Pure Physics Practical: Final notes (2021 Syllabus)

SUMMARY: Common precautions to take care of (and from previous practicals)

General Physics

• Inaccuracy in marking the position of the plumb-line on the lamina

Minimise the distance between the plumb-line and lamina and view the plumb-line directly in front of the lamina when marking it’s position.

• The pivot position may shift when balancing a meter rule

Repeat the experiment and calculate the average of the readings to reduce the error in measurement of the perpendicular distance

• The wooden plank / metre rule used a the ramp may be slightly warped

Place a support under the wooden plank / metre rule at its midpoint to ensure that the ramp is straight

• Use set-square to check if apparatus set up is horizontal or vertical
• Check that a stick/rod/ruler is able to rotate freely about the balancing point or other pivots. (SOE: May not rotate freely due to friction)
• Check that the uniform stick/rod/ruler being used can be balanced at its mid-point (c.g). Otherwise, place some plasticine on one end of the rule to ensure that it balances exactly at a desired mark before carrying out experiment
• Angle of oscillation for pendulum should be small, not more than 10°
• Allow a pendulum to swing for a few oscillations to ensure that it is along the vertical plane so as to brain a regular period of oscillation.
• (Ramp experiment)

• It is difficult to ensure that the starting point of the block is constant

Mark on the side of the cylinder and ensure the line coincides with the starting line (each time the experiment is repeated to obtain new values)

• It is very difficult to ensure that a paper clip is totally straightened, resulting in shorter length measured

Light

Using optical pins

• Use dressmaker pins (to pinpoint positions in experiments for light)

Holes made by optical pins are too big, thus affecting the accuracy of the light rays constructed and hence affecting the angles measured. So dressmaker pins are used to minimise the size of the pin holes.

• 2 Pins used to locate path of light should be more than 5.0 cm apart (to minimise error in location of light rays to reduce angular error when doing alignment) (( use larger pieces of paper to place the pins further apart))

• Use a protractor or set-square to ensure that the pins are placed vertically upright so that the alignment of the pins(s) and images(s) would be accurate.

Using lenses

• The illuminated object should be placed at the same height as the optical centre of the lens so that its image will also be formed at the same level.
• The lens must be placed upright such that it is at right angles to the bench so that the object and image distances are measured along a line parallel to the principal axis.
• The screen should be placed in a plane at right angles to the principal axis to capture the sharp image of the object

• Mirror - the glass layer of the mirror is thick, leading to multiple images being formed.

Replace the mirror and use one with a thinner glass layer

• Ensure that the length of image and object distance measured is as accurate as possible, by ensuring that the metre rule does not move and the lens and screen are the only moving objects.
• An illuminated object can become slanted as the base on the stand is too wide. Blue tack can be used to ensure that the illuminated object remains vertical

Electricity/ circuit

• The contacts between the wires and all the components in the circuit must be tightened to reduce contact resistance in the circuit. (This can be checked by shaking /tapping connection wires lightly at each part of the circuit and observe the ammeter’s reading. If the reading fluctuates, it means there is a poor connection in the circuit.)
• Close the circuit ONLY when you are taking readings, and then open it when you’re not taking readings.

• This is to avoid the heating effect of the current which would cause a rise in temperature and affect the resistance in the circuit.

• When using jockey, the wires should be tapped gently to avoid overexerting on the wire, so that the cross-sectional area of the wire remains uniform and does not affect the resistance.

• Why is a rheostat set to the maximum resistance initially?

To start with a minimum current so that the circuit will not get heated too quickly and affect the readings

• It is hard to fully straighten out a paper clip to measure its length
• The actual length of clip cannot be ensured as it has a certain thickness
• (Take multiple readings of the diameter of a wire and calculate the average length to make measurement of each diameter as accurate as possible)
• Equipment, switches and electrical wires must not be handled with wet hands to prevent electric shock.
• The wire on the bridge is not of uniform thickness hence its resistance will be affected

Replace wire on the bridge whenever possible

• Aluminium induces a greater resistance in circuit than copper (wires)

Thermal physics

• The water or liquid in a vessel must be stirred constantly during the experiment so that the temperature is uniform throughout
• Care must be taken to reduce the heat loss or heat gain through conduction, convection or radiation (if possible) to/from the surroundings

Lag the beaker with an insulating material to minimise heat loss/gain from the surroundings

• Loss in mass of liquid due to evaporation

Increase the mass of liquid used in experiment to minimise the percentage error due to evaporation.

