What is a capacitor?
electrical component that stores charge on 2 separate metallic plates
An insulator, callad a dielectric, placed between the plates to prevent the charge from travelling across the gap
What is capacitance?
the charge stored per unit of potential difference across the two plates
capacitance equation?
C = Q / V
What is capacitance measured in?
Farads, F
Or 1CV^(-1)
What is the relative permittivity?
Ratio of the charge stored with the dielectric between the plates to the charge stored when the dielectric is not present.
What is another name for relative permitivity?
dielectric constant
What is the equation for the total capacitance in series?
1/Ctotal = 1/C1 + 1/C2 +…
What is the equation for the total capacitance in parallel?
Ctotal = C1 + C2 +…
What does the area under the graph of charge against pd represent?
The energy stored by the capacitor
Describe the Q against t graph for the discharging of a capacitor through a resistor
charge decreases with time, curved
Describe the V against t graph for the discharging of a capacitor through a resistor
V decreasing with time, curved line
Describe the I against t graph for the discharging of a capacitor through a resistor
current decreases over time, curved line
Describe the Q against t graph for the charging of a capacitor through a resistor
charge increases with time, curved line
Describe the V against t graph for the charging of a capacitor through a resistor
V increases with time, curved line
What is the time constant?
time it takes for the charge in a capacitor to fall to 37% of the initial value, given by RC
After how many time constant is a capacitor considered fully discharged?
after 5 time constants
How was 37% derived when using the time constant?
Start with the formula Q = Q0e^(-t/RC)
When t = RC (after 1 time constant), the formula becomes Q=Q0e^(-1)
e^(-1) = 0.37, which is where 37% came from
What is the half time of a capacitor?
T 1/2 = 0.69RC
What equations do we require for charging a capacitor?
Charging up a capacitor produces Q = Q0(1-e^(-t/RC)) & V = V0(1-e^(-t/RC)) where V0 is the battery PD and Q0 = CV0
How does a capacitor charge up?
1) Electrons move from neg to pos around circuit
2) electrons deposited on plate A, making it neg charged
3) electrons travel from plate B to pos terminal of battery, giving the plate a pos charge
4) electrons build up on plate A and an equal amount of electrons are removed from plate B, creating a pd across the plates
5) when the pd across plates = source pd, the capacitor is fully charged and current stops flowing
describe & explain in terms of the movement of electrons how the pd across a capacitor changes, when it discharges across a resistor
1) electrons move in opposite direction than when the capacitor was charging up
2) charge on one plate A decreases as it loses electrons, and plate B gains electrons, neutralising them
3) pd decreases exponentially across the plates
state some uses of capacitors
flash photography
nuclear fusion
backup power supplies
DC blocking
smoothing AC to DC
tuning (resonating magnetic field)
state 3 expresssions for the energy stored by a capacitor
E = 1/2 (Q^2 /C) = 1/2QV = 1/2 CV^2
What 2 factors affect the time taken for a capacitor to charge or discharge?
The capacitance of the capacitor, C. This affects the amt of charge that can be stored b y the capacitors at any given pd across it.
The resistance of the circuit, R. This affects the current in the circuit and how quickly it flows, hence how quickly the capacitor charges/discharges
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Essential Formulas5
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Materials31
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Electricity81
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Em Spec7
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3.2 Forces in Action15
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3.1 motion26
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3.3 Work, energy, power8
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3.4 materials24
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3.5 newton's laws of motion12
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4.1 Charge and Current11
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4.2 energy, power and resistance28
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4.3 electrical circuits23
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4.4 waves61
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4.5 Quantum49
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Thermal Physics34
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5.3 Oscillations13
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Gravitational20
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Astrophysics & Cosmology39
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Capacitors26
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Eectric Fields (mine Not Pmt)15
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Electric Fields28
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Module 4 definitions84
