Module 6.1 Flashcards
Capacitance
The charge stored per unit pd in a capacitor
Capacitor
An electrical component that stores charge. A parallel-plate capacitor
is made of two parallel conducting plates with an insulator between them
(dielectric).
Capacitors in Parallel
When capacitors are connected in parallel, their individual
capacitances are summed to give the total capacitance.
Capacitors in Series
When capacitors are connected in series, the total
capacitance is equal to the inverse of the sum of the inverses of the individual
capacitances
Energy Stored by a Capacitor
Equal to half the product of the charge stored
and the capacitance. This can be found from the area under a charge-voltage
graph.
Farad
The unit of capacitance
Time Constant
The product of the circuit resistance and capacitance. It is the
time taken for the voltage to discharge to 1/e (or 36.8%) of its initial charge.
What is the relative permittivity and what is another name for it
- Dielectric constant
- The ratio of charged stored with the dielectric between the plates to the charge stored when the dielectric is not present
- Permitivity = Q/Qo
- The higher the relative permittivity, the higher the capacitance of the capacitor
What does the area under the graph against pd represent
The energy stored by the capacitor
Describe the Q against t graph for the discharging of a capacitor through a resistor
Exponentially decreasing curve
Describe the V against t graph for the discharging of a capacitor through a resistor
Exponentially decreasing curve
Describe the I against t graph for the discharging of a capacitor through a resistor
Exponentially decreasing curve
Describe the Q against t graph for the charging of a capacitor through a resistor
Exponentially increasing curve
Describe the V against t graph for the charging of a capacitor through a resistor
Exponentially increasing curve
What is the time constant
The time taken for the charge in a capacitor to fall to 37% of the initial value given by RC (resistance x capacitance)
- A capacitor is considered fully discharge after 5 time constants
What is the half time of a capacitor
1/2T = 0.69RC
How does a capacitor charge up
1) Electrons move from negative to positive around the circuit
2) The electrons are deposited on plate A, making it negatively charged.
3) Electrons travel from plate B to the positive terminal of the battery giving the plate a positive charge
4) Electrons build up on plate A and equal amount of electrons are removed from plate B, creating a potential difference across the plates
5) when the p.d across plates = source p.d, the capacitor is fully charged and current stops flowing.
Describe and explain in terms of the movement of electrons how the p.d across a capacitor changes, when it discharges across a resistor
1) Electrons move in opposite direction than when the capacitor was charging up
2) Charge on plate A decreases as it loses electrons, and plate B gains electrons, neutralizing them
3) P.d decreases exponentially across the plates
State 6 uses of capacitors
- Flash photography
- Nuclear fusion
- Backup power supplies
- DC blocking
- Smoothing AC to DC
- Tuning (Resonating magnetic field)
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
