EJC Physics 2324 Physics Practical Guide

General Information Examination Physics practical skills are tested in Paper 4, a 2 h 30 min paper that makes up 20% of the overall A-Level score in H2 Physics. Total marks of the paper is 55 marks. In the first 2  1 hr, candidates take turns to attempt either 1 long experiment or 2 short experiments. Candidates will then swap work benches to finish the practical components. In the last 30 min, candidates will attempt a question which tests their ability to plan for an experiment – they will not be allowed to work with any apparatus then. Experimental skills and investigations Candidates should be able to: 1. follow a detailed set or sequence of instructions and use techniques, apparatus and materials safely and effectively 2. make, record and present observations and measurements with due regard for precision and accuracy 3. interpret and evaluate observations and experimental data 4. identify a problem, design and plan investigations 5. evaluate methods and techniques, and suggest possible improvements. Overview of Question Types The range of questions include • single measurements: show repeated measurements. • setup: be careful of the relative positioning of equipment. • table of measurements: you’d likely need to check forward for the equation to better guide your lay-out for optimal presentation. • equation relating the measured quantities: check to see if a straight line (linear) graph is expected or a curve. • graph: plot the graph using the guidelines. • interpretation of the graph data: you should never use the raw data points in your table. Rather, you need to read off the best-fit line to an accuracy of half-smallest-square. • further questions: may look further into uncertainties, improvements and meaning behind quantities.

1. Precision of Laboratory Instruments Example The precision of the laboratory equipment contributes to the uncertainty in measurement. The number of decimal places for raw data must be consistent as it will reflect the precision of the instrument. For digital instruments, record the full value that is provided on the screen. For non-digital instrument (ruler, thermometer…) , record to the precision of the instrument. A non -exhaustive list of apparatus and their precisions is provided below. For vast majority of instruments, precision = smallest division available. There are a few exceptions. It will be to your benefit to understand the principles behind their precision rather than memorise them. No Apparatus Smallest Division Precision of instrument Examples of recording 1 Ammeter (0 - 1 A) 0.02 A 0.01 A 0.20 A, 0.21 A 2 Milliammeter (0 - 100 mA) 2 mA 1 mA 20 mA, 21 mA 3 Voltmeter (0 - 5 V) 0.1 V 0.05 V 2.50 V, 2.55 V 4 Digital multimeter (DMM) Record what is displayed on DMM. 5 Half metre rule or metre rule 0.1 cm 0.1 cm 0.8 cm, 12.1 cm 6 Vernier calipers 0.01 cm 0.01 cm 0.22 cm, 7.51 cm 7 Micrometer 0.01 mm 0.01 mm 0.09 mm, 2.11 mm 8 Protractor 1 1 8, 46 9 Measuring cylinder (100 cm3) 1 cm3 1 cm3 6 cm3, 18 cm3 10 Spring balance (0 - 10 N) 0.1 N 0.1 N 0.4 N, 3.7 N 11 Electronic balance 0.01 g 0.01 g 121.10 g, 121.11 g

*When using a stopwatch to time how long a rolling marble takes to roll down a slope, record the timing stopwatch’s precision of 0.01 s, do not need to consider human reaction time yet (i.e. give to 0.01 s rather than nearest 0.1 s). 2. Units of Laboratory Instruments Units are sometimes provided at the end of the space for your answer, ensure that you record your answer according to the correct units. It is also acceptable to record to the units provided directly by the instrument. Doing so helps to avoid unnecessary conversion errors. Convert if it is genuinely easier to work with the quantities after conversion, or if the units desired are specified by the question: For digital multimeters, the unit of the display is given in the range selection: No Apparatus Smallest Division Precision of instrument Examples of recording 12 *Stopwatch (digital) 0.01 s 0.01 s 0.93 s, 28.15 s 13 Thermometer (–10 C to 110 C) 1C 1 C 7 C, 24 C display gives reading in Volts display is in milliVolts (mV) display is in kiloOhms (kΩ) display reading is in Ohms (Ω) display is in microAmps (μA) display is in milliAmps (mA) display is in Amperes (A) (beeps when there is electrical contact) mm

3. Repeat Measurements (Raw Data) In most cases, 2 measurements are sufficient if the measurements are relatively stable and the uncertainty of the measurements are low. Presentation 1: Show the 2 measurements and the process of calculating average. Example +==6 0 6 1average 6 05 = 6 1 cm2 ..h . . Presentation 2: Tables can also be used for clear presentation. Example 1 / cmh 2 / cmh / cmh 6.0 6.1 6.1 What is critical, • each measurement h1 and h2 (raw data) follows the instrument precision i.e. same d.p.. • the average value h (calculated data) follows the d.p. of the raw data. Take 3 or more repeat measurements if the uncertainty is high. For timing using a stopwatch in particular, one should aim for more than 20 s in a single measurement to bring down the percentage uncertainty to less than 1% (considering that human reaction time is 0.2 s). Example • The addition of column for no. of oscillations n enable us to vary the number of oscillations, so as to ensure that each measurement exceeds 20 s. • t refers to average t. • T refers to period of the oscillations. If the timings involved are less than 15 s, show 3 repeated measurements. Example Let t be time taken for a marble to roll down a slope 1 / st 2 / st 3 / st / st 1.22 1.28 1.33 1.28 Let t be time taken for 22 oscillations n 1 / st 2 / st / st / sT 22 21.64 21.32 21.48 0.9763

4. Linearisation of Equations In practicals, experiments are usually performed to verify certain underlying theoretical equations or to obtain some of the unknown parameters in the theoretical equation. The first step to start an experiment is to analyse the theoretical equation for the experiment and linearise the equation in a way such that the unknown parameters could be found using the raw data collected. Even though the space for table of values typically appears before the equation, we highly recommend that you linear the equation first before drawing your table. Example The period of the oscillation T is given by the equation qTp= L where L is the length, p and q are constants. Plot a suitable graph to determine whether the rel ation is of the for m indicated. Hence find the values for p and q. ( ) ( ) ( ) ( )lg lg lg qT pL T p q L = =+ Plot a graph of lg(T / s) against lg(L / cm), this should give a straight line graph of gradient q and y-intercept lg (p / s cm-q) Structure: Plot a graph “y / units” against “x / units” this should give a straight line graph of gradient “qty / units” and y-intercept “qty / units”.

5. Tables After linearisation, raw data can then be collected and processed accordingly for the graph to be plotted. Since a large number of raw data will be collected and processed to establish trends, they must be presented in a table form which can be easily read and understood. Record all the raw data and processed data collected into a single table as and when they are taken so as to save time. DO NOT split your table into 2 parts! Steps 1. Draw (in pencil) the table outline that maximises the full available space given in the paper. 2. Identify the “y / units” and “x / units” as required from the equation 3. Use first column for the independent variable (don’t need repeat). 4. Use next few columns for repeated raw measurements and average of dependent variable(s).