Capacitors Flashcards
What is a capacitor
Type of electrical component made up of 2 conducting plates which is used to store charge quickly and then discharge quickly
How does a capacitor store charge and cause a uniform electric field
Connect one plate to positive end of power supply and one plate to negative which causes negatively charged electrons to store up on one plate, causing a pd and uniform electric field
Define capacitance
The amount of charge that a capacitor can store per volt applied between the plates
i.e. C = Q/V
Define permittivity
A measure of how difficult it is to produce an electrical field in that material
Why would a dielectric be placed in the between the plates of a capacitor
- Dielectric is a non-conducting material
- Increase the permittivity between plates
- Decrease electric field so less pd required to maintain the same charge
- Increased capacitance
How does the dielectric reduce the pd between the plates
- Dielectric made up of polar molecules which are positive on one end and negative on other
- They orient themselves so -ve side is attracted to +ve plate and +ve side is attracted to negative plate
- This reduces the net electric field and pd between the plates
What is relative permittivity
εr = permittivity of the dielectric / ε0
where ε0 is the permittivity of free space i.e. if there was no dielectric
What is the formula relating capacitance with area of the plates, distance between the plates and permittivity of the dielectric
- As area increases more charged stored so C ∝ A
- As d increases E weakens so more pd required to maintain charge so C ∝ 1/d
- As permittivity of dielectric increases C increases
C = Aε0εr / d
How can you calculate the energy stored by a capacitor
E = 1/2 QV
= 1/2 CV^2
= 1/2 Q^2 / C
How can you work out energy stored by a capacitor with a graph of Q against V
Area under the graph
Describe the graphs of how Q, V and I change over time for a charging capacitor
- Q and V increases but more slowly as time increases until it plateaus at max Q or V
- I decreases from max I but more slowly as time increases until it reaches 0
Describe the graphs of how Q, V and I change over time for a discharging capacitor
- Q, V and I all decrease with time but more slowly as time increases
What is the time constant
Time taken for the charge, pd or current to fall to 37% of max value or rise to 63% of max value
What is the formula for time constant
T = RC
so time taken to charge and discharge is determined by capacitance of capacitor and resistance of circuit
What are the formulas for calculating charge, voltage and current at a given time
If the graph is decreasing:
Q = Q0 e^-t/RC
(Could be I or V as well)
If the graph is increasing:
Q = Q0 (1 - e^-t/RC)
(Could be V)
How can you find the time constant from a graph of ln Q or ln V against t
gradient = -1/RC
How can you work out the time taken for Q, V or I to half
0.69 RC
(Can derive this by subbing in 1/2 Q0 for Q)
