capacitance

ε0

the charge stored on a capacitor consisting of two parallel plates of area 1m^2 separated by 1m when the potential difference between the plates is 1V

define capacitance

the charge stored by a capacitor per unit potential difference

define a capacitor

an electrical component which stores charge

what is a capacitor made up of

two conducting parallel plates with a gap between them

which may be separated by an insulating material known as a dielectric

permittivity

property of dielectrics

measure of the ability to store an electric field in the material

what is relative permittivity

the dielectric constant of a dielectric

what is relative permittivity used for

calculating the capacitance of the capacitor

how to calculate relative permittivity

find the ratio of the permittivity of the dielectric to the permittivity of free space

what is a dielectric formed of

polar molecules

what are polar molecules

molecules with one positive end and one negative end

polar molecules in dielectric with and without electric field

with - polar molecules move and align themselves with the field

without - molecules arranged in random directions

what does each polar molecule have

its own electric field

how do polar molecules in dielectric align when there is an electric field

negative ends rotated towards positive plate

positive ends toward negative plate

what does the strength of a polar molecule’s electric field depend on

the dielectrics permittivity

consequence of polar molecules having their own electric field

field opposes field formed by capacitor

reducing this field

potential difference required to charge capacitor decreases as electric field strength is decreased

capacitance increases

electrical energy stored by capacitor graphically

the area under a graph of charge against potential difference

how to charge a capacitor

connect it in a circuit with a power supply and resistor

how to plot a graph of voltage and current against time

use a data logger to measure the values of potential difference and current

how to draw charge against time graph

measure area under current-time graph

what happens when a capacitor is connected to a power supply

current starts to flow

negative charge build up on plate connected to negative terminal

opposite plate - electrons repelled by negative charge building on initial plate

electrons move to positive terminal

equal but opposite charge formed on each plate

creates potential difference

what happens as charge across plates of capacitor increases

potential difference increases

but electron flow decreases due to electrostatic force of repulsion also increasing

current decreases and eventually reaches zero

how to discharge a capacitor through a resistor

connect it to a closed circuit with just a resistor

capacitor discharging

current flow in opposite direction

I Q and V across capacitor fall exponentially

which I Q or V is the same for charging and discharging

current

define time constant

the product of resistance and capacitance

time taken to discharge a capacitor to 1/e of its initial value (of Q I OR V)

time taken to charge a capacitor to 1 - 1/e of its final value (of Q or V)

define time to halve

time taken for I Q or V of a capacitor to discharge to half of the initial value

time to halve equation

T1/2 = 0.69RC