Capacitance
Q. What is a capacitor?
A capacitor is an electrical component that stores charge on 2 separate metallic plates. An insulator, called a dielectric, is placed between the plates to prevent the charge from travelling across the gap.
Q. What is the time constant?
The time it takes for the charge in a capacitor to fall to 37% of its initial value.
Q. How does a capacitor charge up?
- Electrons move from negative to positive around the circuit.
- The electrons are deposited on plate A, making it negatively charged.
- Electrons travel from plate B to the positive terminal of the battery, giving the plate a positive charge.
- Electrons build up on plate A, and an equal amount of electrons is removed from plate B, creating a potential difference across the plates.
- When the potential difference across plates = source potential difference, the capacitor is fully charged and current stops flowing.
Q. Describe in terms of the movement of electrons how the potential difference across a capacitor changes when it discharges across a resistor.
Electrons move in opposite direction than when the capacitor was charging up. Charge on one plate A decreases as it loses electrons, and plate B gains electrons, neutralising them. Potential difference decreases exponentially across the plates.
Q. Some uses of capacitors.
Flash photography, Nuclear fusion, Backup power supplies, DC blocking, Smoothing AC to DC, Tuning (Resonating magnetic field).
Q. What 2 factors affect the time taken for a capacitor to charge or discharge?
The capacitance of the capacitor, C. This affects the amount of charge that can be stored by the capacitor at any given potential difference across it.
The resistance of the circuit, R. This affects the current in the circuit and how quickly it flows, hence how quickly the capacitor charges/discharges.
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