Physics EMPA hints and tips
Physics EMPA hints and tipsUse the instrument precision.
Instrument precision is defined as the smallest non-zero reading
measured by the instrument, and refers to the actual graduations on
the scale of the instrument. A metre ruler with mm graduations
would have a precision of + 1 mm.For single readings, or multiple
identical readings, taken by this type of ruler the estimate of
uncertainty would be + 1mmUncertainty in a single readingCalculate
the mean valueEstimate the uncertainty using:Uncertainty = 0.5 X
spread
e.g. 5 readings of length: 0.15mm, 0.12mm, 0.16mm, 0.13mm,
0.14mmMean = 0.14mmUncertainty = 0.5 x (0.16-0.12) = 0.02mm Length
= 0.14 0.02mmUncertainty estimates with repeat readingsCalculate
the % uncertainty in the area of a metal wire given that diameter
is 0.16 0.01 mm% uncertainty in diameter = (0.01/0.16) 100 = 6.3
%
area is proportional to diameter squared % uncertainty in area =
6.3 + 6.3 = 12.6 %Combining Uncertainties Example 1Calculate %
uncertainty in the speed measurement given the following
uncertainties in distance and time measurements% uncertainty in
distance = 0.5 %% uncertainty in time = 2.5 %
% uncertainty in speed = 0.5 + 2.5 = 3.0 %Combining
Uncertainties Example 2Use the format:variable symbol solidus
unitYou should not name the variable in full. e.g. eg V/mV is
correct, output pd of solar cell in millivolts is not
Unless directed otherwise, put all data in one table, from raw
data on the left, to processed values for plotting on the
right.
Tables and Graphs1 mark for each of the following
points:Labelling of axesSuitable ScalesLine of best Fit
AS Graphs1 mark for each of the following points:Plotting of
pointsLine of best fitReading data for calculation of gradient, and
suitable size triangleGradient value, with appropriate significant
figures
A2 Graphs100806000123Marking the origin correctly on a graph, eg
PHAB3X Sec A Part 1 Q2(b)Unacceptable: the marking of the origin as
above produces a non-linear scale which will always be
penalised.100806000123Solution: use of the broken scale convention
resolves the problem but watch out if a gradient calculation is
then required.10080600123Unacceptable: leaving an origin unmarked
on either axis will not be accepted; the scale will still be
treated as non-linear since the origin is now
ambiguous.1008060400123Solution: use of a false origin is
acceptable but candidates should be careful if they are then asked
to calculate the gradient.Finding an intercept which cannot be read
directly, eg PHAB3X Section B Q1(a)While the intercept on the
horizontal axis can be read directly, the vertical intercept can
not.Candidates gain no credit for extending the line off the grid
into the margin.00white rectangle represents edge of grid printed
in answer bookletWhen answering Section B, candidates should not be
given the opportunity to re-plot graphs.The use of algebra is
expected if the intercept cannot be read directly.Significant
Figures in a Previous Paper:To test the theory that R = kL2
candidates were expected to evaluate R/L2 for every row of the
table. L/cmR/Wfor use in answering part
(a)6.62.910.67.613.813.017.821.621.430.4L/cmR/Wfor use in answering
part
(a)6.62.90.06710.67.60.06813.813.00.06817.821.60.06821.430.40.066L/cmR/Wfor
use in answering part (a)6.62.90.0666 [0.067]10.67.60.0676
[0.068]13.813.00.068317.821.60.068221.430.40.0664Many candidates
forfeited marks because they truncated their results to2
significant figures; in at least 3 rows of the table, 3 sf was
justified.TablesIndependent variable must be in left hand column of
tableQuote to max possible significant figures (e.g. 1.10 V if
instrument precision is 0.01V)GraphsPoints on a graph should cover
at least half the grid horizontally and vertically.All points must
be plotted Marks are forfeited if plotted > 1mm from correct
positionMarks can be forfeited if > 2mm from trend line
In addition: from recent mark scheme:Use the convention variable
symbol solidus unit for table headings and graph axes; a bracket is
essential if the log of a variable is involved, eg
ln(variable/unit)
When compressing a graph scale use the broken scale convention
if marking the origin (0, 0) otherwise mark a false origin
AS and A2 candidates should be able to calculate the intercept
on a graph if this cannot be read directly
The result of a calculation should be to the same number of
significant figures as the least accurate data used in the
calculation
Tables and Graphs Summary
