NJC Physics practical exam revision notes

Important points for SH1 practical examination 1. Critical skills in measurement, recording measurements and calculations 1.1 Recording measurements instrument How are readings taken? smallest division / decimal place How should measurement be recorded? uncertainty due to resolution$ mass hanger and slotted mass value is engraved on instrument 1 g 1 g 1 g metre rule read graduated markings on the instrument 0.1 cm 0.1 cm 0.001 m 0.1 cm protractor 1o 1o 1o liquid-in-glass thermometers 1 oC 0.5 oC 0.5 oC measuring cylinder (100 cm3) 1 cm3 0.5 cm3 0.5 cm3 newton-meter (or spring balance) 0.4 N 0.2 N 0.2 N vernier callipers value shown on the digital display 0.01 mm value on digital display # last decimal place micrometer screw gauge 0.001 mm value on digital display # last decimal place stopwatch 0.01 s value on digital display @& last decimal place electronic mass balance 0.01 g value on digital display last decimal place $ This uncertainty is due to instrumental limit only. Refer to section 1.4 on estimating percentage uncertainty. # You may record values from vernier callipers to the nearest 0.1 mm and micrometer screw gauge to the nearest 0.01 mm. @ You may record the stopwatch timing to nearest 0.1 s. & When stopwatch is running continuously while you measure how other quantity changes with time, you may record the timings to nearest 1 s or 0.1 s. 1.2 Expressing calculated values to the correct number of significant figures How is the value calculated? How is the calculated value recorded? Example Multiplication and/or division and/or reciprocal and/or power of quantities The s.f. of the final value should follow the least s.f. of the quantities used in the calculation. Example 1 The two measured quantities are A = 10.2 and B = 2.3. The calculated quantity C is related to A and B by the formula C = AB2. C = (10.2)(2.3)2 = 53.958 = 54 B has the least s.f. among the values and so C is expressed as 2 s.f. Example 2 The measured quantity is x = 10.2 cm and C = 1x. C = 110.2 = 0.0980 cm–1 x is 3 s.f., so C is expressed as 3 s.f. Addition and/or subtraction of quantities The d.p. of the final value should follow the quantity with the least d.p. of the quantities used in the calculation. The two measured quantities are X = 10.2 and Y = 2.33. The calculated quantity Z is related to X and Y by the formula Z = X + Y. Z = 10.2 + 2.33 = 12.53 = 12.5 X has the least d.p. among the values and so Z is expressed to one d.p. Average of a set of repeated values The final value should be recorded to the same decimal place (d.p.) as the measured values. The two repeated timings are 20.23 s and 20.30 s. Average timing = 20.23!20.302 = 20.265 s = 20.27 s (same d.p. as the measurement) 1.3 Critical skills Vernier callipers and micrometer screw gauge • Zero the device by pressing the “zero” button (venier callipers) or the “reset” button. • Measurement should be repeated at different positions to account for non-uniformity. Calculate the average value. Electronic mass balance • Zero the device by pressing the “zero” button. • If the reading fluctuates, ensure the balance is away from sources of vibration (e.g. fan or shaky surfaces). If fluctuation persists, record the “average” value you can discern from the display. Stopwatch The starting and stopping of the stopwatch is subjected to error of judgement and reaction. To ensure reproducibility of results, as a rule of thumb, repetitions depends on the duration of the event. Note: Rule of thumb does not apply when the stopwatch is runing continuously while other quantities are measured (e.g. temperature change in a heating experiment over a duration). if the duration is … you should repeat measurements more than 15 s once (i.e. total of two timings) between 5 s and 15 s two times (i.e. total of three timings) less than 5 s four times (i.e. total of five timings)

Using stopwatch for oscillation experiments 1. Conduct trials, where possible, to determine whether you can vary the number of complete oscillations N to achieve a time duration t of at least 20 s over the range of the independent variable. 2. For each independent variable setting, repeat the the timing t, based on the rule of thumb, using the same number of oscillations N. The table of values are as follow: length of pendulum L / cm number of oscillations N timing for N oscillation period T / s t1 / s t2 / s 50.0 15 21.22 21.31 1.418 40.0 17 21.59 21.48 1.267 … … … … Note: If the repeated timings differ more than 0.5 s, you should repeat the measurement to eliminate possible mistakes (e.g. miscount oscillations). 3. Ensure the system oscillates freely and in the desired plane with very little wobble. This is achieved if the oscillation has small amplitudes. 4. Take care not to miscount oscillations, e.g. say “zero” when starting to time. Use a reference such as a pointer (see the figure on the right) to help you judge when the oscillation passes the equilibrium position. Note: The speed of the oscillation is fastest through the equilibrium position, so you should time from this point. If you try to time from the maximum displacement, the system is moving slowly and it becomes difficult to judge when it has reached maximum displacement and is about to reverse direction. 5. Only start the stopwatch when the system has completed one or two oscillations. This will help you to start timing at the correct point and the oscillations are steady when the timings are taken. 1.4 Estimating percentage uncertainties (1) Percentage uncertainty for quantity from a single measurement • Measure & record based on resolution of the instrument. • Assess factors (e.g. set-up, use of the apparatus) that could increase the uncertainty. • Give an estimate of the uncertainty of the measurement. percentage uncertainty = estimated uncertainty of measurementmeasured value ×100% (expressed to 2 s.f) (2) Percentage uncertainty for quantity that is measured at least twice If a quantity is measured repeatedly (at least twice) and the values are not all equal, then percentage uncertainty = 12 × (max value$min value)average value ×100% (expressed to 2 s.f) (3) Percentage uncertainty for a quantity that is calculated from two or more other quantities You need to estimate the uncertainty of each of the quantities and apply the rules for combining uncertainties (refer to Topic 1: Measurements).

2. What you need to know about the different questions? (refer to 9748 Specimen Paper 4) Question 1 Initial measurements (Refer to (a) and (b)) You are guided to set up the apparatus and perform some initial measurements. Marking points: ☐ Accuracy of measurements. ☐ Measurements are recorded to correct d.p. (refer to section 1.1). ☐ Repeated measurements, if required, should be recorded down. • Measurement is repeated for stopwatch, vernier callipers and micrometer screw gauge (refer to section 1.3). • Measurement should be repeated if it is likely to fluctuate (e.g. PT103 (a)(ii)). Data collection (Refer to (c) and (d)(i)) • This table is drawn up in advance of taking readings by converting the given equation into the form of y = mx + c. The linearised equation gives you information on the processed data to include in the table. Refer to equations given in part (d)(i) of the specimen paper. e = P × 1n + Q y-axis gradient x-axis y-intercept What you need to measure? S1 x The table should have the following columns: • All data should be recorded in a single table. When constructing the table, the raw data should be in columns on the left of the table and processed data towards the right. ⟵⎯⎯⎯⎯⎯⎯⎯⎯ measured data ⎯⎯⎯⎯⎯⎯⎯⎯⎯⟶ ⟵⎯⎯⎯ processed data ⎯⎯⎯⟶ independent variable / unit what you vary measurement 1 / unit e.g. t / s measurement 2 / unit (if applicable) calculated value 1 / unit e.g. t2 / s2 Calculated value 2 / unit … Other marking points ☐ headings with variable and units Marking points ☐ Deter