7.2 Capacitors Flashcards
What is a capacitor?
Capacitors store electric charge, which creates a build up of electric potential energy
How do capacitors work?
They are made in the form of two conductive metal plates connected to a voltage supply (parallel plate capacitor)
Electrons are transferred from one plate to another, creating a positive charge on one plate and a negative charge on the other. This process continues until the voltage across the capacitor equals the voltage of the power supply.
Electrons are also pulled off the other plate from the power supply, increasing the charge
What is a dielectric?
A substance that is a poor conductor of electricity, but a good source for electric fields
It ensures charge does not flow across the parallel plates
- so a p.d can be created
What is Capacitance, C?
The charge stored per unit potential difference (between the plates)
So the higher the capacitance, the more charge that the capacitor can store with the potential difference given
C = Q/V
Where C=Capacitance (Farads, F)
How does electrical energy become stored within the plates of the capacitors?
The power supply ‘pushes’ electrons to one of the metal plates
- therefore doing work on the electrons and electrical energy becomes stored on the plates
What happens as more and more electrons are put onto one plate?
The amount of charge increases, so when more electrons are added the electrons already there repulse against it, so more work done is needed to increase the charge
- electrostatic repulsion
Leading to the potential difference across the capacitor increasing as well
How do we calculate the energy stored within a capacitor?
W=1/2 QV
Where W=energy stored
Also:
W=1/2 CV^2
W=Q^2/2C
What’s electrostatic repulsion?
When there becomes so many electrons stored up in the capacitor, the current of them stop as they are being repulsed too hard from entering
How is a capacitor charged?
Electrons get pushed into the capacitor building up a negative charge on one plate
The other plate matches this charge by releasing electrons creating a positive charge
The p.d and Q increase fast then slow down until capacitor is full
- its full when electrostatic repulsion stops the current flowing completely
- the current decreases exponentially
when the p.d across the capacitor becomes the same as the power supply, it stops increasing
How does a capacitor discharge?
Capacitors discharge through a resistor (could be a bulb etc) with no power supply present
Electrons flow backwards from negative terminal to positive
Does this until p.d = 0
During the discharge, current, p.d and charge decrease exponentially
- follow pattern of exponential decay
- rate of change decreases at constant rate, e
What’s the exponential discharge equations?
Q=Q(0)e^(-t/RC)
I=I(0)e^(-t/RC)
V=V(0)e^(-t/RC)
Where Q = charge now
Q(0) = Initial charge
t = time (secs)
R = Resistance (ohms)
C = Capacitance (F)
What’s the time constant of a capacitor during discharge?
The time constant is how long it takes to decrease to 1/e (37%) of it’s initial charge
The time constants are constant
So we can do this several times and get the same constant for that capacitor.
current, p.d, and charge all have the same time constant for a certain capacitor
