Apparatus
1 C-core
1 Iron Nail
1 insulated wire (1m long)
1 power pack
2 power pack cables
2 crocodile clips
1 Petri dish
Paperclips (at least 200)
1 paper cup
Diagram
Method
Firstly the iron nail was taken and wrapped with twenty turns of thin insulated wire, each end of which was attached to a power pack via crocodile clips. We set the power pack to direct current at 2V. A paper cup was approximately half filled with paperclips, and the nail was held by the head over the paper cup. When the power to the power pack was turned on, the nail was dipped into the paper cup and quickly stirred to pick up as many loose paperclips as possible, then taken out of the paper cup and the power turned off so that the number of attracted paperclips could be obtained. This had to be done quickly because the power pack quickly reset due to the short circuit created by the wire. After the number of turns had been recorded the nail was dropped to demagnetise it, the paperclips were placed back into the cup and the experiment was repeated twice (for three tests in total).
This whole process was then repeated, each time adding five turns to the coil, up to a total of fifty turns, producing seven sets of three results. This ensured a good enough range of data to produce accurate averages and hopefully establish a pattern between number of turns and paperclips picked up.
Roughly the same experiment was then repeated using a c-core as the core of the electromagnet rather than a soft iron nail as in the first experiment. The c-core was used because it is a more effective electromagnet than a nail, and because by using multiple types of cores, we could establish that there is, or is not always a direct proportionality between number of turns and strength of electromagnet, regardless of the type of core.
The method of this experiment was very similar, except that we began with ten turns instead of twenty, and went up to thirty (again, in increments of five turns). Because of the physical dimensions of the c-core, we also used a Petri dish in place of the paper cup, which proved to be more practical, and allowed for the paperclips to be more free to move. The c-core was tapped with a steel boss to demagnetise it after each test. As our preliminary results suggested, we also used three volts.
If any anomalies were encountered then that test was taken again and the anomaly disregarded.
Fair Test and Variables
Within each experiment, all of the variables were kept constant except the number of turns, meaning that any change in the results would be due entirely to the change in number of turns. The tests were carried out three times and an average obtained so that anomalies could be identified, repeated and disregarded. Although many of the variables were changed between using an iron nail and using a c-core, this does not matter as we are not directly comparing the two results, merely observing if they have the same pattern when number of turns is changed. In this way it is best to change as many variables as possible so that we can determine whether even in a completely different electromagnet test, changing the number of turns still has a similar effect.
The variables that may have had a bearing on the result are:
Results
These are the results gathered during the two experiments, the ‘Increase’ column shows the increase in the average number of paperclips collected since the previous set of tests.
Results collected using and iron nail core
Results collected using a c-core
Analysis
Once plotted on a graph (below) it is clear that there is a trend whereby an increase of turns yields an increase in strength of the magnet. The trend appears to be linear, and so I have plotted a linear line of best fit, however the line of best fit does not reflect a direct proportionality between number of paperclips and number of turns on either experiment, if it did, the trend lines would pass through (0,0).
The number of paperclips picked up, however, is not an accurate measurement of the magnet’s strength, if we could have measured the magnetic strength on a continuous scale and plotted a chart of number of turns to magnetic strength (eg, in Tesla or gauss), then a more accurate depiction of directly proportional data might be expected.
Conclusion
To conclude, it seems my prediction was accurate. The results show a definite trend between number of turns on the coil and magnetic strength using both types of core. I think had the experiment being an accurate measure of magnetic strength and been performed under perfect conditions, we would have discovered that the two variables were directly proportional. I believe this for the same reasons I stated in my prediction, for the c-core at least, ampere’s law applies and that would produce a (directly proportional) graph.
The theoretical magnetic strength at each stage using the c-core could be derived using this formula if I knew the resistance of the wire used, or had measured the output of the power pack in amps, however this would not have significantly helped because the strength in Tesla cannot be compared with the strength in terms of number of paperclips.
Evaluation
Both experiments obtained good results, both showing strong correlations indicating that the influence of changing variables other than the number of turns was minimal. I think the experiment involving the c-core was performed under more optimal conditions, and the increased number of paperclips allowed us to treat the number of paperclips as something close to a continuous scale. The use of the Petri dish also improved the experiment by stopping the paperclips catching in the c-core experiment, whereas in the cup the paperclips were packed more tightly together.
We did find that in some cases, the number of paperclips picked up may have been inaccurate because we could not ensure the paperclips were equally free to move on each test. This meant that on some tests, paperclips were catching on each other in the cup and could not be pulled out by the electromagnet. This could have been solved by using much smaller objects that had a shape so that they did not catch with each other. Iron filings would have been ideal for the purposes of the experiment, but would have been impractical to use. The large size and weight of the paperclips also meant that very few were picked up with the iron nail at lower number of turns, which would have been solved by using something much smaller also.
In an effort to free more of the paperclips and test the limit of what the electromagnet could carry, we moved the electromagnet in the cup or dish of paperclips. Although this was in the interest of fair testing, by reducing the amount of caught paperclips not being counted, it also introduced another variable, how much the magnet was moved. This was very difficult to do accurately in the short time before we had to turn off the power for each test, and may have produced some unfair results.
