Gravitational Field Lines

• The direction of a gravitational field is represented by gravitational field lines
• The gravitational field lines around a point mass are?radially inwards
• The gravitational field lines of a uniform field, where the field strength is the same at all points, is represented by?equally spaced parallel lines
  • For example, the fields lines on the Earth’s surface

Gravitational field lines for a point mass and a uniform gravitational field

• Radial fields are considered?non-uniform fields
  • The gravitational field strength?g?is different depending on how far you are from the centre
• Parallel field lines on the Earth’s surface are considered a?uniform field
  • The gravitational field strength?g?is the same throughout

Point Mass Approximation

• For a point outside a uniform sphere, the mass of the sphere may be considered to be a?point mass?at its centre
  • A uniform sphere is one where its mass is?distributed evenly
  • The gravitational field lines around a uniform sphere are therefore?identical to those around a point mass

• An object can be regarded as point mass when:

A body covers a very large distance as compared to its size, so, to study its motion, its size or dimensions can be neglected

• An example of this is field lines around planets

Gravitational field lines around a uniform sphere are identical to those on a point mass

• Radial fields are considered?non-uniform?fields
  • So, the gravitational field strength?g?is different depending on how far an object is from the centre of mass of the sphere

Representing Electric Fields

• The direction of electric fields is represented by electric field lines
• Electric field lines are directed from positive to negative
  • Therefore, the field lines must be pointed?away?from the?positive?charge and?towards?the?negative?charge

• A radial field spreads uniformly to or from the charge in all directions
  • e.g. the field around a point charge or sphere

• Around a?point charge, the electric field lines are directly radially inwards or outwards:
  • If the charge is?positive?(+), the field lines are radially?outwards
  • If the charge is?negative?(-), the field lines are radially?inwards

Electric field lines point away from a positive charge and point towards a negative charge

• This shares many similarities to radial gravitational field lines around a point mass
  • Since gravity is only an attractive force, the field lines will look similar to the negative point charge, whilst electric field lines can be in either direction

• A uniform electric field has the same electric field strength?throughout the field
  • For example, the field between oppositely charged parallel plates
  • This is represented by?equally spaced?field lines and shares many similarities to uniform gravitational field lines on the surface of a planet

• A?non-uniform?electric field has varying electric field strength throughout
  • The strength of an electric field is represented by the spacing of the field lines:
  • A?stronger?field is represented by the field lines which are?closer?together
  • A?weaker?field is represented by the field lines which are ?further?apart

• The electric field lines are directed from the?positive?to the?negative?plate

The electric field between two parallel plates is directed from the positive to the negative plate. A uniform E field has equally spaced field lines

• A radial field is considered a?non-uniform?field
  • Electric field strength?E?varies with distance from a charged particle

• Arrows (representing force on a positive test charge) point?from the positive charge to the negative charge