what is a capacitor
- electrical component that stores charge between 2 plates
- insulator called dielectric placed between to prevent charge from travelling across gap
capacitance
charge stored per unit potential difference across 2 plates
relative permittivity
ratio of charge stored with the dielectric between the plates to the charge stored when the dielectric is not present
the greater the relative permittivity the greater the capacitance of the capacitor
er = Q/Q0
area under the graph of charge against pd represent
energy stored by the capacitor
describe the Q against t graph for the discharging of a capacitor through a resistor
exponential decay curve
describe the v against t graph for discharging of a capacitor through a resistor
exponential decay curve
describe the I against t graph for the discharging of a capacitor through a resistor
exponential decay curve
describe the Q against t graph for the charging of a capacitor through a fixed resistor
curves up and over
describe the v against t graph for the charging of a capacitor through a resistor
curves up and over
time constant
time it takes for the charge in a capacitor to fall to 37% of the initial value
given by RC
a capacitor is considered fully charged after 5 time constants
how was 37% derived when using the time constant
- start with formula Q=Q0E^-t/RC
- when t = rc the formula becomes Q=Q0E^-1
- E^-1 = 0.37 which is where 37% came from
half time of a capacitor
T1/2 = 0.69RC
what equations do we require for charging a capacitor
Q = Q0(1-E^-t/rc)
how does a capacitor charge
- electrons move from negative to positive around the circuit
- electrons are deposited on plate A, making it negatively charged
- electrons travel from plate B to the positive terminal of the battery giving 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 a 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, neutralising them
- pd decreases exponentially across the plates
use of capacitors
plash photograph, nuclear fusion, backup power supplies
expressions fro energy stored by a capacitor
E = 1/2(Q^2/c) = 1/2(QV) = 1/2(CV^2)
what factors affect the time taken for a capacitor to charge or discharge
- the capacitance of the capacitor. this affects the amount of charge that can be stored by the capacitors at any given potential difference across it
- the resistance of the circuit. this affects the current in the circuit and how quickly it flows hence how quickly the capacitor charges or discharges
