7.4 Capacitance Flashcards
How is capacitance calculated?
C = Q/V
What is the relative permittivity?
The ratio of the charge stored with the dielectric between the plates to the charge stored when the dialectric is not present.
εr = Q/Q0
The greater the relative permittivity, the greater the capacitance of the capacitor.
What does the area under the graph of charge against pd represent?
The energy stored by the capacitor.
What is the time constant?
The time is takes for the charge in a capacitor to fall to 37% of the initial value given by RC.
A capacitor is considered fully discharged after 5 time constants
How was 37% derived when using the time constant?
- start with the formula Q=Qθe^-t/RC
- when t= RC, the formula becomes Q=Qθe^-1
- e^-1=-0.3680.37, which is where 37% came from
What is the half time of a capacitor?
T1/2 = 0.69RC
What equations do we require for charging a capacitor?
Charging up a capacitor produces Q=Qθ(1-e^-t/RC) & V=Vθ(1-e^-t/RC) where Vθ is the battery pd and Qθ=CVθ.
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 are removed from plate B, creating a potential difference across the plates.
- When the p.d across plates = source p.d, the capacitor is fully charged and current stops flowing
Describe and explain in terms of movement of electrons how the p.d 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.
- P.d. decreases exponentially across the plates.
State the 3 expressions for the energy stored by a capacitor.
E= 1/2(Q^2/C) = 1/2 (QV) = 1/2 (CV^2)
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 capacitors at any given p.d. across it.
- the resistance of the circuit, R. The affects the current in the circuit and how quickly it flows, hence how quickly the capacitor charges/discharges.
