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.

171 AS and A Level Fields & Forces essays

  • Marked by Teachers essays 1
  • Peer Reviewed essays 9
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  1. Marked by a teacher

    Experiment to determine gravity from a spring using digital techniques

    3 star(s)

    Overall this is a good report which addresses many of the key points in a practical.

    It could have been improved further by taking repeated readings and being more…

    • Essay length: 1561 words
    • Submitted: 13/02/2012
    • Marked by teacher: (?) Pete Golton 06/06/2013
  2. Peer reviewed

    Investigating the forces acting on a trolley on a ramp

    5 star(s)

    The author’s response to the question is very good – they have explained the experiment clearly, discussed their results, and evaluated the uncertainties. They have calculated a range of possible…

    • Essay length: 5816 words
    • Submitted: 20/03/2012
    • Reviewed by: (?) dragonkeeper13 21/03/2012
  3. Peer reviewed

    paper cones investigation

    5 star(s)

    The writer has approached the investigation well. As a result, the writer has produced a well written report. All of the writers objectives have been clearly covered, therefore making their…

    • Essay length: 2095 words
    • Submitted: 15/02/2009
    • Reviewed by: (?) cpdavis 13/03/2012
  4. Peer reviewed

    The Compound Pendulum

    5 star(s)

    The candidate has not completed the work so has not answered the question. However, this is a great start. The explanation of theory is good, an important step that is…

    • Essay length: 1972 words
    • Submitted: 19/06/2008
    • Reviewed by: (?) k9markiii 06/03/2012
  5. Peer reviewed

    Falling parachute experiment

    4 star(s)

    This answer is OK but far from perfect. It is customary to put a hypothesis or prediction from theory of what you expect to happen and then link back to…

    • Essay length: 766 words
    • Submitted: 11/05/2012
    • Reviewed by: (?) k9markiii 25/07/2012
  6. Peer reviewed

    Investigate four factors that may affect the strength for electromagnets: the number of turns, the size of the current, the nature of the current (a.c. or d.c.) and the distance between the sensor and the magnet.

    4 star(s)

    The candidate has carried out an experiment to investigate how four factors affect the strength of the magnetic field of an electromagnet. They have successfully answered the question, but there…

    • Essay length: 1503 words
    • Submitted: 07/05/2012
    • Reviewed by: (?) 09/09/2012
  7. Peer reviewed

    Determination of the acceleration due to gravity (g)

    4 star(s)

    There are several misconceptions and lack of precision with physical ideas in the work. Notably, not mentioning that all bodies with mass are affected by gravity, that mass is independent…

    • Essay length: 2347 words
    • Submitted: 15/07/2008
    • Reviewed by: (?) k9markiii 05/03/2012
  8. Peer reviewed

    Energy and its uses

    3 star(s)

    This information booklet contains a nice summary of the different types of energy, and examples of energy changing between these forms. The writer refers to 'applying' different types of potential…

    • Essay length: 2756 words
    • Submitted: 01/12/2008
    • Reviewed by: (?) hassi94 11/03/2012
  9. Peer reviewed

    Electromagnetic Radiation.

    3 star(s)

    The candidate gives a good response and the information provided is scientifically accurate. However their response could be improved by linking together other topics within physics such as polarisation and…

    • Essay length: 569 words
    • Submitted: 02/03/2004
    • Reviewed by: (?) pictureperfect 28/06/2012
  10. Peer reviewed

    Particle accelerators are used to study matter and energy.

    3 star(s)

    The candidate has written a suitable introduction, they clearly state what a particle accelerator is, what it is used for and they have listed the topics they plan to cover.…

    • Essay length: 392 words
    • Submitted: 30/08/2003
    • Reviewed by: (?) pictureperfect 02/07/2012

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