Gravitational Fields

• A gravitational force acts on anything with mass
  • This is always an attractive force and acts on both stationary and moving masses
• Inside a uniform gravitational field, the gravitational force F_g is defined by:

F_g = mg

• Outside a uniform gravitational field (ie. outside the surface of a planet), the force is defined by Newton’s Law of Gravitation:

• Where:
  • G = Newton’s gravitational constant
  • M₁ and M₂ = two point masses (kg)
  • r = separation between the centres of the masses (m)
• This equation tells us that the gravitational force will cause a force of attraction between any two masses

Electric Fields

• An electric force acts on anything with a charge
  • This can be either attractive or repulsive and acts on both stationary and moving charges
• The electric force F_E is defined by:

F_E = QE

• Where:
  • Q = size of the charge (C)
  • E = electric field strength (N C^-1)
• The electric force causes opposite charges to attract and like charges to repel
• When a charge is moving in an electric field, the electric force will cause the particle to move in a parabolic motion

Magnetic Fields

• A magnetic force acts on anything with a charge that is moving within a magnetic field
• The largest force occurs when the particle is moving perpendicular to the magnetic field lines
• The magnetic force F_B for an isolated moving charge is defined by:

F_B = BQv sin(θ)

• Where:
  • B = magnetic flux density (T)
  • v = velocity of the charge (m s^-1)
  • θ = angle between the charge’s velocity and the field lines (degrees)
• When a charge is moving normal to a magnetic field, the electric force will cause the particle to move in circular motion
• The force for a current-carrying wire inside an external B field is defined by:

F_B = BIL sin(θ)

• Where:
  • I = current (A)
  • L = length of the conductor (m)
  • θ = angle between the conductor and the field lines (degrees)