Chapter 21 Capacitance

What is a capacitor

electric component that stores electrical charge across 2 parallel plates. The dielectric prevents charge travelling across the gap.

What is capacitance

charge stored per unit pd. C=Q/V Farads

How does a capacitor work

connect to dc power supply, brief current as power draws e- from one plate and deposits on the other. 1st plate charge Q+, 2nd Q- equal and opposite. Current flows until pd between plates = emf of power supply.

Dielectric increases capacitance

polarises in the electric field effectively increasing charge stored.

electrical permittivity

describes ability to polarise and strengthen charge storage capability. vacuum/ air little to no polarisation

Capacitors in series

1/C total = 1/C 1+2+3 kirchoffs voltage law, resistance in parallel

capacitors in parallel

C total = C 1+2+3 kirchoffs current law, resistance in series

when is work done by the power supply

to deposit e- on -ive plate as like charges repel according to coulombs law. also done removing e- from +ive plate.

voltage defined

electrical potential per unit charge, area under VQ graph is wd in charging.

converting ac to dc

sinusoidal ac wave passed through full wave rectifier, current flows in one direction but varies. current through smoothing circuit capacitor stores energy as pd rises discharges as pd falls

how is it discharged

disconnect power supply and connect to resistor, resistance and time constant know to high degree of accuracy

e- no longer held to +ive plate by emf of power supply

e- repel each other flow around circuit dissipating energy as heat in the resistor

charges on plate equilibrate, no pd e- cease to flow no current

what does time constant depend on

capacitance and resistance

how does resistance affect time constant

lower resistance, higher current ohms law

higher current charge falls to 0 faster as Q=IT

how does capacitance affect time constant

larger capacitance larger charge stored per unit pd, pd proportional to current, larger time constant

pd before resistor connected

V at its max V=Q/C where Q is initial charge

once connected e- flow and charge stored will decrease, decreasing pd and current

for discharing capacitor, when t=t

charge stored decreased to 37% of original value.