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I have done two forms of trial experiment to determine, which wave is best to investigate the inverse square law and which equipment is best suited for experimental work. There are 4 main areas to which the inverse square law applies these are, Gravity, sound, electric fields, and members of the electro-magnetic spectrum. To investigate gravity is impractical as you can do 1 of three things, you will have to work over enormous distances, I. E. several astronomical units, or have instrumentation accurate enough to measure the gravitational attraction of tiny measures, or do a completely theoretical experiment.

All of these options are impractical. It is impractical to investigate electric fields on a safety aspect, you will either need extremely sensitive equipment to detect a field, which is weak enough to safely work in, or risk serious injury and work with a very powerful electric field. Both of these are impractical. It is possible to investigate sound, but in my experience unless your equipment is extremely sensitive, and not at my disposal, it simply doesn’t work. So I chose to investigate two members of the electro-magnetic spectrum, Gamma radiation and visible light.


Ideally I would have investigated gamma radiation and radio waves, the two opposing ends of the spectrum, however we do not have the equipment available to measure radio waves with wavelengths in the region of 103 meters, so I did experiments involving visible light and Gamma radiation. Both members of the electro-magnetic spectrum, gamma being present at the far end of the spectrum with extremely high frequency of anything from 1019 to 1021 Hz and a low wavelength of anything from 10-11 to 10-13 metres.

It is Gammas high frequency, which gives it a high energy level; this is because the energy of a member of an electromagnetic wave is deduced by multiplying the frequency by planks constant. Visible light is present on the right side, with relatively low energies. It has a wavelength of 10-6 meters and a frequency of 1015 Hz. Both experiments were essentially the same, move a detector away from a source of waves, and measure the intensity of the wave at certain distances, in order to determine if the intensity drops of inversely proportional to the square of the distance.

Prior to beginning the Gamma experiment using the rad count I took background counts over a period of a minute, then waited a minute, then took it again over the same time period of 1 minute. I did this at a variety of places in the lab I was using to find if there was a space with a less fluctuating background count, so my experiment would be more accurate, however I discovered the variation in the background count to be practically identical in all areas of the lab.

Gamma Experiment The purpose of a trial experiment is not to prove the validity of a hypothesis but rather to determine which practical apparatus and working practises are best suited to the main experiment. Practical issues The majority of practical issues are mainly concerned with which apparatus should be used. What must first be decided is what actual source are you going to use. We only had access to one source of Gamma radiation this was cobalt 60. So I used cobalt 60.

When dealing with the inverse square law the most obvious aspect is that you are measuring the intensity, and distance of a source, so these are the two factors which are most important. To measure the distance of the source, I have three kinds of apparatus at my disposal, a simple metre rule, vernier scale callipers, and digital vernier scale callipers. I ruled out the sight-read vernier callipers first because of their impracticality, I feel my inexperience with them may lead to high inaccuracies, as I find it difficult to read from them.

I feel I can measure a metre rule to +- 0. 5 mm, the digital vernier callipers can read to an accuracy of a hundredth of a millimetre. It is of a much higher level of accuracy, however is actually very difficult to physically use and measure small gaps without knocking the equipment and rendering the reading useless, it is easier to use over lengths of 5cms and above, but the percentage error here is so small I may as well use a ruler anyway. Also this is only a trial experiment so accuracy isn’t really that important. So I feel I will use a ruler due to its logistical benefits.

The next thing to decide is what kind of dector should be used, obviously it will be a Geiger muller counter and tube, but what kind should be used? There are two ‘Families’ of Geiger muller counter and tubes these are Scalar counters and rate meters. We only had access to two kinds of ratemeter, a large analogue Meter and a digital rad count I choose to use the digital rad count because simply it was the most recent radiation dector we had, and in previous experiments the large analogue meter had proven to be unreliable.

So the digital rad count it is. Methods Over lengths of 15cm it is hard to ensure the point source of radiation is directed exactly at the dector, if it is not pointing directly, the error will be very small, but as distances increase the error will increase, the greater the distance the greater the error due to the angled nature of the source. To combat this I propose two things, the radiation source will be placed in a ‘L-frame’ or rig the dector will then be placed in a retort stand at an equal height to keep the source and dector at an even height.

Secondly I will securely attach the meter rule to the work top I will be working on, I will ‘brace’ the rig against the ruler and then position the retort stand and clamp to be in line with the rig, which will then be left untouched, and the position of the rig in relation with the stand will be kept in place with the bracing against the straight ruler. The presence of a securely fixed ruler also helps with the actuality of taking readings.

When measuring from point source to dector it is easy, if the ruler is being held in mid-air and not held against anything, to knock the equipment, or take erroneous readings. However with the ruler secured to the worktop, it not only can’t knock the equipment but makes taking measurements easy. As long as you take the first reading with the source at zero distance to the dector rather than measuring to the point source, it is now possible to use the vertical strut of the l-frame a your reference point to measurement.

This makes it more accurate, and reduces the dangers of parallax. Also what must be decided is the range of distances I will work across and the increments by which I will increase my distance. I feel the best distance to measure across will be from zero distance, through to 15 cms. And I also feel it is advisable to increase in stages of 5mm. I believe if we plot distance against 1/Vc then a straight line will be described, so a sufficient number of results are necessary to accurately plot this.

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Kylie Garcia

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