Studied by 2 people
What is a capacitor?
An electrical component that stores charge
What does a capacitor look like?
2 separated metallic plates which store charge, with an insulator placed between them to prevent charge travelling across the gap
What is capacitance? (C)
Charge stored per unit pd across the 2 plates
What equation links capacitance, charge, and pd?
C=Q/V
What is capacitance measured in?
Farads (f)
What is the relative permittivity?
Ratio of charge stored with the dielectric between plates to charge stored without dielectric present
What happens to pd across a capacitor as it is charged?
Initial pd is 0
Increases with time
What happens to pd across a resistor as a capacitor is charged?
Initial pd = pd of cell
Decreases with time
What happens to charge when a capacitor is connected to a dc power supply?
Power supply draws electrons from 1 plate and deposits them on the other
First plate has charge +Q second has charge -Q
These charges are equal and opposite
What happens to current as a capacitor charges?
Decreases at the same rate as the pd across the resister
Eventually reaches 0
What is a dielectric?
An insulator
How does the dielectric affect the capacitance of the device?
Increases the capacitance: polarises in the electric field, increases charge stored on the plates
How is a capacitor charged?
Elections flow from negative terminal of cell to negative plate
Excess electrons on negative plate repel electrons off positive plate which move towards positive terminal of cell
Plates acquire an equal and opposite charge
What is kirchoff's voltage law?
The sum of the EMFs in any closed loop is equal to the sum of the potential differences in that loop
How can we find the total capacitance in series?
1/CT = 1/C1 + 1/C2 + 1/C3
What is the total charge in series?
QT=Q1=Q2
What is kirchhoff's current law?
Total current flowing into a node = total current flowing out of node
How can we find the total capacitance in parallel?
CT = C1 + C2 + C3
What is the total charge in parallel?
QT = Q1+Q2
When is work done in a capacitor?
To push electrons to the negative plate
To pull electrons away from positive plate
What is voltage?
Electrical potential energy per unit charge
What does the area under a voltage-charge graph represent?
Work done in charging up the capacitor - the energy stored in the capacitor
What are the energy equations for capacitors?
W = ½QV
W = ½CV
W =Q²/2C
Why are capacitors useful?
They can store and discharge large quantities of energy in a short time
What are some applications of capacitors?
Short pulses of energy: camera flashes, touch screens
UPSs (backup power supplies): in data centres to protect hardware, hospitals in life support machines
To convert AC to DC: in a smoothing circuit a capacitor stores energy as pd rises and discharges as it falls
How can a capacitor be discharged?
Disconnect the power supply
Connect a resister across capacitor
Excess electrons flow from negative plate to positive plate, decreasing charge
What is the time constant?
A measure of how long exponential decay will take in a circuit
Time taken for pd, current, or charge to fall to 0.37 of its original amount
What is time constant equal to?
t= CR
What are the discharging equations for pd, current, and charge?
V = V0e^(-t/CR)
Replace V with I or Q
Describe the discharging graphs of V, I, and Q against time
Exponential decreasing curve
How can a graph of lnV against t be compared to y=mx+c?
lnV/t = lnV0 - 1/ť
1/ť = m
lnV0 = c
Describe iterative modelling in capacitors?
(Change in Q)/( change in t) = - Q/CR
Start with known Q0 and ť
Pick t much smaller than ť
Calculate charge leaving capacitor: (change in Q) = (change in t)xQ / CR
Charge left: subtract change in Q
What is the charging equation for pd or charge of capacitor?
Vc = V0(1-e^(-t/CR))
Describe the charging graph of pd and charge against time
Increasing, exponential decrease
