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Experimental work in Physics

Since the very basis of much of the study of Physics is by experiment it is important to know how to carry experiments properly and how to present your results in the correct way. Very little faith can be put in a theory if it can never be backed up by an experiment. They are of two main types:-

(a) standard investigations that are designed to prove accepted quantities or laws
(b) "open ended" experiments which are designed to investigate previously "unknown" areas of the subject where there may be no previously recorded answers

Possible open ended experimental investigations

1. Energy in a balloon 20. Drag on small boats – use a piece of guttering 39. Friction in curtain rails
2. Viscosity of liquids 21. Extension of the wire experiment – surface area/bolometer 40. Cars and carpet – braking experiments
3. How bubbles arise in liquids 22. The properties of a small water pump 41. The strength of adhesives
4. Forces in athletics 23. Electromagnetic induction – separated coils – thickness measurement 42. Bouncing balls
5. Vibrations in a soap film 24. Energy in a light bulb – LDR etc. – colour effects 43. Optical properties of treacle
6. Defects of a lens - astigmatism 25. Mixing water and methanol 44. Stretching a rubber band – heat effects
7. The propeller and drag 26. Expansion of a loaded rubber band when it is heated 45. Strength of plants – bending in the wind
8. Mechanical resonance 27. Resonance in buildings – earthquake simulations 46. The bifilar suspension
9. Growth of current in a light bulb 28. Absorption of light by glass 47. Waves on a stretched spring
10. Diffraction and resolution 29. Potential on a Van der Graaff – paper levitation 48. Falling magnet
11. Singing tube 30. Attractive forces in electromagnets 49. Fatigue in aluminium strips
12. Craters 31. Resonance in buildings – earthquake simulations 50. The bifilar suspension
13. Damping and resonance 32. Reflection of microwaves from mesh of different sizes 51. Effect of polish on friction
14. Strength of a polythene bag 33. Transmission of light through glass and water 52. The silt meter
15. Conductivity of salt solution 34. Thrust of model aircraft propellers 53. Transformer efficiency
16. Capillarity in sugar cubes 35. Electromagnetic clutch 54. Strengths of wet and dry tissues
17. Bending of a beam 36. Photochromic sunglasses 55. Penetration of nails into wood
18. The jumping ring 37. Standing waves on a vibrating wire hoop 56. Objects in an air stream
19. Viscosity of liquids 38. Vibrations in a soap film  

Some further comments on "open ended" experiments

In these experiments you cannot be sure what you will find - you won't find them written up in any book so it is especially necessary that you plan them carefully.
Before starting ask yourself these questions and be able to answer them!

What am I going to measure or investigate?
Do I need a control experiment?
What apparatus will I need and is it available?
Are my aims realistic?
How much time will I need for the investigation?
What is my overall plan of action?
Will the readings be taken manually or automatically?
How will I present my results?
Are there any safety considerations that I should think about?

Suggested procedure for practical work

You should treat all your experiments carefully, no matter how simple they appear to be. Although it is unlikely that you will stumble on a new law of physics during your a level course there is always a chance and careful experimental work just opens up that possibility! A good experimental technique gained at this stage could be of immense use later on.

When embarking on an experiment try and follow something close to the following suggestions:

1. Read all the instructions carefully before starting your practical work

2. Check that you have all the apparatus that you need and that you know how to use it. This is especially true of meters and power supplies - ask for help if you are not sure which output or range to use before switching on.

3. Plan your work before doing anything, that includes the setting up of the apparatus and the presentation of results

4. Decide on the both the spread and the number of readings that you are going to take of the various quantities.

5. Do not "set" your values to exact numbers - i.e. when making a series of measurements of the period of swing of a pendulum take readings at roughly 5 cm intervals - do not try to adjust the length to exact multiples, it will be very difficult and you are likely to measure inaccurately

6. Always repeat your readings if there is time and certainly check any reading that is to become a "base" value used in many later parts of the experiment. Do not rush your work!

7. All results should be quoted to a sensible degree of accuracy and this accuracy recorded. Don't claim an unreasonable degree of accuracy. Although a stop clock may show times to within 0.01 s you may only be able to judge when to press it to +/- 0.1 s and it this that should be quoted as an accuracy.

8. If you are asked for the gradient of a line in a practical question it is likely that the line is a straight one, but if the slope at a specified point is asked for then the line will probably be a curve.

9. Include a comment of all errors and difficulties and how you overcame them or suggestions for their solution.

10. A full and complete conclusion should appear at the end of each experimental account. Remember that experiments without a mathematical answer are just as important as those that do have a numerical result and a conclusion is still needed.

Tables of results

When writing up your practical work it is essential that tables of results are presented properly. They should:
Be fully ruled out using a ruler
Contain the correct information at the top of each column
Contain only figures in the actual measurement and calculation section
Contain the quantity at the top of each column
Contain the correct units at the top of each column
Contain an estimate of the accuracy of measured quantities at the top of each column

An example of part of such a table is set out below:

Drawing a graph

When drawing graphs the following points should be remembered:

1. The graph should fill the page if possible
2. It should have a sensible scale (e.g. 5 units matching five squares etc.)
3. The scale should be labelled with the quantity, symbol and units
4. The points should be clearly plotted, either as a small cross or a dot with a circle round it
5. The graph should be given a title
6. The line should always be the best fit curve (maybe a straight line) unless you are specifically requested otherwise.
7. At least eight points should be plotted for a straight line graph and ten for curve
8. The gradient should be taken over as large a section of the graph as possible


© Keith Gibbs