What is a capacitor?
A pair of conducting plates separated by an insulator
What is a capacitor used for?
Used to store energy
How does a capacitor store charge
When a p.d is applied across the plates, they acquire equal and opposite charges. The electrons are forced to move, so the capacitor stores energy until the electrons can flow back.
What is capacitance
A conductors ability to store charge
C=Charge on either plate/P.D between the plates
C=Q/V
How can capacitance be increased without changing the dimensions of the capacitor?
Inserting a layer of insulating material between the plates known as a dielectric
Does a capacitor store charge?
No, as the combined charge of both the plates is 0. It does however, store energy
How can the energy be calculated?
U=1/2 QV
By combining the equation for internal energy and capacitance, what equations arise from this
U=1/2 CV^2 and U=1/2 Q^2/C
What is created due to one plate being positive and the other being negative?
A uniform Electric field, E=V/D
What is true about capacitors in series
P.d is shared across the capacitors but the charge is equal
What is true about capacitors in parallel
P.d is equal across each capacitor but the charge will be shared
How to calculate total capacitance for capacitors in parallel
Ct=C1+C2+…
How to calculate total capacitance for capacitors in series
1/Ct=1/C1 + 1/C2 + …
Explain how a capacitor charges up
- Switch is closed
- Electrons flow to the right plate from left plate
- Opposite charges build up on plates
- P.d across capacitor increases
- Capacitor charges
To investigate a charging capacitor, is the resistor in series or in parallel with the capacitor?
Series
What happens to current when the capacitor is charging?
Current decreases due to increasing opposing voltage
State the relationship between Q and V.
Q is directly proportional to V.
So can substitute V for Q in the Charge equation
Explain how a capacitor discharges
- Close switch to charge the capacitor
- Open the switch and the capacitor begins to discharge
- It is an example of exponential decay
- Electrons flow from the negative plate, through the resistor and back to the positive plate.
To investigate a discharging capacitor, is the resistor in series or in parallel with the capacitor?
Parallel
What is RC
The time constant. The time taken for the charge to decrease to 1/e of its initial value
