Capacitance (Unit 4) Flashcards
Series?
-capacitance - 1/c
- voltage- size
-charge - equal
Parallel?
-capacitance- c1+c2
- voltage - equal
-charge- add
What is a capacitor?
-a pair of conducting plates separated by an insulator
How does a capacitor work (store energy)?
-When no p.d. across the capacitor the plates are neutral
and they have an equal number of electrons and positive ions
-When a p.d. is applied to the plates, electrons are forced to move and the plates acquire equal and opposite charge (net charge is zero).This stores energy until the electrons can flow back
Definition of capacitance?
C - charge on plate/ pd between plates
C - Q/V
Units of capacitance?
- F - CV^-1
How to increase capacitance of a vacuum spaced capacitor?
- using a dielectric
What is a dielectric?
- an insulator between the plates of the capacitor
Electric field strength units?
Vm^-1
Field lines in a capacictor?
- equal and parallel
- uniform in between plates
How is a capacitor charged?
- 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 positive charge
-electrons build up on plate A and an equal amount of electrons are removed from plate B, creating a PD across the plates
-when the PD across plates= source PD, the capacitor is fully charged and current stops flowing
How does PD across a capacitor change, when it discharges across a resistor?
-electrons move in opposite direction than when the capacitor was charging up
-charge on plate A decreases as it loses electrons and plate B gains electron, so A and B become neutral
-PD decreases exponentially across the plates
Equation for charging a capacitor?
Q=Qo(1−e^−t/RC)
Equation for discharging a capacitor?
Q=Qoe^-t/RC
What is the time constant?
- RC
- time for charge to decrease to 37% of it’s original value
Investigation of the charging and discharging of a capacitor to determine the time constant practical ?
- set up prac
- start stopwatch
- record initial voltage
- record voltage every 5 seconds until voltage reaches 0v
- plot a graph voltage over time
- find what the time when the voltage is 37 percent of the initial
-this gives the time constant
What does the investigation of the charging and discharging of a capacitor to determine the time constant practical look like?
- Battery connected to parallel with a capacitor first then a resistor connected to a voltmeter
-when charging capacitor switch makes first loop closed - when discharging first loop is open
Investigation of the Energy Stored in a Capacitor practical?
-Charge the capacitor by closing the switch and then record the
reading on the voltmeter.
- Repeat this for different capacitors so that the effect of
capacitance on energy stored can be evaluated.
- The energy stored by the capacitor (E) is found by E=½ CV 2
where C is capacitance and V is the voltage across the
capacitor.
- Work out the energy stored by each of the capacitors
Investigation of the Energy Stored in a Capacitor what does it look like?
- battery connected to capacitor and voltmeter in parallel with switch in series for first loop
V over t graph for discharging of a capacitor through a fixed resistor?
- downwards slide hitting x axis and levelling off
I over t graph for discharging of a capacitor through a fixed resistor?
- downwards slide hitting x axis and levelling off
Q over t graph for discharging of a capacitor through a fixed resistor?
- log graph
time constant percentage?
37 percent
How is ammeter and voltmeter connected?
-ammeter connected series to whatever measuring
- voltmeter in parallel
time constant for LOG GRAPH?
- 63 percent
