SNGS P3 PRACTICAL NOTES

1 Physics Practical Summary Summary of common Precaution and Sources of Error Precautions Precautions are those taken during your experiment that improve the accuracy and reliability of your experimental results. These typically are not the explicit steps provided in the question paper. Sources of error Sources of error are inherent to the experiment. In another word, these errors will always be present no matter how careful you are with the experiment or after you have taken the precautions. Human error (parallax error), instrumental error (zero error), environmental factor (wind) are NOT considered as significant sources of error. The following summarizes common precautions and sources of error. This list is not exhaustive. Those stated here might not be applicable to every experiment. So, use them as a guide and modify them accordingly to the experiment that you are doing. LIGHT Precautions for Light Optical Pins 1. Pins must be placed at least 5 cm apart. 2. Pins must be vertically upright. Mirrors 3. The reflecting silvered surface and not the glass surface of the mirror, must be placed on the line. 4. Plane mirror must be placed vertically upright on the line. Glass Block 5. Trace block outline correctly and avoid error due to uneven edges. Lens 6. Ensures that the planes of the lens, the screen and the light source are parallel. 7. Place metre rule alongside the setup to measure the various lengths accurately. 8. Ensure that the centre of the illuminated object is at the same height as the centre of the converging lens by measuring with a ruler. 9. Image and object distance must be measured from the centre of the lens. Key Sources of Errors for Light 1. The holes made by the optical pins provided are rather large (may not be true for thin pins). This affects the accurate construction of the paths of the incident and emergent rays. 2. The mirror used is very light and its position shifts easily during the experiment. This causes error in the measured angle of incidence and hence affects the path of the reflected ray. 3. The pin cannot be at exactly on the outline of the block as instructed. This affects the accuracy of the positions of the search pins and hence affects reading of angle θ. 4. The object is thick and its surface is uneven. This makes it difficult to obtain a sharp image of the entire object by positioning the lens at one location. 5. For large values of ‘u’, the images formed are very small. It is difficult to determine the exact position of the sharpest image as there is a range of values of v for which the image is sharp.

2 MECHANICS Precautions for Mechanics Moments 1. The metre rule must be able to move about the pivot freely. There should be no obstruction to its movement. 2. The metre rule is kept horizontal by ensuring the distance from the table top to both ends of the metre rule are the same. Centre of Gravity 3. The plumbline must be positioned close to the cardboard when taking reading. 4. The cardboard must be able to move freely about the pivot. Spring 5. The spring and thread holding the ruler should hang vertically downwards and perpendicular to the table top. A set-square can be used to ensure this. 6. The displacement for oscillation of spring should be kept small. Density 7. The plasticine must be completely submerged in the water when taking its volume. 8. The volume of the liquid in the measuring cylinder should be read at the bottom of the meniscus of the liquid at eye-level. Pendulum 9. The angle of oscillation must be kept small by ensuring the angle is not more than 10 ° from equilibrium. Other oscillations (e.g. oscillating mass/rod on a spring) 10. The mass was pulled down vertically and before being released to ensure that the oscillation is only vertical (without swaying left and right). 11. Waited for the oscillations to stabilize for a few oscillations before starting the stopwatch. Measuring short length and timing 12. The experiment is repeated twice and the average of 2 readings is calculated. Key Sources of Errors for Mechanics Moments For metre rule hanging from a retort stand, 1. It is difficult to make very small changes to height of the metre rule by adjusting the boss of retort stand. Hence, the metre rule may not be exactly horizontal. This will affect the accurate reading of ____. For metre rule balanced on a knife-edge, 2. It is difficult to keep the ruler balanced and maintained at horizontal position. This affects the accurate reading of the position of the centre of gravity. Forces 3. It is difficult to measure the angles between the threads/lines/strings during equilibrium. The threads/lines/strings will move if the protractor touches it. 4. It is difficult to position the centre of the protractor on the knot. This affects the accuracy of measurement of θ. 5. The protractor is not steady when being read while held in the hands. This affects the accuracy of measurement of θ.

3 Centre of Gravity 6. The plumbline and the card will move if the ruler/pen/pencil touches it. This makes it difficult to take measurement of ‘x’ accurately as the ruler cannot be placed against the card. Spring 7. It is difficult to determine the same point from which the length of the spring is measured, when the spring stretches. This affects the accurate measurement of the length / extension of the spring. 8. It is difficult to hold a ruler steadily against the spring. This will affect the accurate measurement of the length/ extension of the spring. Density 9. The mass of the paper clips/ marbles provided may not be the same. This affects the accurate measurement of m. Pendulum and other oscillating/vibrating mass 10. It is difficult to estimate the centre of mass of the pendulum bob. The affects the accurate measurement of pendulum’s length. 11. Oscillations with total time of only 2 to 3 seconds, the percentage error in t due to human reaction time is significant. Timing for a moving object 12. As the distance travelled by object decreases, the time taken is shorter and thus percentage error due to human reaction time will be greater. ELECTRICITY Precaution for Electricity 1. Ensures that the jockey is placed perpendicularly to the wire. 2. Switch off in between experiments to prevent the wires and components from heating up which will affect their resistance. Key Sources of Errors for Electricity 1. The wire becomes hot after some time and the resistance will increase. Thus value of current flowing through and potential difference across the resistor will be different, causing the values of I and V measured to be inaccurate. 2. There are kinks in the resistance wire and thus it cannot be fully straightened on ruler and will affect the values of I and/or V recorded. 3. Contact of jockey on the resistance wire is not consistent thus causing some variations in the reading of I and/or V. 4. The precision of digital calipers are less precise than digital micrometer. So, the accuracy of the resistivity is limited by the precision of the digital calipers.

4 Precision of instruments • The precision of your raw data (measured values) are as shown in the table below (not exhaustive): Apparatus Record up to Examples Ammeter (0 –1 A) 0.01 A 0.22 A, 0.50A Ammeter (0 – 3A) 0.05 A 2.55 A, 0.50 A Voltmeter (0 – 1V) 0.01 V 0.59 V, 0.20 V Voltmeter (0 – 5 V) 0.05 V 0.15 V, 3.30 V Measuring cylinder (100 ml) 0.5 ml 18.5 ml, 90.0 ml Measuring cylinder (250 ml) 1 ml 70 ml, 23 ml Metre rule, half-metre rule 0.1 cm / 0.001 m 12.5 cm, 12.0 cm, 0.125 m Digital micrometer 0.01 mm 2.34 mm, 2.50 mm Digital calipers 0.01 cm 2.25 cm Electronic balance 0.01 g 12.41 g Protractor 1  20  Spring balance (0 – 10 N) 0.05 N 5.20 N Stopwatch (digital) 0.1 s 20.2 s Thermometer (-10 C to 110 C) 0.5 C 17.0 C, 50.5 C