Section 10 - Capacitors Flashcards
(24 cards)
What is the relative permittivity (aka dielectric constant)?
- The ratio of the charge stored with the dielectric between the plates to the charge stored when the dielectric is not present
- εr = Q/Q0
- The greater the relative permittivity, the greater the capacitance of the capacitor
What is the mark scheme definition of the dielectric constant?
permittivity of medium/permittivity of free space
How does the dielectric constant affect capacitance?
With a dielectric, the capacitance of the capacitor is multiplied by the dielectric constant
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 capcitor through a resistor
A graph that looks like a half life graph, the graph hits the y axis
Describe the V agaisnt t graph for the discharging of a capacitor through a resistor
A graph that looks like a half life graph, the graph hits the y axis
Describe the I agaisnt t graph for the discharging and charging of a capacitor through a resistor
A graph that looks like a half life graph, the graph hits the y axis
Describe the Q against t graph for the charging of a capacitor through a fixed resistor
An upside down half life graph that meets the origin
Describe the V agaisnt t graph for the charging of a capacitor through a fixed resistor
An upside down half life graph that meets the origin
What is the time constant?
The 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 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 is approximately 0.37
What is the half time of a capacitor?
T1/2 = 0.69RC
What equations do we require for charging a capacitor?
Q=Q0(1-e^-t/RC)
V=V0(1-e^-t/RC)
V0 is the battery PD and Q0=CV0
How does a capacitor charge up?
- Electrons move from negative to positive around the circuit
- The 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 postitive charge
- Electrons build up on plate A and an equal amount of electrons are removed from plate B, creating a potential difference across the plates
- When the p.d. across plates = source p.d., the capacitor is fully charged and the current stops flowing
Describe and explain in terms of the movement of electrons how the p.d across a capacitor changed, when it discharges across a resistor
- Electrons move in opposite direction than when the capacitor was charging up
- Charge on one plate decreases as it loses electrons and the other plate gains elecrons, neutralising them
- P.d decreases exponentially across the plates
State the 3 expressions for the energy stored by a capacitor
E = 1/2(Q^2/C) = 1/2(QV) = 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 amount of charge that can be stored by the capacitors at any given potential difference 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
How does inserting a charged dielectric change the capacitance?
- The dielectric aligns with their positive side facing the capacitor’s plates negative side
- This produces a counter electric field
- The p.d. reduces between the capacitor plates, but the charge stays the same
- C=Q/V therefore capacitance increases
What is the energy difference equation?
E=1/2Q^2 (1/c2-1/c1)
If the same maximum capicitance is required using plates with half the diameter, what are 2 ways to acheive this? Include numerical data
- Insert a dielectric with a relative permittivity of 4
- Reduce the air gap by 1/4
How do you draw a dc supply oscilloscope with the same rate of energy dissipation as an ac.
Draw a sraight horizontal line at the Vrms
How do you find the frequency from an oscilloscope?
T = Number of divisions across the whole wave x time per division
f=1/T
State and explain a change to one control setting on the oscilloscope that would reduce the uncertainty in the value of the time constant
- Reduce the time base setting
- Uncertainty is due to the smallness of the divisions and this means the waveform will be stretched horizontally
What is the use of a mirror when doing an analogue reading?
- Move position until the needle is aligned with its reflection in the mirror
- Reduces parallax error
