Capacitors Flashcards
(30 cards)
What is a capacitor and what does it store?
A device that stores charge and energy when a potential difference is applied.
What does the gradient of a Q–V graph represent for a capacitor?
The capacitance, which tells you how many coulombs of charge are stored per volt (unit: farad).
What is the basic equation relating charge, voltage, and capacitance?
Q = VC
How do you calculate the energy stored in a capacitor from a Q–V graph?
The energy is the area under the graph, which forms a triangle, so: E = 1/2 QV
What are the alternative formulas for energy stored in a capacitor?
E = 1/2 CV²
E = 1/2 Q²/C
Why do we often calculate charge using Q = It?
Because charge is hard to measure directly. This only works if current is constant.
Why do we use a variable resistor when charging a capacitor?
To keep the charging current constant, since it naturally starts high and falls to zero.
What happens to the PD across a capacitor when it’s fully charged?
It equals the PD of the battery.
Before full charge, where does the rest of the battery’s PD go in a circuit?
Across the resistor.
What is the equation for exponential decay of PD during capacitor discharge?
V/V₀ = e^(-t/RC)
What do the variables in the decay equation represent?
V: PD at time t
V₀: initial PD
R: resistance
C: capacitance
t: time
Does the decay equation work for charge and current as well?
Yes, the same exponential form applies.
What is the equation for charging a capacitor?
V/V₀ = 1 - e^(-t/RC)
What happens to current during capacitor charging?
It decays (does not increase).
How can you rearrange the exponential decay equation to solve for R or C?
Take the natural log: ln(V/V₀) = -t/(RC)
How do you graphically find the time constant from discharge data?
Plot -ln(V/V₀) vs. t: gradient = 1/(RC)
Or plot ln(V) vs. t: gradient = -1/(RC)
What is the time constant and how is it defined?
τ = RC. It’s the time for PD to drop to 37% of its initial value during discharge.
How do the units confirm that RC is a time constant?
Ω × Farads = seconds
What is the basic physical structure of a capacitor?
Two parallel plates that become oppositely charged when a PD is applied.
Is charge allowed to flow between the plates of a capacitor?
No.
How are capacitors often built physically in labs?
The plates (foils) are wound into cylinders.
What is capacitance proportional and inversely proportional to?
Proportional to plate area
Inversely proportional to plate separation
What is permittivity (ε) and how is it used?
A measure of how easily an electric field forms in a medium. Capacitance increases with higher permittivity.
How do we express permittivity with different materials?
ε = ε₀ × εᵣ
Where: ε₀: permittivity of free space
εᵣ: relative permittivity (>1)
