capacitors Flashcards
what is capacitance
the capacitance of a capacitor is the amount of charge it can store per unit potential difference
what is capacitance measured in
farads
equation for capacitance
C= Q/V
what are capacitors
they consist of two metal plates separates by a dielectric
what does the capacitance of a capacitor depend on?
surface area of plates,
separation of plates
dielectric being used
what does a polar dielectric consist of?
lots of polarised molecules each with a positive end and a negative end.
what happens when a dialectic is placed in a capacitor
when places in capacitorf the molecules align and all the positive ends are attracted to the negative plate and all negative ends are attracted to positive plates - results in capacitance increasing
Why do the capacitance results in the capacitor increase
Polarised molecules each have in an electric field around them
This electric field opposes the electric field of the capacitor plates
Therefore potential difference required to change the plates decreases
What happens to capacitance as potential difference is decreased
Capacitance increases
equations to calculate energy stored in capacitor?
E = 1/2 QV
E = 1/2 CV ^2
E = Q^2/C
how do we calculate energy stored by a capacitor using a graph?
area under a charge voltage graph
what does the gradient of a charge voltage give us?
capacitance
what is the time constant equal to?
product of resistance in the circuit and the capacitance of the capacitor
time taken to charge capacitor to (1 -1/e) of its _____ value
final
time taken to discharge the capacitor to 1/e of its ____ value
initial
what is relative permittivity (dielectric constant)
Ratio of the charge stored with the dielectric between the plates to the charge stored when the dielectric is not present
what increases when the relative permittivity increases
capacitance of the capacitor
describe graph of discharging capacitor v against t
negative exponential graph
describe graph of discharging capacitor I against t
negative exponential graph
describe Q against t graph for charging a capacitor
starts from orgin and curves
describe v against t graph for charging a capacitor
starts from orgin and curves
what is the time constant
time it takes for charge In capacitor to fall to 37% of the initial value
RC = resistance x capacitance
how many constant to fully discharge a capacitor
after 5 time constants
what is the half time of a capacitor
T1/2 = 0.69RC
how does a capacitor charge
- electrons move from negative to positive around the circuit
-electrons are deposited on Plate A which makes it negatively charged
-electrons travel from plate B to positive terminal of the battery which gives the plate a positive charge
-electrons build up on plate A and an equal amount of electrons are removed from plate B creating a pd across the plates
-when the pd across plates = source pd , the capacitor is fully charged and current stops flowing
describe and explain in terms of the movement of electrons how the pd across the capacitor changes when it discharges across a resistor
- electrons move in opposite direction than when the capacitor was charging up
- charge on one plate A decreases as it loses electrons and plate B gains electrons (making them neutral)
- pd decreases exponentially across plates
2 factors that affect time taken for capacitor to charge/discharge
- capacitance of capacitor
- resistance of the circuit as it affects the current
