electric fields and capacitance

Coulomb’s Law

magnitude of force between two point charges in a vacuum is directly proportional to the product of their charges and inversely proportional to the square of the distance between the charges

electric field strength

force per unit charge experienced by a charged object in an electric field

electric potential

work done per unit charge on a positive charge in bringing it from infinity to that point in the field

greatest at the surface of a charge

0 at infinity

electric potential difference

energy needed to move a unit charge between two points

capacitance

charge stored by a capacitor per unit potential difference

what is a capacitor

2 conducting parallel plates with a gap between them

they may be separated by a dielectric

when connected to a power supply, opposite charges build up on the plates so a uniform electric field is formed

permittivity

property of dielectrics

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

relative permittivity formula

ε = 

dielectric

formed of polar molecules, one end is positively charged and the other is negative

when there is no electric field, the molecules are arranged in random directions

when there is an electric field, they move and align themselves with the field

each molecule has its own electric field (the strength depends on the permittivity)

these oppose the field formed by the capacitor, so the electric field strength decreases

potential difference required to charge the capacitor decreases

capacitance increases as C = Q/V

how do you charge a capacitor

connect it in a circuit with a power supply and resistor

what happens in the charging of a capacitor

current flows and a negative charge builds up on the plate connected to the negative terminal

on the opposite plate electrons are repelled by the negative charge building up on the other plate

electrons move to the positive terminal

equal but opposite charges formed in each plate

potential difference is created

as the charge across the plates increases, the potential difference increases but electron flow decreases due to the increasing force of electrostatic repulsion

the current decreases and eventually reaches zero

how do you discharge a capacitor

connect it to a closed circuit with a resistor

what does the time constant (RC) represent in charging

time taken to charge a capacitor to 1 - 1/e of its initial value

what does the time constant (RC) represent in discharging

time taken to discharge a capacitor to 1/e of its initial value