Electrostatics

Charge of an electron

1.6 × 10^-19 C

Relative mass of protons/neutrons and electrons

Protons/neutrons are 2000 times heavier than electrons

Coulomb’s force

F = k (q1. q2 / r²)

k is constant: 9 × 10 ^9, N m² C^-2

Dielectric constant (Er)

Dictates how much insulating material weakens an electric field compared to a vacuum

dielectric constant formula

Er = E / E0

where: E is the absolute permittivity of the medium, E0 is the permittivity of free space/ the vacuum

electrostatic force and dielectric constant formula (Coulomb’s Law)

units: N

where ke: Coulomb’s constant = 9 × 10 ^9, Er: dielectric constant,

• in a vacuum Er is equal to 1

notes on dielectric material in electrostatic force

Inserting a dielectric material always decreases the net electrostatic force between two charges by a factor of Er

ke (Coulomb’s Constant) in formula ( for electrostatic force and dielectric constant)

where E0: permittivity of free space, Er: dielectric constant/ relative permittivity

dielectric material in an electric field

Em = Eo / Er

where Em: electric field inside a material, E0: electric field in a vaccum (or air), Er: dielectric constant

notes on dielctric material in an electric field

Adding a dielectric material weakens the internal electric field because its internal polarization opposes the external field.

capacitance for dielectric material in parallel plate capacitor

C = Er . C0

where C : capacitance, C0: capacitance in a vacuum, Er: dieelctric constant

capacitance

C = Q/V, in F

capacitance in a vacum formula (C0)

C0 = (E0 . A)/ d

where C0: capacitance in a vacuum, A: Area of the plates, d: Distance between plates

notes on dielectric material and capacitance

Inserting a dielectric increases the capacitance by a factor of Er

electric field formula

E = F / q, units: N/C

electric field due to point charge

E = k ( q / r²)

electric potential energy

U = (k . q1. q2) / r, units: J

uniform electric field

E = V/d, units V/m

energy and potential formula

U = q . V or ∆U = q. ∆V, units: J

where q is charge in C, V is voltage/electric potential in V

formulas for energy stored in a capacitor

• U = ½ CV²

• U = ½ QV

• U = Q² / 2C

If the capacitor is still connected to a battery…

the Voltage (V) stays constant

Use U = ½ CV²

If the capacitor is disconnected from the battery…

the Charge (Q) stays constant

Use U = Q² / 2C

dielectric in a capacitor (simpler formula)

C = Er . C0