electric fields

what is coulombs law

that the force between two point charges separated by a distance r in a vacuum is:

directly proportional to the product of the two charges

inversely proportional to the square of their separation

assumption made when calculating force between charges

the air is treated as a vacuum

compare electrostatic forces between subatomic particles and gravitational forces

the electrostatic forces between subatomic particles are many orders of magnitude greater than the gravitational forces

what do electric field lines show

the direction of the force on a point positive charge

define electric field strength. vector or scalar?

force acting per unit charge on a point positive charge

vector

magnitude of E in a uniform field

E=V/d

direction of electric field

higher potential to lower potential

magnitude of E in a radial field

E = Q/4pie0r²

how to find the work done by the electric field in moving the particle from one plate to the other

W = fd (force x distance in the direction of the force)

define electrical potential difference

the work done per unit charge in moving the charge between two points

describe the trajectory of moving charged particles entering a uniform electric field initially at right angles

shape of a parabola

define electric potential

work done per unit charge to move a small point positive charge from infinity to a point in the field

what is electrical potential at infinity

0

units for electrical potential

v or jc^-1

when charge is positive, electrical potential is….

when charge is positive, electrical potential is positive at point, the charge has more energy than at infinity because work must be done against the resistive force

when charge is negative, electrical potential is…

when charge is negative, electrical potential is negative as charge loses energy falling from infinity (electric field itself does work accelerating the point positive charge from infinity)

define electrical potential difference (deltaV). vector or scalar?

the work done per unit charge to move a point positive charge between two points

scalar

define equipotentials

lines joining points of equal potential

describe equipotentials

no work is done moving charge along on equipotential surface

equipotentials meet field lines at 90 degrees

describe equipotentials in uniform field

equally spaced and parallel

describe equipotentials in radial field

spherical equipotential surfaces, in 2D these appear as concentric circles

deltaV between equipotentials is constant

electrical potential in radial field features

charge on the surface of the conductor will always distribute so that the potential is the same over the whole surface

no change in the pd from one side of the sphere to the other so electric field strength inside the sphere is zero

area under e against r graph

= Edeltar = change in electric potential

accelerating charges in uniform fields

use E=V/d and F=QE to find force and hence acceleration (F=ma) on charged particles

as field is uniform, acceleration is constant so can use SUVAT

energy transfers in uniform fields

loss of electrical potential energy = gain in kinetic energy

motion of a charged particle moving perpendicularly across a uniform field

constant horizontal component of velocity

increasing vertical component of velocity caused by the electrostatic force due to the electric field

how to keep electron in constant motion without any work being done

move electron in a circle with a point charge at centre

as this is an equipotential surface

deltaV = 0 so deltaW = 0

when exxperimenting wire in magnetic field

choose non-magnetic material so only investigating effect of current

if applied alternating current instead of dc: wire would vibrate

when comparing size of deflection:

comment on charge, hence direction

compare masses - for larger mass, deflection is smaller

define electrical permittivity

flux of an electric field in a medium generated per unit charge

describe dielectric insulator

work is done on capacity separating positively charged surface of the dielectric from the negatively charged plate