It is very likely that most of the practical work you will do will involve finding the value of a physical quantity. We call this an experiment. However, there are going to be cases where you will be asked to ‘Find out whether …’ or to ‘Find out how …’. For example, you may be asked to find out whether the extension of a rubber band is proportional to the force stretching it. If the activity is to find out ‘whether …’ or ‘how ...’, we call it an investigation.
Whether the activity is an experiment or an investigation, however, there will always be an aim and a conclusion. The aim is an opening statement of the purpose of the activity. The conclusion is a closing statement of the result or the outcome of carrying out the activity.
For any activity, experiment or investigation, the conclusion should always be related to the aim.
Undertaking a practical activity may be divided up into three distinct parts: 1 Preparing, i.e. planning and designing the activity.
2 Executing, i.e. carrying out the activity.
3 Reporting, i.e. compiling an accurate account of all that was done in relation to the activity and stating the result.
Preparing
To prepare an experiment or an investigation means to plan it and then to design it. To plan it is to prepare an outline of what you are going to do, whereas to design it is to draw up a detailed account of how you are going to carry out the plan. The plan will be based on the theory associated with the activity. The design will go much further. It will be concerned with all or most of the following points
1 Deciding on the list of apparatus to be used, and a diagram showing how the apparatus is to be arranged.
2 The range of values to be employed, if appropriate.
3 The range and sensitivity of the items of apparatus (see figure 3.6).
experiment ❯
investigation ❯
aim conclusion ❯
An activity might be based on a familiar situation such as finding the period of a pendulum of given length or the sizes of the feet of your classmates) or a situation that is less familiar and whose results are harder to predict. For example: What is the effect on the period of a pendulum if we keep themass of the bob constant but alter its radius? Is there a relation between the height of males and the size of their feet?
preparing ❯
design ❯
range ❯
0 – 10 A
NO !
0 – 1 A
YES !
Figure 3.6 An example of a decision you might have to make when planning an experiment. Why
is the 1 A meter a better choice?
4 Precautions to be taken to achieve the best accuracy and to ensure against accidents and damage to any person or to property.
5 The sizes of quantities that might make for precision in your readings, where possible.
6 Limitations are circumstances or conditions that might prevail during the
conduct of the activity that might adversely affect readings obtained. Very often these conditions are beyond the control of the experimenter, and steps must be taken to reduce their effects.
7 The presentation of your results. This is an important consideration. Should the results be tabulated? If so, how should the table be drawn up and what should the headings be? What should be the degree of significance in each column?
8 The question of whether or not a graph should be drawn and, if so, of what variable against what other variable. If a graph is drawn, some use will be made of it. In many cases a slope is taken. If this is the case, the design will include the way in which this slope is to be used to help you to arrive at a conclusion. The steps between the graph and its use in arriving at a conclusion will all have to be carefully considered in the design.
The variables are the quantities that change as the experiment or
investigation proceeds. One quantity is given a convenient value to start with. (This is called the independent variable) and as a result another quantity in the activity changes. This quantity is called the dependent variable. When we draw graphs, we usually plot the dependent variable on the vertical axis (see figure 3.16).
Executing
After you have drawn up your plan and your design, you are ready to carry out the activity. This should be done in two parts:
• the first part should be a trial (sometimes called a ‘dry run’); • the second part is the activity proper.
To carry out the dry run, the apparatus is set up in accordance with your design. If, during this dry run, you find that it is necessary to make changes to your design, then you should feel free to do so. You must be sure, however, that the changes you introduce would produce a better result at the end.
If the dry run goes as expected and you encounter no hitches along the way (such as conditions you did not anticipate and therefore made no allowance for), then you will be ready to carry out the ‘real’ experiment.
The sensitivity of an instrument is the value of the quantity measured by that instrument, which is represented by 1 scale division. For example, if there are 100 whole scale divisions on the scale of a newton-meter that measures forces up to
10 N, then 1 scale division would represent 10
100 N
or 0.1 N. The sensitivity of this newton-meter would therefore be 0.1 newton per division or
0.1 N div–1. Sensitivity is not the same thing as
precision or accuracy (see chapter 2).
limitations ❯
Try to get into the habit of presenting your results in a neat and orderly manner. Do not scatter results all over the page.
variables ❯
independent variable ❯
dependent variable ❯
Two helpful hints
1 Analogue meters must be chosen so that
maximum readings obtained on them occupy a fairly large fraction of the full range. If a digital meter is used, the range selected should give as large as possible a number of significant figures in the reading (better precision).
2 Never take your apparatus apart before you
are satisfied that your work was brought to a satisfactory conclusion. You may need to use it for a further check!
which a graph is to be plotted, this graph should be plotted as the values of measurements are obtained. Axes for the graph can be set up once the maximum and minimum values of the independent variable are used in the trial. Be sure to check that the corresponding values of the dependent variable can be measured on the instruments selected for use in the experiment or investigation. If this is not the case, you will have to adjust the range of values you use to suit your instruments or to change the range of the instruments to suit the range of values envisaged in your plan. Plot these two extreme points on the axes.
Having plotted these two points, you should now plot the intermediate values and look for the shape of the emerging graph. You should find that your graph is either a straight line or a smooth curve (figures 3.7 and 3.8). You may find that a point or two do not quite ‘fit in’ with the trend that the graph shows. In a case like this, you should go
back to your apparatus and check the co-ordinates of the point again. If you get the same values, then you should go ahead and plot them. Do not reject the point at this stage.
Sometimes plotted points are so arranged that it is difficult to tell whether the graph to be drawn among them should be a smooth curve or a straight line. Guidelines that will help you to decide how to draw the best line or the line of best fit are given on pages 28 and 29).
Reporting
Your report should be an accurate, dated account of all that was done in carrying out your practical activity. If the activity is an experiment, you can use some or all of the following headings.
The aim
This is a statement of the purpose of the experiment. The method to be used should be stated, where possible (for example: ‘To determine the density of a liquid by Archimedes’ principle’).
Apparatus
You should write down all the items of apparatus used in the experiment, together (where possible) with a statement of particulars of each (e.g. range and precision for ammeters, thermometers and newton-meters).