. The capacitance is a measure of the capacitor’s ability to store charge per unit voltage.
  4. Insulating Effect of Dielectric:
    • The dielectric material prevents the flow of current between the capacitor plates. It acts as an insulator, allowing the capacitor to hold the stored charge even when disconnected from the supply. The dielectric effectively isolates the charges on the plates, maintaining the electric field and the stored energy.
  5. Time Constant:
    • The time constant of a capacitor, determined by the product of capacitance (C) and resistance (R) in the circuit, influences the rate at which the capacitor charges and discharges. When disconnected from the supply, the capacitor discharges slowly over time due to the resistance in the circuit, but it retains the stored charge for an extended period.
  6. Conservation of Energy:
    • The principle of conservation of energy applies to capacitors. The energy stored in the electric field during the charging process remains in the capacitor as potential energy. Even when disconnected from the supply, this energy is conserved, and the capacitor continues to hold the charge.
  7. Voltage Potential:
    • The voltage potential across the capacitor terminals, established during the charging process, is maintained when disconnected from the supply. This potential difference represents the stored charge’s ability to do work when the capacitor is reconnected to a circuit.

In summary, a capacitor holds charge when disconnected from the supply because of the energy stored in its electric field during the charging process. The dielectric material insulates the charges, preventing their immediate discharge, and the capacitor retains the stored charge until it is reconnected to a circuit or allowed to discharge over time through its inherent resistance. The ability of a capacitor to store and retain charge is fundamental to its various applications in electronic circuits.

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