Capacitance Flashcards
Explain what is meant by the capacitance of an object.
The amount of charge it is able to store per unit potential difference across it.
Describe the structure of a capacitor.
Two electrical conducting plates.
Separated by an insulator known as a dielectric.
What determines the voltage a capacitor will charge up to?
A capacitor will only charge up to the voltage of the cell or battery charging it.
Why do capacitors have a voltage rating?
This is the maximum potential difference that can be safely put across it.
What does a Q-V graph look like for a capacitor and what does the gradient represent and the area underneath represent?
Gradient = capactitance
Area = (electrical potential) energy stored
Why is energy stored by a capacitor?
Work has to be done to push electrons onto the negative plate and to remove electrons from the positive plate.
What does permittivity mean?
Measure of how difficult it is to generate an electric field in a certain material.
Higher value = more difficult = weaker field strength for a given charge.
What is εr?
Relative permittivity (or dielectric constant)
Ratio of the permittivity of a material to the permittivity of free space.
Has no units.
How can εr be calculated for a material?
εr = ε1 / ε0
Describe the molecules in a dielectric.
They are polar molecules.
So have a positive end and a negative end.
Describe the orientation of the dielectric molecules when no charge is stored by the capacitor.
Molecules are aligned randomly.
Describe and explain the orientation of the dielectric molecules when charge is stored by the capacitor.
An electric field is generated across the dielectric.
Negative ends of molecules attracted to the positively charged plate and vice versa.
Molecules rotate and align with electric field.
Explain the effect that the dielectric molecules have on the overall electric field.
The dielectric molecules each have their own electric field.
Which opposes the electric field of the capacitor.
Causing the overall field to be reduced.
How does the permittivity of the dielectric affect the overall field?
Larger permittivity of the dielectric = larger opposing field.
Greater reduction in overall field.
Explain the effect this has on the capacitance of the capacitor.
Overall electric field E reduces and E = V/d, so this reduces the p.d. needed for a given charge.
As C = Q/V, the capacitance increases.
If a change is made to a capacitor whilst still connected to the battery, what happens?
Voltage across it must stay the same.
Capacitance changes, which causes charge stored to change.
If a capacitor is disconnected from a battery and a change is made to it, what happens?
Charge stored must stay the same.
Capacitance changes, which causes voltage across it to change.
Explain, in terms of electrons, what happens when a capacitor is charged.
Electrons are repelled by the negative terminal and build up on one plate making it negative.
Electrons are also attracted to the positive terminal and removed from the other plate making it positive.
Plates gain an equal and opposite charge.
No electrons flow directly between plates due to insulation.
Describe the energy transfers that take place when charging through a fixed resistor.
Chemical energy in battery is transferred to electrical potential energy in the capacitor and to thermal energy in the resistor which is dissipated to the surroundings.
(Half the energy to the capacitor, half to the resistor).
When charging through a fixed resistor, sketch a graph to show how current changes over time and give the equation. What does the area under the graph represent?
Graph of I = I0e-t / RC
Area under graph = charge stored.
How can the initial current I0 be determined?
Applying Kirchoff’s voltage law: 𝜀 = 𝑉𝑅 + 𝑉𝐶
Which gives: 𝜀 = 𝐼𝑅 + 𝑄/𝐶
Initially Q = 0, so this gives 𝐼𝑜 = 𝜀 / 𝑅
When charging through a fixed resistor, sketch a graph to show how potential difference changes over time and give the equation.
V = V0(1 - e-t / RC)
When charging through a fixed resistor, sketch a graph to show how charge changes over time and give the equation. What does the gradient of the graph represent?
Q = Q0(1 - e-t / RC)
Gradient = current
Define time constant (RC) in terms of current, potential difference and charge when charging through a fixed resistor.
Time for the current to decrease to (1 / e) or 37% of its original value.
Time for potential difference or charge to increase to 1 − (1 / e) or 63% of its final value.
