10.2 - capacitors Flashcards
(29 cards)
What do capacitors do?
Store electric charge
What is a capacitor made from?
A pair of electrical conductors close together
sheets of metal foil with an insulating layer between them
How does a capacitor store charge?
Charges come in positive/negative pairs. Charging a capacitor involves using a potential difference to pull pairs apart and put a lot of positive charge on one plate and a lot of negative charge on the other. The charge then has nowhere to go if the circuit is not closed, so is stored on the plates
What happens to the potential difference as the amount of charge stored increases?
Potential difference increases
What is capacitance?
The charge stored per volt
What is the symbol for capacitance?
C
What is the equation used to work out capacitance?
C = Q/V
What are the units of capacitance?
Farads, F
What are the equivalent units to one Farad, F?
CV^-1
What does a larger capacitance mean for the amount for charge that can be stored?
A greater charge can be stored for a given potential difference
How does the charge on a capacitor decay when it is discharged?
Exponential decay
What effect does moving the plates of a capacitor closer together have? Which equation shows this?
More energy can be stored
E = V/d
What is the differential equation for rate of flow of charge when discharging a capacitor?
dQ/dt = -Q/RC
What conditions do there have to be for charge to decay exponentially?
Current is proportional to potential difference, capacitance is constant
What is the breakdown voltage of a capacitor?
The voltage at which a spark will cross between the plates
What is the equation used to find the remaining charge on the plates of a capacitor at a given time?
Q = Q0 x e^-t/RC
What is the symbol for the time constant?
τ (tau)
What is the equation for the time constant?
τ = RC
What does the time constant tell you?
Q is equal to Q0 x 1/e when t = τ
Q is equal to roughly 37% of its original value
What is the equation for the energy stored on a capacitor?
E = 1/2QV, where Q and V are the initial values
NOT E=QV
Why is E=QV NOT used for the energy stored on a capacitor?
E = 1/2QV is used because the potential difference drops as the capacitor discharges, the energy per unit charge falls during discharge. Energy is lost to resistance in the circuit.
The average is found be using 1/2
What are the units of the time contant, RC
s^-1
What are the units of -t/RC
No units (you cannot raise a number to the power of something which has any units)
What does the area under the curve represent on a charge/time graph?
The charge stored/moved
