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About Physics Practical

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  • Moderator
    eagle's Avatar
    23,328 posts since Aug '01
    • This is from this thread http://www.sgforums.com/forums/2297/topics/326238 which has gone out of point... So I'm starting anew... Will compile the above into my list as I go along these few days...

      When I complete it, it will be placed in ExamWorld as well.

      This is a list I will give to my tuition students. However, I'm not that selfish to keep it for them only.... I share with everyone :D
      Hopefully, it can help forumers do well in their Physics SPA.


      A) Display of Data

      Each student is advised to
      (a) present data by tabulating them neatly, showing ALL observations which have been taken.
      (b) head tables using the GIVEN symbols and GIVEN unit. If no units are given, use the most convenient unit.
      (c) note that all physical quantities should be given units.
      (d) give at least 5 sets of readings unless otherwise stated.
      (e) plan the readings so that they are uniformly spread out within the given limits. In any case, try to use the maximum range.


      B) Accuracy of Readings

      Data recorded should reflect the sensitivity of the measuring instruments used, e.g...
      (a) metre rule ===> to the nearest mm, e.g. 0.500 m, 40.0 cm, etc
      (b) thermometer ===> to the nearest half degree, e.g. 35.5 °C, 60 °C, 76 °C (98.0 °C, 32 °C however are incorrect)
      (c) stopwatch ===> to the nearest hundredth of a second, e.g. 34.34 s, 0.63 s
      (d) voltmeter readings and resistance ===> to one decimal place, e.g. 2.0V, 0.6V, 3.5 Ω, 0.7 Ω
      (e) ammeter readings ===> to two decimal places, e.g. 0.20 A, 0.34 A


      1) Readings or calculated values in general do not exceed 2 significant figures as we cannot plot values of a quantity to more than 2 significant figures on graph paper in most cases.
      2) The calculated quantity cannot be more accurate than the least accurate quantity measured.
      E.g. V = 2.0 V ,I = 0.12A, R = V/I = 2.0/0.12 = 16.7Ω or 17Ω


      C) Graph

      The majority of experiments in Physics require the drawing of a graph, for a graph is usually the best way of obtaining the average of a set readings.

      When a graph is plotted, always remember
      (a) Write down the title of the graph plotted
      (b) Label both axes with the quantities and their units
      (c) Scaled used should occupy more than half of the area of the space available.
      (d) If a scale is suggested, follow it. If not, scales based on multiples of 2 or 5 are used. Avoid using multiples of 3.
      (e) Each point should be clearly and neatly marked with a dot and circled (diameter of circle should not exceed 4 mm) OR marked with cross (diagonals of cross should not exceed a 2 mm by 2 mm square)
      (f) All points must be correctly plotted within the grid on the graph paper.
      (g) Draw the graph line or curve as a single fine pencil line or a smooth curve, through the best mean position defined by the points. (If one of the points is far from the line, repeat the experiment for a value near this point.)


      D) Calculation of Gradient (for straight line only)

      (a) A large enough triangle must be chosen (at least 5 cm by 5 cm) and clearly indicated on the graph to work out the gradient, preferable about 3/4 of the line drawn.
      (b) The method of calculating the gradient should be properly shown.
      (c) If possible, give the unit to the final answer of the gradient.
      (d) Try to fill up more than half of the graph paper by adjusting your scales.


      E) Evidence on graph

      Evidence on how a reading is obtained from a graph must be shown clearly with dotted reference lines.

      Edited by eagle 14 Jan `09, 12:13AM
  • Moderator
    eagle's Avatar
    23,328 posts since Aug '01
    • F) Some COMMON PRECAUTIONS in Physics Experiments


      1) Using pins
      (a) the pins were placed vertically upright. (Check with protractor or set-square.)
      (b) the 2 pins used to locate the path of a light ray were placed more than 6 cm apart to minimise errors. (Check that pin-pricked points are clearl marked and labelled)
      (c) View the base of the pins when aligning pins in a straight line as the pins may not be perfectly upright.

      2) Using lenses
      (a) Object and image distances should not be measured along a line parallel to the principal axis. A set-square is used to minimise parallax errors.
      (b) The lens must be upright and parallel to the screen.
      (c) The illuminated object should be placed at the same height as the optical centre of the lens.


      1) Timing oscillations
      (a) Disturbance due to wind was avoided by switching off the fans and closing windows and doors.
      (b) Ignored the first few oscillations and started timing only when oscillations were steady.
      (c) Made sure that the angle of swing was small during timing (less than 5°).
      (d) The length of the pendulum was measured from the point of suspension to the centre of the bob.
      (e) When the oscillations of the pendulum became elliptical, the timing was rejected.

