O Level Physics Practical Precautions

When stating precautions, think of the sources that make it easy for errors in measurements to occur.

Example: In an experiment to measure the time taken for a cylinder to roll down a ramp, the human reaction time is a source of error. One way to minimize this error is to modify the experimental set up by using a longer ramp so that it takes a longer time for the cylinder to roll down. In any case, you will have to repeat the measurements and take the average.

Scroll down to see Fig. 1

Reduce human reaction time...

Measuring the dimension of an object

- To determine the volume of a piece of wire, one method is to measure its dimensions and then calculate the volume using length x π x (diameter/2)². From this formula we know that we have to measure the length and diameter of the wire using appropriate instruments. In measuring the length, we straighten the wire and lay it against the scale of the metre rule. This avoids parallax error. If there are some kinks in the wire so that it is not perfectly straight, this is a key source of error which we cannot overcome. One end of the wire may be placed at the zero end. If the zero end of the metre rule is worn off, this will be a source of error called the zero error.

Lay wire on the scale to avoid parallax error... 

- The diameter of the wire is measured using the micrometer screw gauge. Other than the possible zero error in the micrometer screw gauge which should be checked, another error that may arise is due to that the wire may not be uniformly thick.

Check for zero error in micrometer screw gauge... 

Density experiments

- To determine the density of a stone, we usually measure its mass on an electronic balance before immersing it into a measuring cylinder of water to take the volume of the displaced water. If the stone is wet, this will introduce an error in the measurement of its mass.

- While immersing the stone into the water, bubbles of air may stick to the stone. This is a possible source of error in measuring the volume.

- The position of our eye while taking measurements on the measuring cylinder may also be another source of error called parallax error.

Eye at the same level as the bottom of the meniscus...

Scroll down to see Fig. 2 


Oscillation experiments

- The simple pendulum experiment is used to determine the acceleration g due to gravity using the equation period T = 2π√(L/g) where L = length of the pendulum. This equation is derived mathematically by assuming that the angle of displacement (or amplitude) is very small so that the bob oscillates in a straight line. Therefore, if the pendulum’s swing becomes elliptical, the period measured would not be accurate.

Angle of displacement must be small and swing must not be elliptical... 

- Air resistance is a key source of error which cannot be avoided.

- In any experiment that involves taking time measurements, there will be error due to human reaction unless an automated detection system is used. This error cannot be eliminated but can be minimised.

Scroll down to see Fig. 3 


Finding CG of a lamina

- In an experiment to find the CG of an L-shaped card, you suspend the card from a pin that is inserted at the edge of the card. A plumb line is, at the same time, suspended from the pin. The CG of the card would lie along the straight line along the plumb line. If, however, the card is sticking at the pin such that the card is not suspended freely, the CG may not be along the plumb line.

Card must suspend freely...

Scroll down to see Fig. 4 


Light experiments

- In glass block experiments, pins are used to locate images although ray boxes may be used too. This feat is difficult as it is always subject to parallax error. So, make sure your pins are vertical.

Optical pins must be vertical... 

- After that, you remove the pins and join the holes made by the pins. If the pins are large causing large holes, it would be difficult to draw an accurate line. But this is a key source of error which cannot be avoided.

- It is not easy to see a clearly-defined image from a lens. That is why in lens experiments, the room is often darkened and the object, often a cross wire, is illuminated by a torch. Even then, it is difficult to determine the exact position for the sharpest image as our visual perception of the sharp image differs. This error is an example of a key source of error which cannot be avoided.

- When forming an image on a screen using a lens, the object and image must be along the principle axis of the lens.

Align object and image with optical centre of lens... 

- A metre rule is often used to measure object and image distances. If the object, lens or screen is a distance away from the metre rule, estimating its position would be a source of error for the object distance.

- The lens is typically placed on a lens holder. If the lens is not perfectly vertical or is not parallel to the screen, this would be a source of error for the measurement of image distance.

Scroll down to see Fig. 5


Electricity experiments

- If electricity appears in a practical exam, either you are asked to set up a circuit based on a given circuit diagram or the set up has already been done for you.

- There is a small piece of mirror underneath the pointer of the ammeter or voltmeter to help avoid parallax error when taking measurements. The pointers of the ammeter and voltmeter may fluctuate which means there is a bad connection.

- If the experiment asks to find the resistance of a conductor, you must take care that the conductor must not heat up. This is because resistance changes with temperature.

Switch off to prevent resistance from heating up... 

- We know that resistance of a wire R = resistivity x length L/cross-sectional areal A. The following is one possible set up to investigate the relationship between R and L.

Scroll down to see Fig. 6 

- Typically, the wire is laid straight on the scale of the metre rule to avoid parallax error when measuring the positions of contacts across which voltage is measured.

- If there are kinks on the wire, the wire will not be perfectly straight. This is a key source of error which cannot be avoided.

- One variable which should be constant is the cross-sectional area of the wire. If the wire, however, is not uniformly thick throughout, this is another key source of error.