Capacitors Flashcards
What is capacitance?
The amount of charge an object is able to store per unit potential difference across it
What is the unit for capacitance?
Farads
What are capacitors made up of?
Two electrical conducting plates separated by an electric insulator(dielectric)
Describe the current-time graph for a capacitor
- Current in microamps
- As time increases, current stays the same
- Area under the graph is charge stored on plates, Q = It
Describe the charge-p.d graph for a capacitor
- Charge in microcoulombs
- Charge calculated by Q = It
- Q and p.d are directly proportional
- Gradient of the graph = Capacitance
State 2 applications of a capacitor and describe how they works
- Camera flash: camera battery charges the capacitor for a few seconds and all the charge is put into the flash almost instantly
- Power supplies: The capacitor smooths out the variations in the current supplied
What happens to the plates of a capacitor while it charges?
One becomes positively charged while the other becomes negatively charged
Derive the energy stored equation for a capacitor
- charge-p.d graph is directly proportional
- Area under graph = Energy stored by capacitor
- Area of a triangle = 1/2 x base x height
- Energy stored by capacitor = 1/2QV
What is permittivity?
A measure of how difficult it is to generate an electric field in a medium
What are polar molecules?
Molecules with one end being positively charged and the other end being negatively charged
Describe the alignment of molecules in a dielectric without an electric field
The molecules are aligned randomly throughout the dielectric
Describe the alignment of molecules in a dielectric with an electric field
- The negative ends of the molecules are attracted to the positively charged plate
- The positive ends of the molecules are attracted to the negatively charged plate
- Each molecule has its own electric field, which opposed the applied electric field of the capacitor
Describe the flow of electrons as a capacitor charges
- Electrons flow from the negative terminal of the supply onto the plate connected, building a negative charge on the plate
- This repels the electrons on the other plate, moving them to the positive terminal of the supply, making that plate positive
Describe the current-time graph for a charging capacitor through a fixed resistor
- y intercept = Initial current
- As time increases, current decreases
- L shaped
Describe the p.d-time graph for a charging capacitor through a fixed resistor
- As time increases, voltage increases
- Exponential for max voltage
- r shaped
Describe the charge-time graph for a charging capacitor through a fixed resistor
- As time increases, charge increases
- Exponential for max charge
- r shaped
Describe the discharging current-time graph for a discharging capacitor through a fixed resistor
- y intercept = initial charge
- As time increases, current decreases
- L shaped
Describe the p.d-time graph for a discharging capacitor through a fixed resistor
- y intercept = initial p.d
- As time increases, p.d decreases
- L shaped
Describe the charge-time graph for a discharging capacitor through a fixed resistor
- y intercept = initial charge
- As time increases, charge decreases
- L shaped
State the log-linear equation for charge of a discharging capacitor
ln(Q) = (-1/RC)t + ln(Qo)
- y axis = ln(Q)
- gradient = (-1/RC)
- x axis = t
- y intercept = ln(Qo)
What is the time constant?
- When t = RC
- Also the time taken for the charge of a discharging capacitor to fall to about 37% of initial charge
State the time to halve equation
T(1/2) = ln(2)RC = 0.69RC
Derive the equation for the time to halve
- Q = 1/2Qo = Qoe^t/RC
- 1/2 = e^-t/RC
- ln(1/2) -> ln(2^-1) -> -ln(2) = -t/RC
- t = ln(2)RC
