Stage 6 Physics - Practical Work


More often than not, practical work in Physics will involve determining and using the mathetical relationship between two variables: the independant and dependant variables. The material below describes how such an investigation would be carried out and what to include in a write-up of the investigation as a report.

Key Terms

Be sure to familiarise yourself with each of the following terms regarding experimental work and investigations.

Term Definition Comments
Hypothesis A possible answer to a research question Always develop the hypothesis using your background research
Materials Substances, chemicals or organic matter used in an experiment
Apparatus Equipment, glassware, measuring devices, etc used in an experiment
Independent Variable The variable being changed by the experimenter Operates over a range of values in an experiment
Dependant Variable The variable being measured by the experimenter Depends on the value for the independent variable
Controlled Variable A variable kept constant in an experiment to make it a fair test Kept constant to ensure the dependant variable is only affected by the independent variable
Validity A measure of how precisely an experimental method measures what it purports to measure Can be improved by controlling variables, measuring more precisely and reducing errors and weaknesses in the method
Reliability A measure of the consistency of an experiment and/or its results Can be assessed and improved by taking many measurements and/or repeating the experiment
Accuracy A measure of how close a value is to an accepted value Relevant when measuring a value such as the acceleration due to gravity


Conducting Investigations

In Stage 6 Physics, experimental work will involve all or part of a process that starts with the design of an experiment and ends with a conclusion and evaluation. Each of the stages in the process is outlined below as well as the headings you would use for each when writing up your report.

 

Stage Aspects Outcomes Report Heading
Planning Defining the problem or research question Identifies a focused problem or research question linking the dependant and independent variables Research Question
Doing background research Sources information on each variable and the investigation generally Background
Formulating a hypothesis or prediction States and relates the hypothesis to the research question using the background research Hypothesis
Design Identifies variables Identifies the relevant independent, dependant and controlled variables Variables
Selecting appropriate apparatus and materials Selects appropriate apparatus and/or materials Apparatus and Materials
Designing a method for the control of variables Describes a method that allows for the control of variables Method
Designing a suitable procedure to change the independent variable Describes a method describes how the independent variable will be changed Method
Designing a suitable procedure to measure the dependant variable Describes a method that describes how the dependant variable will be measured Method
Data Collection Collecting and recording raw data Records appropriate raw data (qualitative and/or quantitative), including units Results
Organising and presenting raw data Presents raw data clearly, allowing for easy interpretation (e.g. well organised tables) Results
Data Processing and Presentation Processing raw data Processes raw data correctly (e.g. calculations, gradients and conversions) Results
Presenting processed data Presents processed data appropriately, helping interpretation (e.g. graphs) Results
Conclusion and Evaluation Drawing conclusions Gives a valid conclusion, based on the correct interpretation of the results, with an explanation and, where appropriate, compares results with literature values Discussion
Evaluating procedure and results Evaluates procedure and results including limitations, weaknesses and errors (compromises to validity) Discussion
Improving the investigation Identifies compromises to validity and reliability and states realistic suggestions to improve the investigation Discussion

 

Writing Practical Reports

Your practical reports should include each of the following sub-sections, where appropriate. Sometimes, for example, it may not be necessary to include some sections (such as Hypothesis or Background Theory). It is up to you to decide which sections are relevant. If you are not sure, ask your teacher.

Title

Write a title for your investigation that gives the reader a clear idea of what your experiment is about. The title will appear on the cover page of formal tasks by default but you should also include it at the beginning of your report.

Research Question

This must be clearly stated and be quite specific. This is the scientific question to be solved. It is best expressed as a focused question, which is a question that is answered with a statement, not just a yes or a no. For example, "How does varying the length of a fixed mass pendulum affect its period of oscillation?" is better than "Does the length of a pendulum affect its period?" It is best to avoid questions that can only be answered with yes or no answers. Start your question with words like how, why or when.

Try to limit your problem and keep it simple. Note that the previous question contains only two variables. To find the answer to a question such as "What factors and how do they affect the period of a pendulum?" would require that you test several different independent variables.

