Paper 3

Key Terms

  • error - the difference between measured and the true value

  • uncertainty - the interval which the true value can lie in

  • accuracy - how close the measured/calculated value to the true value

  • precision - how consistence the values are, obtained from repeat measurements

  • Why result might be precise but not accurate?

    • there might be systematic error in the measuring instruments

    • so values are consistent when taken repeats, but not close to the actual value

Uncertainty

  • Calculating percentage uncertainty

    • if given uncertainty, just use this divide by the measured value

    • if given resolution, divide by 2 to get the uncertainty

    • for set of data and need to use the mean, percentage uncertainty = 100*(½ range/ mean)

  • Measuring instruments (resolution), high resolution so has low uncertainty

    • micrometer - 0.01mm

    • digital/vernier calliper - 0.01/0.1mm

    • if ask which one to use, almost always say micrometer

  • Why is there uncertainty?

    • the way measurements is made

    • reaction time

    • parallax error

  • reduce uncertainty for diffraction grating experiment

    • measure from nth order on one side to nth order the other side

    • use a larger grating to screen distance

    • use vernier calliper to record distance

    • use a grating with more lines per mm

Criticise

  • Criticising result table

    • data not recorded to the same d.p/s.f.

    • data should be recorded to more d.p/s.f.

    • not enough data recorded

    • repeats and mean not taken

    • the readings of independent variables are not in the same interval

    • position of the mass holder is not recorded for oscillating spring experiment

  • When asked to criticise a graph of log, mention

    • whether the graph is supposed to follow the observed trend

    • usually yes, if no, could be because of passing through/not passing through origin

    • measuring instruments not in high resolutions, so there might be systematic error, affecting the data recorded

    • see if there’s extrapolation of graph

    • usually more data points are needed to give a valid conclusion

  • graph where line supposed to go through the origin, if doesn’t there’s systematic error(e.g. zero error, parallax error); if line of best fits does not cover all/most points, then there’s random error

Improve Experiments

  • how to improve experiment of oscillating springs also mention

    • ruler may move during oscillations

    • move ruler closer to the spring

    • it is difficult to measure from the same position of the spring each time of the spring, so measure the position after adding mass and when mass is removed

  • In experiment of oscillating springs, use multiple oscillations and taking a mean, increasing time so reduces uncertainty

  • use set square to ensure spring is parallel, use fiducial marker to determine the equilibrium position

  • how to ensure accurate measurements?

    • measure from different places/orientations

    • repeats then take a mean reduces random error also always mention reduce percentage uncertainty

    • when taking measurements, if object very thin, take stacks of them and calculate a mean, this reduces percentage uncertainty

  • relationship between volume/length vs temperature

    • remember to stir water

    • parallax error when reading length/temperature

    • digital thermometer

    • let tube reach thermal equilibrium before taking reading because temperature of air otherwise may not be the same as temperature of the water because time is taken for the glass tube to reach the same temperature

Using other apparatus instead of standard ones

  • Using a voltmeter instead of oscilloscope

    • a.c. voltmeter allows reading to be taken directly

    • Gives a better resolution than measuring trace height on an oscilloscope

    • but voltmeter may draw current and affect the circuit it connects to

    • accuracy would depends on the calibration of the voltmeter

  • Using video instead of stopwatch

    • can use slow motion, so use when time is very short, can view motion more slowly

    • can record motion, so easy to judge when an oscillation is complete

  • why use a data logger with light gates instead of stopwatch?

    • Stopwatch - reaction time

    • so stopwatch has an uncertainty bigger than data logger

    • but repeats timings/taking a mean using a stopwatch can reduce uncertainty

    • light gate is difficult to set up in an oscillating spring system

    • datalogger used when need to record for a long period of time

    • higher sample rate

Other things to be aware of

  • when explain experiments involving recording sets of data, need to mention at least 5 sets

  • When using equations to explain, remember to define symbols, e.g. phi = work function, or just state in words, and in question with rearrange to use log and plot graph, always remember to write explicitly compare with y=mx +c

  • always always always mention reduce random error when taking mean!!!