Flashcards · YPH11 · Topic 6

Practical Skills in Physics II flashcards

17 cards on Topic 6, subtopics 6.3 to 6.5. Pick a subtopic or study them all.

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The seven things a Unit 6 plan is assessed on: Apparatus, calibration, measuring, control of variables, repeats, safety, and use of the data.
Why time 20 oscillations rather than one?: A reaction time of about 0.2 s is about 20% of a 1 s period but about 0.5% of a 40 s total, so the percentage uncertainty is much smaller.
A plan to test y = kxⁿ: what graph should be plotted?: lg y against lg x. The gradient is n and the intercept is lg k.
A plan to test y = A e^(bx): what graph should be plotted?: ln y against x. The gradient is b and the intercept is ln A.
How should calibration be stated in a plan?: A zero check on each instrument, or a comparison with a known value, before readings are taken.

Improvements to timing an oscillation: Use a fiducial marker at the equilibrium position for a consistent start and stop, and time more oscillations to cut the reaction-time percentage.
Number and range of readings: At least six readings, over a wide, evenly spaced range. A narrow range gives a poor gradient.
Significant figures in a table of raw readings: Keep the instrument’s resolution, with trailing zeros, and the same number of decimal places down a column.
How is the heading of a logged column written?: With the unit inside the bracket, for example lg (L / m) or ln (V / V). The logarithm itself has no unit.
A reading lies off the line. What should you do?: Re-read and repeat it, and compare with the value the line predicts. Keep it if the repeat is still off the line, because the physics may change there.

Repeatability vs reproducibility: Repeatability: precision with one operator, one method and a short timescale. Reproducibility: precision when different operators or apparatus are used.
Combining uncertainties: Add or subtract quantities: add absolute uncertainties. Multiply or divide: add percentage uncertainties. Raise to a power n: multiply the percentage uncertainty by n.
m = 0.240 ± 0.002 kg, v = 3.40 ± 0.05 m s⁻¹. Kinetic energy and its uncertainty?: KE = ½ × 0.240 × 3.40² = 1.39 J % = 0.83% + 2 × 1.5% = 3.8% KE = 1.39 ± 0.05 J
A lg T against lg L graph has gradient 0.50. What does this show?: T ∝ L^0.50: the period is proportional to the square root of the length.
How do you judge whether a result is accurate?: Find the range allowed by its uncertainty: if the accepted value lies inside it, the result is accurate within the uncertainty. Without an uncertainty, use percentage difference = (measured − accepted) ÷ accepted × 100%.
Error (in measurement): The difference between a result and the true value. It is not a mistake by the operator.
Valid measurement: A measurement that measures what it is meant to measure.