An Experiment to Investigate How Fast Electrons Move
Method
We set up the apparatus as shown below but instead of a microscope slide we used a petri dish. When the switch was closed a coloured band of manganate (VII) ions moved towards the positive terminal.
In order to measure the drift velocity of these ions we drew two lines on the filter paper and dropped the potassium manganate crystals between the negative terminal and first line. As the crystals drifted across the first line, we started stopwatches, one for each crystal. We stopped the stopwatches as they drifted across the second line. We then divided displacement by time to find the velocity of each crystal.
Measurements
Before commencing the experiment, we considered what precautions we could take to ensure accuracy. We drew start and finish lines on each piece of filter paper used, measuring the distance between the lines with a vernier calliper to achieve the most accurate reading that we could with the equipment available. We adjusted the voltage from the power supply whilst reading from a voltmeter (set to 600v) so that we could ensure the best accuracy possible from our voltage readings. We used stopwatches which measured to hundredths of a second as we were unsure as to how close the time differences would be between the crystals. We were unable to acquire scales that would measure a small enough amount of the crystals to use in each experiment so we had to judge the size by eye. We were however advised that the size of the crystal would not affect the experiment and so we considered that this potential source of error may be negligible. We recorded all of our measurements which are shown below.
We felt that we could improve our accuracy further by performing a couple of trial experiments to see how we could improve upon our initial ideas. In our trial, we used a distance of 3.08cm and a voltage of 100v. We discovered that the crystal moved extremely slowly. We then increased the voltage to 250v and found that whilst the crystal appeared to move faster at this voltage, it was still very slow. We tried using different quantities of ammonia solution to see if this would affect the results. We found that the drift speed appeared faster with a larger quantity of solution but it was unclear as to whether some of this was due to diffusion. We tried varying quantities of ammonia solution in the Petri dish with the paper and concluded that the experiment would work best with a quantity of 2ml of ammonia solution and so this amount was fixed for our investigation to minimise the error due to the ammonia solution. We noted that there may be timing errors due to the thickness of the pencil lines and so used a sharp pencil with as fine a point as we could achieve. We were unable to remove the Petri dish and so had to clean and dry it as best as possible whilst it was in situ. We had to accept that this may lead to an error but we that we had minimised the error as best we could. We therefore decided to place three crystals in the dish at one time so that we would have comparable readings for each voltage and that the conditions for all three readings at a particular voltage would be as close to identical as we could achieve. We each timed one of the crystals which increased the error in timing due to differing reaction times but we felt this would be better than having one person attempt to time all three crystals to avoid confusion and further human error. We also noticed that the voltage changed during the experiment and so we decided to note the varying voltages and take an average.
