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Guidance notes on experimental work.

Section 1 ­ Treatment of uncertainties in Physics at AS and A2 level


One of the main aims of the practical work undertaken in GCE Physics is for candidates to
develop a feeling for uncertainty in scientific data. Some of the treatment that follows…

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This is the misreading of a scale or faulty equipment.

Suspect results
These are results that lie well outside the normal range e.g. points well away from a line or
curve of best fit. They often arise from mistakes in measurement. These should be recorded
and reason for discarding…

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2. Estimation of uncertainty from a single reading

Sometimes there may only be a single reading. Sometimes all the readings may be
identical. Clearly it cannot be therefore assumed that there is zero uncertainty in the
With analogue instruments, it is not expected that interpolated readings will be taken…

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The period of oscillation of a Pendulum/Spring

The resolution of a stop watch, used for measuring a period, is usually 0.01s. Reaction time
would increase the uncertainty and, although in making measurements on oscillating
quantities it is possible to anticipate, the uncertainty derived from repeat readings is likely to

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Determining the uncertainties in derived quantities.

Please note that candidates entered for AS award will now be required to combine
percentage uncertainties.

Very frequently in Physics, the values of two or more quantities are measured and then these
are combined to determine another quantity; e.g. the density of a material…

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So T2 = 961 ± 4%.
4% × 961 = 40 (to 1.s.f)
So the period is expressed as T = 960 ± 40 s.
Note: x 1 is the same as 1/x. So the percentage uncertainty in 1/x is the same as that in x.
Can you see…

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Notes for purists:
1. When working at a high academic level, where many repeat measurements are taken,
scientists often use "standard error" , a.k.a. "standard uncertainty". Where this is used,
the expression x0 ± is taken to mean that there is a 67% probability that the value of x is…

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GRAPHS [derivation of uncertainties from graphs is only expected in A2]

The following remarks apply to linear graphs:

The points should be plotted with error bars. These should be centred on the plotted point and
have a length equal to ymax - ymin [for uncertainties in the y values of…

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Section 2 ­ Ideas for practical work

Prac Context
Density of regular solids [cuboids, cylinders] Use of metre rule, callipers, micrometer,
Identification of material using density. balance
Initial work on uncertainties
Density of liquids and irregular solids Use of measuring cylinders
Weighing a rule by balancing a loaded rule Use…

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Section 3 ­ Experimental techniques

The following is a selection of experimental techniques which it is anticipated that candidates
will acquire during their AS and A2 studies. It is not exhaustive, but is intended to provide
some guidance into the expectations of the PH3 and PH6 experimental tasks.

Measuring instruments…


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