Chapter 21: Capacitors Flashcards
capacitor
- electrical component that stores charge on separate metallic plates
- insulator, dielectric, between plates to prevent charge from travelling across the gap
Capacitance
charge stored per unit potential difference across the two plates
Relative permittivity
- Ratio of charge stored with dielectric between two plates to charge stored without it
- greater relative permittivity, greater capacitance of capacitor
How to you add capacitors in series?
1/Ct = 1/C1 + 1/C2…
How do you add capacitors in parralel
Ct = C1 +C2 +…
What does the area under a charge against pd graph represent?
energy stored by the capacitor
Time constant
time taken for the charge of a capacitor to fall to 37% of the initial value, given by RC
How do capacitors charge up?
1) Electrons move from negative to positive around circuit
2) Electrons deposited on plate A, making it negatively charged
3) Electrons travel from plate B to the positive terminal of the battery, giving the plate a positive charge
4) Electrons deposited on plate A and an equal amount of electrons are removed from plate B, creating a potential difference across the plates
5) When the pd across the plates = source pd, the capacitor is fully charged and the current stops flowing
How does the PD across a capacitor change when discharging. Describe in terms of electron movement.
1) Electrons move in the opposite direction than when the capacitor was charging
2) Charge on one plate A decreases as it loses electrons, and plate B gains electrons, neutralising them
3) PD decreases exponentially across the plates
Uses of Capacitors
- flash photography
- nuclear fusion
- backup power supply
- smoothing AC to DC
Energy equation for Capacitor
E = 1/2 Q V
What factors affect the time taken to discharge/charge a capacitor?
- Capacitance, affects the amount of charge that can be stored by the capacitors at any given potential difference
- Resistance, affects the current in the circuit and how quickly it flows, hence how fast it charges/discharges
