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Force in Uniform Fields

Physics: Cambridge International A Level, topic 4.1.2

Force on a Mass in a Uniform Gravitational Field

Objects with mass feel a force when they are placed in a uniform gravitational field. This force is called the weight of the object.

Uniform gravitational field

Uniform gravitational field

  • The gravitational field lines of the Earth are directed radially inwards towards the centre of the Earth.
  • Near the surface of the Earth, the field lines are approximately parallel.
  • We can consider the gravitational field of the Earth as being a uniform field close to the surface.
  • The magnitude of the gravitational field strength gg near the surface of the Earth is approximately constant and equal to 9.8 m/s.
Weight

Weight

  • A particle of mass mm in a uniform gravitational field feels a force WW that is given by:
  • W=m×gW=m\times g
  • This force is known as the weight of the object.

A Uniform Electric Field

A uniform field is one which has the same strength at every point in the field.

Uniform field

Uniform field

  • A uniform field has the same strength everywhere, it can be made from parallel charged plates.
  • If two charged plates are held a distance apart the field between them will be uniform
  • The field lines will travel straight from the positive plate into the negative plate.
Calculating the field

Calculating the field

  • If two plates are held apart with different charges they will have a voltage difference between them. The equation for the field strength is:
  • Field strength = voltage drop ÷ distance between the plates
  • E=V/dE=V/d
Force on a charged particle

Force on a charged particle

  • Similarly to an object of mass in a uniform gravitational field, a charged particle feels a force in a uniform electric field.
  • The force is equal to the size of the charge qq multiplied by the strength of the electric field EE:
  • F=E×qF=E\times q

Test yourself on Force in Uniform Fields

  • What is the value of acceleration due to gravity at the Earth's surface?

  • An object's weight is 100 N. If the gravitational field strength is 2.5 N/kg, what is its mass?

  • Which of the following are true of weight?

  • What are the units for gravitational field strength?

  • What is the equation for the field strength of a uniform field?

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