Choose a problem that is focused and can be solved experimentally. For example, the question "What is the period of a pendulum?" can be answered by finding the definition in a dictionary.

This part of the scientific method is the most important and often the hardest part. Defining the question means that the scientist must narrow the scope of their research down to one area. Often, scientists will pose a series of questions all relating to the same topic area. Doing more background research will usually help them to narrow their focus down onto one question.

Background Theory

Give a brief account of the theory behind the experiment(s) being done. Ensure that it is relevant to the research question and hypothesis (or hypotheses) under investigation and is scientifically accurate. Many investigations will involve a number of separate experiments each testing the effect of a different independent variable, e.g. temperature, force. Use separate paragraphs to explain clearly and distinctly each.

Hypothesis

Making a hypothesis is an important initial step in finding out something by experimenting. A scientist will always have an answer in mind before designing an experiment. This possible answer is called a hypothesis. A hypothesis is not just a guess or inference. It is made after considering all of the available information. Scientists will use their initial research or background theory and any other knowledge they have before making a hypothesis. Also, a hypothesis is a general statement. It doesn't just try to explain one observation or one situation. It will explain many observations or instances related to a problem or question.

A hypothesis is, therefore, an idea about the solution to a problem based on knowledge and research. While the hypothesis is a single statement, it is the key to a successful research project. All of your project research should be done with the goal of expressing a problem, proposing an answer to it (the hypothesis) and designing project experimentation. Then all of your project experimenting will be performed to test the hypothesis.

The hypothesis should make a claim about how two factors relate and should be:

  • Stated in the form “If the ‘independent variable’ is increased/decreased then the ‘dependent variable’ will increase/decrease (see below for explanation of variables). Write a statement such as “It is hypothesized (or predicted) that ... because ...” It is directly related to the research question and if necessary rewrite your research question so that the two are related.
  • Where possible, the hypothesis should represent a quantitative, mathematical relationship between two variables. For example, “When mass is kept constant, the acceleration produced is directly proportional to the applied force.”
  • Clearly explained in terms of relevant physical theory. This should be done in the section above “Background Theory”.

If your investigation involves several separate experiments, each separate experiment must have its own heading eg Experiment 1 - The effect of temperature on ... etc, its own hypothesis, method, results and conclusion. Identify each separate experiment clearly.

Variables

The variables should be clearly identified and categorized as follows (use these as subheadings):

  • Independent (the variable that you are manipulating to observe its effect on the dependent variable)
  • Dependent (the variable that changes as a result of the independent variable being changed)
  • Controlled (these are the variables which have been kept constant - all relevant controlled variables should be included

Apparatus and Materials

Apparatus

List the equipment used in the experiment. In most cases you should include a fully labelled diagram.

Materials

List the materials (chemicals), if any, used in the investigation - be specific with quantities eg approximately 2 g of magnesium whose mass is accurately known.

Method

It is preferable that your method should be written in third person past tense, e.g. The pendulum was pulled back to an angle of 30° with the vertical. The method should also specifically address:

  • how the independent variable was changed.
  • how controlled variables were kept constant and what their constant values were, e.g. the temperature was kept constant at 25°C using a water bath.
  • how the dependent variable was measured.
  • how many times the experiment was repeated (or if only done once why this was the case eg. time/equipment constraints).

If you are following a procedure given to you on a sheet you do not have to rewrite the sheet. It is perfectly acceptable to write that the Apparatus, Materials and Method provided on the sheet were followed. Additional statements should only be provided if you varied from the given procedure.

Results

Data Collection

Both qualitative and quantitative data should be recorded. Be sure to record relevant controlled data as well as the measurements and observations. The data collection assessment criterion applies specifically to this section of your report.

Data will usually need to be tabulated (placed in a table). Tables should be well set out with clear headings and units. Where appropriate, quantitative data should include uncertainties.

Always include a title for your table so that you can reference it later in your report, eg. ‘Table 1 - Voltage versus current for an ohmic conductor’.

