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AS and A Level: Fields & Forces

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What are gravitational fields?

  1. 1 A gravitational field is a region where a mass experiences a force. The field strength, g, at any point in the field is given by g=F/m and the value of g on the Earth’s surface is taken to be 9.81Nkg-1.
  2. 2 Field lines point towards the centre of the Earth and are radial. Over small distances, near Earth's surface, g can be considered constant so field lines are parallel and the field is uniform.
  3. 3 G was calculated by Henry Cavendish by measuring the force of attraction between two lead spheres of known mass and separation. The force between two masses is given by F = Gm1m2/r2 and this is called Newton’s law of universal gravitation.
  4. 4 Inside the Earth, g falls from 9.81 to 0 Nkg-1 so we cannot use the inverse square law for r < RE.
  5. 5 Combining Newton’s law with circular motion can be used to calculate distance to geostationary satellites.

What are electric fields?

  1. 1 An electric field is a region where a charge experiences a force. The field strength E at any point in the field is given by E = F/Q. The force between two charges is given by Coulomb’s law.
  2. 2 For radial fields, E = 1/ Q/r2 and this is another inverse square law. For uniform fields, E = V/d.
  3. 3 Uniform electric fields can be set up to accelerate charges. The work done accelerating a charge through a p.d. V is given by W = QV. The unit of energy can be given in Joules (J) or electronvolts(eV).
  4. 4 When a charge enters a uniform electric field, such as between the deflection plates of an oscilloscope, there will constant acceleration and so suvat equations can be used.
  5. 5 For all electric fields, equipotential lines are drawn perpendicular to field lines. For radial fields, always show at least 3 equipotential lines as concentric circles with increased spacing.

    The equipotential lines can be experimentally determined using conductive paper, metal electodes and a voltmeter to map out points of equal potential. You should be able to draw equipotential patterns for two point charges.

Similarities and differences between gravitational and electric fields.

  1. 1 Gravitational forces are always attractive but electric forces can be both attractive and repulsive. There are no negative masses but there are negative charges.
  2. 2 The ratio of the strength of the two forces is huge. For two electrons, FE/FG is approximately 1042. This demonstrates how much stronger the electric force is compared to the gravitational force over the same distance.
  3. 3 Both fields obey an inverse square law.
  4. 4 Over short ranges, electric forces dominate but over much larger distances, say between planets and their moons, gravitational forces dominate because the attractive and repulsive electric forces tend to cancel out.

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