Note: Readings for voltmeter are usually in 2 d.p. Depending on the experiment, write your answer to the appropriate number of decimal places. Although it is mostly in 2 d.p. Be careful!

Accuracy and precision:

• Vernier callipers are able to measure to a precision 0.01 cm. (Usual measurement is to cm)
• Micrometer screw gauge can measure to a precision of 0.01 mm. (Meaning readings on main scale are to mm)
• Take readings on stopwatches to the nearest one decimal place.

• Unlike electronic sensors in data loggers, stopwatches need to be started and stopped by hand. This manual operation introduces a random error called human reaction time. Human reaction time is about 0.3-0.5 s.

• The period of a pendulum T is found by dividing t (average) by

• Why we need to take time for 20 oscillations: When we divide the measured time by 20, the inaccuracy associated with period T due to human reaction will be only 1/20 of the human reaction time.

• A more precise instrument can measure smaller intervals, so the measured values are likely to be closer together, and is thus more precise.

• If an experiment has small random errors, it is said to have high precision, the readings are close to the average value

• The number of significant figures quoted for measurement gives an indication of its precision. (The greater number of s.f.s implies greater precision)
• For any measurement, the number of decimal places will depend on the unit used. Whereas the number of significant figures shows the precision to which the measurement has been made and is independent of the unit. (higher number of significant figures means a higher degree of precision

• The number 3500 is ambiguous; it can either be 2,3 or 4 significant figures. To remove ambiguity, use standard form or scientific notation for representation
• Choose to express a number in standard form/notation if the number requires too many zeros. (In scientific notation, all digits before the multiplication sign are significant)

Errors

Errors that are beyond your control

• Reaction time in starting/stopping stopwatch
• Human judgement in determining the exact position for the sharpest focus on the image on the screen
• Human judgement in reading to the smallest division on the scale

Stating precautions

(Include)

1. Steps to be taken
2. When the step is to be taken
3. Why the step should be taken (how it affects results)

(Do not quote precautions off the steps/procedure)

Planning experiment

• Plan an experiment to investigate a relationship
• Give a set of instructions for the user to carry out
• Give specific instructions for user to comment/conclude certain relationship
• Instruct the user on how to use the graph to find unknown values

• Controlling values - Values that are kept constant
• Independent variable - Variables that you can make changes
• Dependent variable - Variables that you measure due to the change
• Procedure
• Graph
• Relationship
• Equation

• Procedure ⇒ 1. Set up the experiment in Fig.1.1

                      2. ...

                      3. ...

                      4. Repeat steps 1 to 3 for 5 different readings of (x)...

                      5. Record readings in table as shown

(Revise all the formulas as much as possible. Some experiments may construct equations that may form a derived quantity. Linking back to the original definition and formula for the value, remember to write it down using the exam)

• Graph ⇛ Instruct user what graph to plot

           ⇛ Graph must ALWAYS be dependent vs independent (dependent against independent)

           ⇒ Use a sensible ratio (e.g. 2 cm rep 1, 2 or 5 units)

                                                (Do NOT use odd number of units e.g 3)

• Conclude/ check relationships based on the graph.

Final notes

• ALWAYS be aware of the units you are working with (always)
• Remember to manipulate equations
• STOP and think

• Is this acceptable?
• There should be other things to pay attention to too
• Always work back

• Risks and precautions do not improve accuracy of a result
• Remember to state and error or precaution, and EXPLAIN how it helps to avoid the problem

2023 edit from author: All the best!

2024 edit from author: Accidentally privated document, anyone with the link should have access now. Study Hard!