Cranbourne East
Physics
Lab Manual
Units One to Four
CESC
VCE Physics Lab Manual
Format of Prac reports
The following is the format expected for all short prac reports submitted.
Prac design
Each prac undertaken should show evidence of thought in the design of the prac in your prac logbook. The logbook should contain rough notes outlining the following information:
- Choose a topic (in normal pracs, this will be predefined)
- Define a question you want to answer. This is a critical step. The question will be the driving force behind the investigation. (again this will be defined for you in normal pracs, but not in EPI’s)
- Define your variables : this means – What will you need to measure in order to answer your question.
- Equipment : Can you access the equipment you need to measure the Variables you need to measure?
- Define the Independent and Dependant variables. Decide how you will control the variables
- Form an initial hypothesis as an answer to your question. Decide exactly how this hypothesis will be supported/not supported by the results of your measurements.
The prac is now performed using your design as a guide. All results should be recorded in your log book. Any results digitally stored must be referred to in your logbook.
Formal Write-up
The formal write-up can be performed in the logbook or in another form
Abstract
The abstract is a short statement of the aim and findings of the experiment. No more than two or three paragraphs long it should provide the reader with a brief overview of the prac.
Method
A description of the way in which the prac was carried out and the equipment used. For the short prac reports the description of the method can be brief, and include a reference to your prac design (if it is comprehensive enough) , but a list of equipment and a diagram is essential.
Results
Here the main results of the experiment are presented in tabular or graphical format. Long lists of results should be put in an appendix. Numerical results must always be accompanied by an uncertainty.
Discussion
A discussion of the results and the answers to any questions posed in the prac sheet.
Conclusions
A summary of the conclusions you draw from the prac.
Errors and Uncertainties- A discussion
One of the most difficult concepts to grasp in senior science is how to calculate and report uncertainties in Pracs. This discussion is a guide to the sorts of calculations and considerations that you will need at VCE level.
The first thing we must do is carefully define the terms used in the discussion, since sloppy use of the terms can only lead to confusion.
Errors / Errors are mistakes that have been made during the performance of the experiment. They include misreading data, pushing the wrong button, and calculation errors. Errors usually stand out in numerical data as outliers. In a perfect prac, errors do not occur.Uncertainties / Uncertainties are unavoidable results of taking any measurements. No measuring instrument or technique is free of uncertainty. There are many types of uncertainty. Any quoted numerical result must be accompanied by its uncertainty.
Error of reading / This is an uncertainty related to human use of some equipment. A good example is a ruler used to measure the length of many objects by a human operator. It would be difficult for the operator to measure more accurately than 1 or 2 mm and so every measurement made would have an uncertainty of 1-2 mm or more.
Limits of resolution / All equipment has limits. The resolution of a measuring device is the smallest unit it can measure down to. For example a temperature probe may only measure 1/10ths of a degree not 100ths of a degree. Therefore a temperature of 23.44 degrees would be measured as 23.4 degrees. The uncertainty of the measurement would be 5/100ths of a degree, since the probe is “rounding up and down”
Instrumental inaccuracies / Any measuring device can be “out” due to malfunction or fault. It can be difficult to estimate the uncertainty produced by instrumental inaccuracies, but a careful calibration procedure can pinpoint inaccuracy.
Reducing Uncertainties and Errors
- Good experimental procedure can allow us to reduce errors and the size of the uncertainty associated with measurements. It is important that you apply these ideas to the pracs that you perform.
- Work carefully and record all results IMMEDIATELY .
- Understand the limits of the equipment you are using
- Take multiple measurements and use averages
- Look for results that are very different from the others, these outliers may be errors.
- In the EPI, evaluate the possible uncertainties of your experiment before you do it; you may find there are other ways to achieve your goals that lead to smaller uncertainties.
Reporting uncertainties
Uncertainties should be reported in one of two formats, absolute uncertainty or percentage uncertainty.
Example : A measurement is made of the height of a balls rebound. The measurement is 1.24 m. The experimenter feels his measurement could be 5 cm out each way.
Absolute uncertainty 1.24m +- 5 cm
Or 1.24 +- 0.05m
Percentage uncertainty = 0.05/1.24 x 100% = 4%
1.24m +- 4%
Uncertainties in further calculations
Most of the time, the measurements made in a prac will be used for further calculations. We must therefore be able to assign an uncertainty to the results of such calculations.
Bracket method
The simplest method to do this is to use the “bracket method” to see how a calculation will affect uncertainties. The largest and smallest possible values are used in the calculation to determine the resultant uncertainty.
Eg) In a mechanics experiment a student measures the velocity of a dynamics trolley to be 4.3+-0.3 m/s . Its mass is 0.3 kg exactly. She calculates the Kinetic Energy :
But the Velocity could be as high as 4.6 m/s or as low as 4 m/s, subbing these values means the Kinetic energy could be:
A reasonable uncertainty would be about 0.4 joules. Clearly quoting the result as 2.7735 J when the uncertainty is 0.4 J is using too many figures. A better result would be:
KE= 2.8+- 0.4 Joules
ALWAYS BE AWARE OF TOO MANY DECIMAL PLACES.
Percentage Method
If you have converted your uncertainties to percentages you can deal with uncertainties in further calculations easily. The rule is : When multiplying or dividing two or more quantities that have uncertainties, add the % uncertainty to arrive at the uncertainty for the new number. When two numbers with uncertainty are added or subtracted, the largest uncertainty applies to the result.
Eg)
A student has measured the mass and acceleration of a dynamics cart to be :
0.50kg +- 5% and 5 m/s +- 3%
Calculating the force using F= ma
F = 0.5 x 5 = 2.5 Newtons
Uncertainties are added 5% + 3% = 8%
Answer is 2.5N +- 8%
X:\Science\VCE\Yr 11 Physics 2015\Physics lab manual_CESC.doc
Ian Porter