Qualitative and quantitative comments about errors and uncertainties may be relevant in this section of your report. Qualitative problems might include parallax problems in reading a scale, reaction time in starting or stopping a timer, random fluctuation in a readout or difficulties in knowing when a moving object actually passes a point.

Data Processing and Presentation

Quantitative data will usually need some sort of processing and should always be processed and included in your report.

If appropriate, your results should be presented as a graph. Graphs of the data or processed data are often used to illustrate an equation or a relationship between data.

Graphs can be hand drawn on graph paper or drawn with a graphing program such as MS Excel. If using a graphing program this should be stated. Choose the appropriate type of graph (line graph, bar graph etc), label axes, include units. Use an appropriate scale, plot points clearly (use a circle with a dot inside it), draw lines of best fit (do not join the dots) and if there are multiple values include a clear key. Often graphs are used to extract information, eg. gradient, or for interpolation or extrapolation. If this is the case it must be shown clearly on the graph.

You should always include a title for your graph such as ‘Figure 1 - Voltage versus current for an ohmic conductor’.

Calculations - A fully worked example of each type of calculation performed can be included here. You should make clear statements about where the values you have used actually come from (eg. from the gradient of the graph, from Table 1, etc.) If many repetitive calculations need to be carried out only one should be shown, the results of other calculations could be included in a table in the Data Collection section.

You must analyse the results. You should comment on their accuracy and relate them to your hypothesis. If you referred to literature in your hypothesis you must also relate to the literature. Remember you can never prove a hypothesis and you can’t conclusively disprove one either, so avoid using those words. Alternatives are the results do not support the hypothesis or the results agree with the hypothesis. If you don’t think your results are reliable then say so: the results are inconclusive. If the results are inconclusive or unexpected, you should suggest that further experimentation is recommended.

Conclusion and Evaluation

The evaluation and conclusion assessment criterion relates specifically to the Evaluation and Conclusion sections of your report.

Your evaluation should follow the format of the assessment criteria. As a general rubric, you should do the following:

  • Write a conclusion, which is based on the correct evaluation and analysis of your results, and give a clear explanation of how you came to this conclusion using your results and analysis.
  • Compare your value with literature values or compare the relationship you have determined with the commonly accepted one or with a particular theory or law (eg. force is directly proportional to the acceleration produced).
  • Evaluate your procedures and results by pointing out and explaining any limitations, weaknesses and errors. Limitations can be thought of as things that you cannot change. Weaknesses are compromises that can be improved and errors are those associated with measurement.
  • Suggest how the limitations and weaknesses described above affected your results.
  • Suggest ways of improving your investigation so that any weaknesses and errors can be minimised.

Do not be trivial eg. “We did not have good equipment” and do not discuss personal errors eg. “Rob could not operate the stopwatch because his prefrontal cortex had been severed during a lobotomy.”

Improving Your Investigation

Suggestions:

  • could include suggesting more repetition only if this would help improve the accuracy of the results.
  • may suggest using more accurate equipment but be specific.
  • should be realistic (a useful way of thinking about this is imagine you were asked to do the investigation again, what would you do differently/better to improve the experiment)

Suggest reasonable modifications to improve the procedure or suggest alternative procedures. Remember you are evaluating the procedure itself; this includes the use of equipment, the management of time and the processes themselves. It is inappropriate to criticize the instructions you are given - this will just irritate the marker. In your evaluation, you should not evaluate safety or how you worked as a group etc. Do not include any mistakes that do not affect the outcome of your experiment e.g., surgically removing your partner’s appendix during the experiment because you were bored. If you make mistakes that cause you to run out of time, write this up as poor time management on the part of the experimenter but don’t go in to too much detail or better still try to avoid admitting to poor time management by suggesting an improvement such as “If we could travel faster than the speed of light and time could go backwards….” Your evaluation is not a story; keep it concise. A suggestion such as conducting the experiment in groups instead of individually does not count.