E fields

Coulomb's Law

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. Attractive for opposite charges and repulsive for like charges

Magnitude of electric force compared to gravitational

Electric force > Gravitational force

(masses are small but much larger charges)

Ratio = electric/gravitational

Electric Field Strength

Force per unit charge experienced by an object in an electric field

Constant in uniform field (equally spaced field lines), varies in radial

Field line direction

Shows direction of force acting on a positive charge

How to find out charge of particle via uniform field

Fire particle perpendicular to field lines, charged particle experiences constant electric force, causes particle to accelerate so follows parabolic shape

Positive charge = follows direction of field

Negative charge = opposite direction of field

Absolute electric potential

Potential energy per unit charge of a positive test charge at that point in the field

Where is absolute electric potential greatest?

At the surface of the charge

As distance increases, potential decreases so potential at infinity is zero

What does the sign of Absolute Potential Energy depend on?

Depends on sign of the charge:

Positive charge = Positive potential (force is repulsive)

Negative charge = Negative potential (force is attractive)

Significance of gradient on potential against distance graph

Gradient of tangent = electric field strength at that point

Electric Potential Difference

Energy required to move a unit charge between two points

Equipotentials in radial and uniform fields

Uniform fields - Equipotentials are equally spaced parallel to the plates

Radial fields - Equipotentials are concentric circles around a point charge

(no work is done moving a charge along an equipotential surface)

Area under Electric Field Strength against distance graph significance

Area = Electric Potential Difference

Capacitance

Charge stored by a capacitor per unit potential difference

Capacitor

Electrical component which stores charge made up of two conducting parallel plates, separated by dielectric (insulating material). Creates uniform field

Permittivity

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

Relative permittivity/dieletric constant

Used to calculate capacitance of a capacitor

𝜖𝑟=𝜖/𝜖0

where er = dielectric constant, e = permittivity of substance

Electric Potential

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

Permittivity of free space

A measure of the ability of a vacuum to allow an electric field to pass through it

Electric field

A region surrounding a charged object which causes a force to be exerted on any charged object placed within the field