capacitance

Definition of capacitance and equation

The ability of the capacitor to store charge

C=Q/V

C=Ax free space x permittivity/d (separation of plates)

What are polar molecules and what does it do

Partial + and - at each end, electrically neutral

Rotates in presence of electric field and cancels field to reduce p.d and increase capacitance

What is permittivity

Resistance to an electrical field passing through it

What is the relative permittivity in air at 1 atmosphere and vacuum

1.0006 and 1

What happens with the energy stored in a charging capacitor

Chemical energy of battery to electrical potential energy in capacitor and therm energy to resistor and surroundings

Or half energy from battery converted into thermal and half stored in capacitor

What happens with the energy stored in a discharging capacitor

Electrical energy from capacitor is all converted into thermal

How to calculate energy stored in a capacitor

E=1/2 QV

½ CV²

½ Q²/C

What happens in a charging capacitor

No charge or p.d across capacitor

Current flow is at max

Electrons removed from positive plate to negative

Charge on negative plate builds up and makes it hard for electrons to get on

Current reduces and p.d across capacitor =source of emf

What happens in a discharging capacitor

Max p.d and max current

Electrons flow from negative to positive

Charge, p.d and current reduces

All electrons are on positive plate and everything is equal to 0 now

What is time constant

Time taken for p.d. And Q to rise 63% of max value when charging

Discharging is a fall of 37%

What is the equation of time constant

RC or Q/I

How to calculate half life

Time constant x ln2

RC x ln2