CIE A Level Physics (9702) 2019-2021

Revision Notes

25.4.3 Forces in G, E & B Fields

Forces in G, E & B Fields

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 Fg is defined by:

Fg = mg

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

Forces in G, E & B Fields equation

  • 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 FE is defined by:

FE = 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 FB for an isolated moving charge is defined by:

FB = 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:

FB = BIL sin(θ)

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

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