6.1 Capacitors Flashcards
Define capacitance
Charge stored per unit p.d across the capacitor
What is the unit of capacitance
Farad
Define the farad
1 coulomb per volt
What happens when a capacitor is charging
-Switch closed
-Capacitor starts to charge as there is max current
-P.d across capacitor increases
-P.d across resistor decreases as p.d across capacitor increases
-Capacitor fully charged
-Current is 0
-Circuit set up with battery in series with resistor, capacitor and switch
What happens when a capacitor is discharging
-Switch opened
-Capacitor discharges through the resistor
-Charge stored by capacitor decreases
-P.d decreases
-Current decreases
-Eventually all reach 0
-Circuit set up with battery in parallel with capacitor, resistor and voltmeter and series with switch
How do you calculate the capacitance of capacitors in series
1/C = 1/C1 + 1/C2 …
How do you calculate the capacitance of capacitors in parallel
C = C1 + C2 …
Describe an experiment to investigate charge stored on a capacitor
-Set up a circuit with 2 capacitors in series with a switch, an ammeter and a resistor and voltmeters in parallel to capacitors
-Close the switch
-Current will fall to 0 quickly
-Measure the p.d on both capacitors
-Calculate charge using Q=VC
-Repeat to see that charge should be same on both capaciors
What does the area under a p.d - charge graph represent
Energy stored in capacitor
How can you increase the energy stored by a capacitor
Increase the p.d as work done is directly proportional to p.d squared
What are some uses of capacitors
Camera flashes
Touch screen devices like mobile phones
Back up power supplies
What does the graph of Q-t I-t V-t look like
Exponential decrease
Define the time constant for a discharging capacitor
Time taken for the p.d/Q/I to decrease to 37% of its initial value
When is the equation
X = x0 ( 1 - e^-t/RC ) used
When a capacitor is charging and for charge and pd only
