Capacitance Flashcards
Capacitors
Electrical components in which charge is separated
Dielectric
Insulator which separates plates in a capacitor
How does charge build in a capacitor
Electrons flow due to the potential difference across the cell. Electrons are removed from one plate and flow around the circuit to the other but cannot cross the dielectric.
The two plates build up an equal and opposite charge
Current falls to zero when pd across capacitor is equal to the emf
Capacitance
The amount of charge a capacitor can store for a given pd. C=Q/V
Capacitance in parallel
Pd across each capacitor is the same
Charge is conserved - Q=Q1+Q2+Q3…
C=Q/V
C=C1+C2+C3…
Capacitance in series
V= V1+V2+V3…
All capacitors have the same charge
1/C = 1/C1 +1/C2 +1/C3 …
emf is split between components depending on capacitance - Q is equal so V is proportional to 1/C
Work done in a capacitor (explanation)
Work must be done to pull electrons to negative plate and push electrons away from positive plate
Work is done by battery/power supply
Energy stored by capacitor(equation)
Work done = area under V against Q graph
W=1/2 QV = 1/2 V^2C = 1/2 Q^2/C
Discharging capacitors
When capacitor is connected to a resistor (no power pack), the capacitor will discharge
x = x0e^(– t/CR)
Where x= I, V or Q
Time constant
CR
the time taken for pd to decrease to e^-1 (37%) of its initial value
Calculating capacitance from graph
v= v0e^(-t/CR)
ln(v) = ln(v0) - t/CR
ln(v) against t
gradient = -1/CR
Charging capacitors
I =I0e^(-t/CR) same as discharging
x= x0(1-e^(-t/CR))
where x is V or Q
Uses of capacitors
Used to store and release large amounts of energy very quickly
Used for camera flashes, particle accelerators, defibrillators, back up power for computers, emergency lights, hospitals
Smoothing alternating current
How can a capacitor smooth alternating current
Diode is used to only allow current in one direction
Capacitor smooths out curve because current decreases exponentially
Effect is larger with a larger time constant
