Chapter 21 - Capacitance Flashcards
Describe how capacitors are charged
- When a capacitor is connected to a cell, electrons flow from the cell.
- The electrons cannot pass from plate to plate due to the insulation, and so electrons build up on one plate and become deficient on the other
- the plate with lack of electrons has a positive charge and the plate with electrons has a net negative charge
- As the current must be the same at all points, charge is conserved meaning each plate has equal and opposite charge Q+ and Q-
- therefore a capacitor is much like a device used to separate charge
define capacitance
charge stored per unit potential difference across it
- C=Q/V so Q=VC
what is the unit of capacitance
Farad
1 farad = 1 coulomb/volt
Outline capacitors in parallel
C1+C2+C3 = Ctotal
Q1+Q2+Q3 = Qtotal
p.d. across each capacitor is the same
outline capacitors in series
1/C1 + 1/C2 = Ctotal
V1 + V2 = Vtotal
When investigating capacitors, why is there an initial spike in current when the switch is closed?
Electrons flow to capacitor until it is full then stop
where does the energy stored in the capacitor come from
the work done in pushing electrons to the negative plate and pulling them away from the positive plate
how to find the energy stored by a capacitor
- plot a p.d. against charge graph
- as C = V/Q
- area under graph is equal to the work done or energy stored
- work done = 1/2QV
which capacitor equation is used for charging capacitors
the 1 - … one
which capacitor equation is used for discharging
the normal one
what can discharging capacitors be modelled as
exponential decay
what is the time constant for a discharging capacitor
time taken for the p.d. to decrease to about 37% of it’s original value or 1/e
state two applications of capacitors
smoothing to allow a constant smooth charge for phones
flashes for cameras etc.
