Planning experiments and measuring instruments flashcards

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Resolution of an instrument
The smallest change in a quantity that the instrument can show — one scale division or the last digit of a display.
Range of an instrument
The span of values, smallest to largest, that the instrument can measure.
Zero error
A non-zero reading shown by an instrument when the true value is zero. It is a systematic error.
Calibration
Checking an instrument's readings against known values (e.g. known masses, melting ice) and correcting for any difference.
Parallax error
A shift in a reading caused by viewing a pointer that is not in contact with the scale from an angle instead of straight on.
Fiducial mark
A fixed reference line or pin against which a moving object is timed or a position is read, e.g. at the centre of an oscillation.
Independent, dependent and control variables
Independent: deliberately changed. Dependent: measured as a result. Control: kept constant so they do not affect the result.
Systematic error
An error that shifts every reading in the same direction by the same amount (or proportion); repeating does not remove it.
Random error
An error that scatters readings unpredictably either side of the true value; its effect is reduced by repeating and averaging.
Uncertainty in a single reading (IAS)
Half the resolution of the instrument, e.g. ± 0.05 mm for a vernier caliper. (No — learn it)
Percentage uncertainty
(uncertainty ÷ measured value) × 100%. (No — learn it)
Reading a vernier caliper
Main-scale mm to the left of the vernier zero + (number of the vernier line that lines up × 0.1 mm). (No — learn it)
Reading a micrometer screw gauge
Sleeve reading (to 0.5 mm, check the half-mm mark) + (thimble division on the datum line × 0.01 mm). (No — learn it)
Correcting for a zero error
Corrected reading = reading − zero error (keep the sign: a negative zero error is added back). (No — learn it)
Vernier scale: why 10 divisions in 9 mm?
Each vernier division is 0.9 mm, 0.1 mm shorter than a main division, so line n lines up when the zero is n × 0.1 mm past a mm mark.
Micrometer thimble: why 0.01 mm per division?
One turn moves the spindle 0.50 mm (the pitch) and the thimble has 50 divisions: 0.50 ÷ 50 = 0.01 mm.
Effect of a zero error on a straight-line graph
The line moves up or down: the intercept changes but the gradient does not.
Which graph tests y = kL³ (e.g. a loaded cantilever)?
Plot y against L³: a straight line through the origin with gradient k.
Which graph tests I = P/4πr² for a small lamp?
Plot I against 1/r²: a straight line through the origin with gradient P/4π.
How to set up a graph in a plan
Rearrange into y = mx + c, state what is plotted on each axis, the expected shape, and what the gradient or intercept equals.
Why time 20 oscillations rather than one?
The reaction-time error at start and stop stays the same but is a much smaller fraction of the longer time, so the percentage uncertainty falls.
Why use the ratchet on a micrometer?
It slips at the same small force every time, so the object is not squashed and readings are consistent.
Why measure a wire's diameter at several places and in two directions?
The wire may vary along its length or not be perfectly round; the mean is a better value.
How do you avoid parallax?
Put the eye level with the pointer, looking perpendicular to the scale; bring scale and pointer close together; use a mirror scale on meters.
What is a set square used for in a practical?
Checking that a rule is vertical (or at 90° to the bench), or lining up an object's edge with a scale.
When are repeat readings NOT appropriate?
When the reading cannot change (steady digital value) or the measurement changes the object (plastic deformation, destructive test). Take more values instead.
How do you reduce the biggest uncertainty in a plan?
Find the measurement with the largest percentage uncertainty; make it larger, use a finer instrument, or measure many units together.
Commenting on a result in context
Check it is sensible for the material, whether the conditions of use lie within the range tested, and whether the model's assumptions still hold.
Vernier: jaws closed reads −0.3 mm; with the object it reads 15.8 mm. True value?
15.8 − (−0.3) = 16.1 mm.
Micrometer: sleeve shows 2 mm and the half-mm mark after it; thimble 17 on the datum line. Reading?
2.5 + 17 × 0.01 = 2.67 mm.
Percentage uncertainty in a 0.45 mm wire diameter: vernier vs micrometer?
Vernier 0.05 ÷ 0.45 = 11%; micrometer 0.005 ÷ 0.45 = 1.1%. Choose the micrometer.
10 oscillations timed as 6.42 s, 6.58 s, 6.50 s. Period and % uncertainty?
Mean 6.50 s → T = 0.650 s; uncertainty = half range = 0.08 s → 0.08 ÷ 6.50 = 1.2%.
Pointer 15 mm in front of a scale; eye 250 mm from the scale and 40 mm off level. Parallax shift?
Similar triangles: 40 × 15 ÷ (250 − 15) = 2.6 mm.
A spring balance reads 0.08 N unloaded and 2.62 N with an object. Mass of the object?
(2.62 − 0.08) ÷ 9.81 = 0.259 kg.
Naming an instrument in a plan
Always add its resolution with a unit: “micrometer screw gauge, resolution 0.01 mm”.
Missing the half-millimetre mark on a micrometer
If a mark below the datum line shows after the last whole mm, add 0.50 mm before the thimble reading.
“Fair test” as an answer
Not enough. Name each control variable and say how it is kept constant.
Safety answers
Give a hazard that belongs to this experiment and the precaution that removes it — not general lab rules.
Recording a vernier reading as 23.40 mm
Wrong number of decimal places: a vernier resolves 0.1 mm, so write 23.4 mm.

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