3.7.3.1

Electric and gravitational fields share structure but differ in sign

Electric Fields & Potential — AQA A-Level Physics

Similarities
  • Both forces follow an inverse square law with separation.
  • Both field strengths follow 1/\(r^{2}\) relationships (radial field).
  • Both potentials follow 1/r relationships.
  • Field lines around a point mass and a negative point chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). are identical (both point inward).
  • Uniform gravitational fieldA region of space in which a mass experiences a gravitational force. lines and uniform electric field lines are identical (parallel, equally spaced).
  • Equipotential surfaces are spherical around a point source and parallel lines in a uniform field.
  • Work $done = (mass$ or chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).) x (change in potential): $DeltaW = M DeltaV$ or $DeltaW = Q DeltaV$.
Differences
  • Gravitational force acts on mass. Electric force acts on chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C)..
  • Gravitational force is always attractive. Electric force can be attractive or repulsive.
  • Gravitational potentialThe work doneEnergy transferred when a force moves an object. In electrical circuits, W = QV (charge times potential difference). per unit mass in bringing a small test mass from infinity to that point. Always negative. Measured in J kg⁻¹. is always negative. Electric potentialThe work doneEnergy transferred when a force moves an object. In electrical circuits, W = QV (charge times potential difference). per unit positive charge in bringing a small test charge from infinity to that point. can be positive or negative.
  • Gravitational fields are relatively weak: G is much smaller than k.
  • Gravitational fieldA region of space in which a mass experiences a gravitational force. has no equivalent of a 'neutral point' between two masses (both attract).
Examiner Tips and Tricks
  • This comparison can appear as a 6-mark extended answer.
  • Prepare a structured list of similarities and differences.
Related:Current & Charge Newton's Law of Gravitation Gravitational Field Strength Gravitational Potential Gf1
Electric Fields & Potential Overview
Equipotential surfaces are perpendicular to field lines 13 of 14 Worked example: Coulomb's law with an alpha particle and gold nucleus