chapter 21 - capacitance Flashcards
capacitor
stores charge
- 2 conducting plates
- separated. by an insulator - dielectric
electrolytic capacitor
- dielectric is an electrolyte
- only works when connected right way round
capacitance
charge stored per unit pd
C = Q/V
measured in Farads
parallel plate capacitor
C = Aε/d
as A increases more charge can build up on the plate so C increases
as d decreases plates exert a greater force so more electrons move so charge increases C increases
dielectric
between plates of a capacitor
- molecules become polarised in an electric field
- align with the field
- attract more electrons onto the plate increasing charge increasing C
permitivity ε
is larger with a dielectric
capacitor in series
1/Ct = 1/C1 + 1/C2 ….
capacitor in parallel
Ct = C1 + C2….
energy stored in a capacitor
= 1/2QV
= 1/2CV^2
= 1/2 Q^2/C
less than E = QV as other half of energy lost through heat so not stored
discharging a capacitor
initially there is a voltage across the plates
V decreases as Q moves from the plates Q = CV
initially I when charge is moving from plates as Q = It when no pd no moving charge so no I
discharging equations
Q = Q0 e^-t/RC
V = V0 e^-t/RC
I = I0 e^-t/RC
time constant
RC
time taken for the pd to decrease to 1/e of its initial value
discharging when t = RC
Q = Q0/e
V = V0/e
I = I0/e
1/e = 0.37
discharging graphs
all downwards curve start fast then slows
can ln and plot
ln against t
eg lnQ = lnQ0 - t/RC
grad = -1/RC
charging a capacitor
starts with 0 charge so 0 pd
electrons repelled from neg terminal and attracted from pos
charge builds up on plates increasing pd
but as I = V/R it starts at max I
as pd builds up harder to force charge on the plates so rate of flow of charge is lower until I = 0 and pd = emf
charging equations
Q = Q0 (1-e^-t/RC)
V = V0 (1-e^-t/RC)
I = I0 w^-t/RC
charging when t = RC
Q = (1- 1/e) Q0
V = (1- 1/e) V0
I = 1/e I0
1- 1/e = 0.63
charging graphs
V and Q upward curves start fast then slow
I same as discharging
V across resistor same as I ( emf - V)
capacitor uses
- releasing energy quickly eg flash
- can store energy
- can smooth voltage
rectifier
4 diodes in a diamond
only allows +ve current to flow - makes ac source become dc
but instead of not allowing neg current just makes it become positive
capacitor use in a rectifier
can be used to smooth current
when V starts to decrease the capacitor is charged so holds some charge and V - keeping it flowing between the humps
making to become almost a constant direct voltage
reduce bumps by increasing RC
