4.2 Electric and Gravitational Fields

• Electric fields

  regions where an object experiences electric force
• Radial electric fields

  exist around point charges and uniform spheres
• Electric field strength, E

  force per unit test charge

  E=F/q

  units= NC^-1
• Coulombs Law

  the force between 2 point charges is

  1. directly proportional to the product of their charges
  2. inversely proportional to the square of their distance apart
  3. at its maximum when the particles are separated by a vacuum
• Electric Potential, V

  the electrical potential of a point within an electric field is equal to the work done per coulomb of positive charge in bringing the charge from 'infinity' to that point
• Electric Potential Gradient

  the change in potential per metre at a point within an electric field

  Potential gradient, E= ΔV/Δd

  units: JC^-1 or Vm^-1
• Gravitational Field Strength, g

  a gravitational field is a region within which an object experiences a gravitational force proportional to its mass, M

  g= force on mass/mass

  Units: Nkg^-1
• Newton's Law of gravitation

  every particle of matter in the universe attracts every other particle with a force that is

  1. directly proportional to their masses
  2. inversely proportional to the square of their distance apart

  F= Gm(1)m(2)/r^2

  where G= universal constant of gravitation
• Gravitational Potential, V

  the gravitational potential of a point within a gravitational field is equal to the work done per kg of mass in bringing the mass from 'infinity' to that point

  Units: Jkg^-1
• Conservation of energy in a gravitational field

  when objects move in a gravitational field, the principle of conservation of energy applies:

  gravitational P.E + K.E at point a = gravitational P.E + K.E at point b
• Escape Velocity

  the minimum velocity at which a projectile must be launched to completely escape the influence of the planet it is launched from
• Potential gradient

  the change in potential per metre at a point within a gravitational field

  potential gradient= ΔV/Δr

  Units= Jkg^-1