      2) Parallax Errors
      Whenever there is a gap between the scale and the mark to be read, parallax error is present.
      (a) When reading the measuring cylinder or thermometer - parallax errors were minimised by viewing the reading at eye-level and at the meniscus.
      (b) When reading meteres such as voltmeter, ammeter, etc - parallax errors were minimsed by viewing the pointer from directly above such that the pointer coincided with its image.

      3) When judging whether apparatus is horizontal or vertical,
      (a) Make use of the vertical edge of a door, blackboard, a wall or a plumbline to judge whether apparatus is vertical a not.
      (b) Use the teachers' bench or the horizontal edge of the blackboad to judge whether or not your apparatus is horizontal.


      1) The water or liquid in the vessel was stirred constantly during the experiment so that the temperature was uniform throughout the water. (Note: Do not use the thermometer as a stirrer)

      2) The thermometer was not placed at the bottom of the beaker when the water was heated. This is because the temperature of the beaker is usually higher than that of water. Overheating may lead to over-expansion of the mercury thread thus breaking the thermometer.

      3) Temperature readings should be taken only when they are steady.

      4) When comparing heat loss due to a change, e.g. the colour of the cooling vessel, make sure that all parameters remain the same except for the colour of the vessel.
      (Make sure that starting temperature for both experiments are the same.)

      5) Disturbance due to wind was avoided by switching off the fans and closing windows and doors.

      6) When calorimeter was used,
      a) it was covered with a lid to reduce heat loss by convection,

      b) it was placed in a shiny jacket lined with good insulating material (e.g. cotton wool) to minimise heat loss by radiation and conduction.


      1) Main
      The meters (voltmeter and ammeter) must be read with the eye directly above the pointer such that the image of the pointer in the mirror cannot be seen, to avoid parallax error.

      2) The contacts between the wires and the components and the plug of the switch must be tightened* to reduce contact resistance in the circuit.
      (* This can be checked by shaking/tapping the connecting wires lightly at each part of the circuit and observe the ammeter's reading. If the reading fluctuates, it means there is a bad connection in the circuit.)

      3) Make sure there is no 'kink' in the bare resistance wire especially when you have to measure the length of the wire accurately.

      4) The jockey should be held vertically so that the readings obtained are more precise. Do not exert too much pressure on the resistance wire, otherwise the cross-sectional area will no longer be uniform.

      5) Switch on the circuit for a few seconds before taking a reading and then switch it off immediately. This is to avoid unnecessary heating in the resistance wire as a change in temperature can affect the resistance.

      Note: If you find that the connecting wires are not long enough, do not join up two pieces of wire. The contact resistance at the joint can be quite significant. Instead, ask for a longer piece of wire.

      Edited by eagle 14 Jan `09, 12:12AM
  • Moderator
    eagle's Avatar
    23,328 posts since Aug '01
    • *reserved for drawing of graphs, once I think of how to display the data*

      Edited by eagle 14 Jan `09, 12:12AM
  • Moderator
    eagle's Avatar
    23,328 posts since Aug '01
  • whoseline's Avatar
    20 posts since Feb '09
  • Moderator
    eagle's Avatar
    23,328 posts since Aug '01
  • HiAy3Captain's Avatar
    1,120 posts since Apr '09
  • Mr.ms.chairperson's Avatar
    7 posts since Apr '09
    • hmm eagle what do you mean by key sources of error. It looks like you have covered the key sources of error in itself when you discussed about the precautions. i.e if you don't take the following precautins, error will occur. =)

  • Moderator
    eagle's Avatar
    23,328 posts since Aug '01
    • key sources of errors means errors that cannot be avoided

      e.g. take precautions also no use type.

  • SBS n SMRT's Avatar
    4,506 posts since Mar '08
    • Key sources of error


      For Pins: It is hard to determine the exact duplicate image of the pins / The pinhead is too large, cause a large hole, therefore, hard to craft a accurate line.

      For lens: Hard to get a correct sharp image, human visual preception of the clear image will differ


      Heat lost to the surrounding, temperature recorded lower than expected


      Flactuating in A / V meter reading due to uneven contact/ kinks in the wire/ Joule heating cause resistance in the wire to differ/ in perfect battery, with resistance in it


      Air resistance, wind in the lab cannot be controlled...........non perfect pendulum

      these are the most common, remember pallrax error, zero error all cannot...

  • AngRunHao's Avatar
    6 posts since Jul '09
    • Thanks, though i always have problems displaying principal of moments by balancing the ruler or something like that

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