Module 6.1 - Capacitors ✓ Flashcards
How do capacitors in parallel add?
Ct = C1 + C2 + …
How do capacitors in series add?
1/Ct = 1/C1 + 1/C2 …
What is the structure of a capacitor?
Two large metal plates with a dielectric (insulator) between them
What is the equation for capacitance?
C = Q/V
Define capacitance
Capacitance is the charge per unit voltage stored by a capacitor
How does a capacitor work?
When connected to a power source, electrons move away from one plate and collect on the other, leaving one positively charged and one negative. A p.d. is created across the two plates, with a uniform electric field between them. When connected to a load, electrons from the -ive plate will flow round the circuit to the positive plate, powering the load.
What is the equation for work done removing negative charge form one plate of a capacitor and adding it to the other?
W = ½ * Q * V
What are the three equations for energy stored by a capacitor?
W = ½QV W = ½V^2C W = (Q^2)/(2C)
What are some of the uses of capacitors
- Flash photography
- Backup power supplies (e.g. give pc time to shut down properly in case of a power outage)
- AC to DC converters
What does a graph of charge against time for a charging capacitor look like?
Q l _ _ _ l / l l -------------- t
same thing for V against t
What two factors does the time taken to charge/discharge a capacitor depend on?
- The capacitance (C) of the capacitor (affects the charge that can be transferred at a given voltage)
- The resistance of the circuit (affects the current in the circuit)
What are the equations for _____ of a discharging capacitor?
1) Charge
2) Current
3) p.d.
Q = Q₀e^(-t/RC) I = I₀e^(-t/RC) V = V₀e^(-t/RC)
What are the equations for _____ of a charging capacitor?
1) Charge
2) Current
3) p.d.
Q = Q₀(1 - e^(-t/RC))
I = I₀e^(-t/RC)
(but travels in opposite direction to charging current)
V = V₀(1 - e^(-t/RC))
What is taken as the time for a capacitor to discharge ‘fully’
5RC (5 * time constant)
What is special relationship occurs in exponential decay?
For a given proportion, it always takes the same amount of time for that proportion (e.g. of charge) to be lost
For example:
It always takes the same length of time for the charge on a capacitor to halve regardless of the starting charge
