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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.

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

    Experiment to determine gravity from a spring using digital techniques

    3 star(s)

    Procedure Apparatus * Stand * Motion sensor * Computer with datastudio installed * Slotted masses and mass holder * CD * Pointer * Half metre stick * Balance accurate to 1g * Lab jack * Spring Part One * The experiment is set up as shown. * Datastudio is opened on the computer, and "create experiment" is clicked. * In the "create experiment" window, the motion sensor is selected. * In the measurement window, "Position, Ch1 & 2 (m)" is selected. * In the "display" window, the "graph" selection was double clicked to set up a graph of the results.

    • Word count: 1561
  2. Peer reviewed

    Investigating the forces acting on a trolley on a ramp

    5 star(s)

    Finally, the angle of the ramp was altered and the measurements taken again for three different angles. By measuring the vertical height at the point where the light gate was and a fixed distance from this, it was possible to calculate the exact angle at which the ramp rested by using simple trigonometry. Theory There were two ways to approach the experiment. It is possible to calculate gravity (g) by considering the 'conservation of energy', which calculates the gravitational potential energy of the trolley and uses this information to find the acceleration of the trolley. In this experiment, the variable is the vertical component of the ramp.

    • Word count: 5816
  3. Peer reviewed

    paper cones investigation

    5 star(s)

    air C Displacement of fall C Time taken to fall D Prediction Since the theory suggests that So the square of time should be directly proportional to the inverse of mass. A straight line graph of t2 versus m-1 should give a straight line graph with a gradient of All of these values will be measurable or known, except for the drag coefficient, c. Method Preliminary experiments Determining a size of the paper cone: Three unweighted paper cones were constructed from A4 paper and selotape: w is the width (diameter)

    • Word count: 2095
  4. Peer reviewed

    The Compound Pendulum

    5 star(s)

    * 'h' is the distance of the pivot from the centre of gravity in metres - independent variable * 'g' is the acceleration due to gravity - dependant variable By plotting T against h at this point, the graph will be a parabola. This is because T is proportional to h, and the equation is not in the form of y = mx + c, in which case the graph would be a straight line. The above equation can also be written including the mass of the pendulum.

    • Word count: 1972
  5. Peer reviewed

    Falling parachute experiment

    4 star(s)

    The object will accelerate to higher speeds before reaching a terminal velocity. Thus, more massive objects fall faster than less massive objects because they are acted upon by a larger force of gravity; for this reason, they accelerate to higher speeds until the air resistance force equals the gravity force [1]. Method The apparatus used in the experiment are a plastic bag, scissors, a set of 5 paperclips, a ruler, stopwatch or wristwatch with ability to read to at least 0.1 s, notebook and pencil.

    • Word count: 766
  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)

    3 Connect the circuit as the diagram showed. 4 Twine the wire on the magnet with 20 turns. 5 Turn on the switch and record 5 successive readings on the graphical calculator as 'X1 T' (since the reading changes all the time) 6 Turn off the switch and change only and increase the number of turns on the magnet by 10 turns. 7 Turn on the switch and record the new 5 successive readings on the graphical calculator as 'X2 T' 8 Repeat step 6&7 for another 4 times and correlatively get X3 X4 X5 X6 T. Part 2: Size of the current 1 Measure the room temperature and record as 't'.

    • Word count: 1503
  7. Peer reviewed

    Determination of the acceleration due to gravity (g)

    4 star(s)

    this investigation, I am going to determine the acceleration due to gravity on the earth by using an electronic timer and varying its height of dropping. In this method, a steel ball is hold by a free fall adaptor (ball release mechanism) , when we release the ball, the current to the circuit is switched on and the ball begins to fall. At the same time an electronic timer starts. The ball falls through a receptor pad and this will break down the circuit to stop the electronic timer.

    • Word count: 2347
  8. Peer reviewed

    Energy and its uses

    3 star(s)

    Elastic potential energy (EPE) is the energy stored in bodies such as springs, elastic and rubber bands. An archer drawing a bow applies EPE to the bow string. science.howstuffworks.com 4. Chemical energy is the stored energy possessed by foods, fuels and batteries. A human eating food is taking in chemical energy. http://en.wikipedia.org/wiki/Battery_ (electricity) 5. Thermal or heat energy is energy that flows by conduction, convection or radiation from hot areas to cold it can also be a by product of wasted energy during energy conversions http://www.oxfordreference.com/pages/VED_samples 6.

    • Word count: 2756

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