While writing up your lab report, you will often have to mention various sources of errors that you encountered while doing your physics experiment that could’ve affected your final results. They make for good material to put in the discussion part of the lab report and usually indicate that you have a fair understanding of the limitations of the particular physics experiment that you were doing.
Here is a summary of some of these sources of errors and how to avoid them.
Sources of errors in mechanics experiments
- Avoid disturbance due to wind by switching off the fans and closing windows and doors
- Ignore the first few oscillations and start timing only when oscillations are steady
- Make sure that the angle of swing is small during timing (less than 5°)
- Measure the length of the pendulum from the point of suspension to the centre of the bob
- When the oscillations of the pendulum become elliptical (wobbling), reject the timing
- Whenever there is a gap between the scale and the mark to be read, a parallax error is present
- When reading a measuring cylinder or thermometer, parallax errors can be minimised by viewing the reading at eye-level and at the meniscus.
- When reading meters such as voltmeters, ammeters etc, parallax errors can be minimsed by viewing the pointer from directly above such that the pointer coincides with its image reflection in the glass. (This may or may not be possible depending on the lighting and how reflective the glass covering the pointer is)
When judging whether apparatus is horizontal or vertical
- Make use of the vertical edge of a door, blackboard, a wall or a plumb-line to judge whether apparatus is vertical or not.
- Use the teacher’s bench or the horizontal edge of the blackboard to judge whether or not your apparatus is horizontal.
- Also use a level when available
Sources of errors when working with experiment concerning heat
When working with heat
- Stir the water or liquid in the vessel constantly during the experiment so that the temperature is uniform throughout the water. (Note: Do not use the thermometer as a stirrer)
- Do not place the thermometer at the bottom of the beaker when the water is being heated. This is because the temperature of the beaker is usually higher than that of water. Overheating may lead to over-expansion of the mercury thread resulting in a wrong reading and in some cases breaking the thermometer
- Take temperature readings only when they are steady
- When comparing heat loss due to a change in, for example, the colour of a cooling vessel, make sure that all parameters remain the same except for the colour of the vessel. (Make sure that starting temperature for both experiments are the same)
- Avoid heat transfer by convection by switching off the fans and closing windows and doors
- Use lids where possible to reduce heat loss by convection
- Perform experiments in good insulating material such as a Styrofoam cup to minimise heat loss by radiation and conduction. Also lining the container with good insulating material such as cotton or wool can help to minimise heat loss.
Sources of errors when working with experiments dealing with light
- Place the pins vertically upright (Check with a set-square)
- Place the the pins used to locate the path of the light ray more than 6 cm apart to minimise errors (Check that pin-pricked points are clearly marked and labelled)
- Use the view from the base of the pins when aligning pins in a straight line as the pins may not be perfectly upright
- Measure the object and image distances along a line parallel to the principal axis. A set-square can be used to minimise parallax errors
- Make sure the lens is upright and parallel to the screen
- The illuminated object should be placed at the same height as the optical centre of the lens
Sources of errors when working with experiments dealing with Electricity
General sources of errors
- Read the meters (voltmeters and ammeters) in such a way to avoid parallax. Put your eye directly above the pointer such that the reflection of the pointer in the glass cannot be seen or they both line up
- Tighten the contacts between the wires and the components to reduce contact resistance in the circuit. Check this by shaking/tapping the connecting wires lightly at each part of the circuit and observe the ammeter’s reading. If the reading fluctuates, it means there is a bad connection in the circuit
- Make sure there is no ‘kink’ in a bare resistance wire especially when you have to measure the length of the wire accurately
- Switch on the circuit for a few seconds before taking a reading and then switch it off immediately. This is to avoid unnecessary heating in the resistance wire as a change in temperature can affect the resistance
- If you find that the connecting wires are not long enough, do not join two pieces of wire. The contact resistance at the joint can be quite significant. Instead, ask for a longer piece of wire.
Remember your physics experiment lab report should include some sort of error discussion. it’s not possible to conduct a perfect experiment. Show that you are aware of the limitations in your practical and include them where necessary.