Investigating the Damping of Motion in a Simple Pendulum through Induced Eddy Currents

Background:

When a conducting material passes through a magnetic field, it experiences what are known as eddy currents. The relative motion of the conductor to the magnetic field causes a circulating flow of electrons inside the conductor. These 'eddy currents' create electromagnets. According to Lenz's law the induced current produced in a conductor will always flow in such a way that the produced magnetic field opposes the field which produced it. An increase in magnetic field strength, or an increase in conductivity of the material, results in an increase in eddy currents.

Risk Assessment:

Electrocution:

Likelihood = 1(Low)

Danger = 3 (Mild electrocution)

Risk Factor = 3x1 = 3 (Acceptable)

Dropping heavy equipment:

Likelihood = 1 (Low)

Danger = 3 (Possible fracture of toes, etc.)

Risk Factor = 3x1 = 3 (Acceptable)

Overall, the experiment is not a particularly dangerous one. There aren't any very risky procedures, and hence the likelihood of all accidents is low. The currents and voltages being used may, at worst, cause pain and burns, but the chances of electrocution are very low. Although heavy equipment is being used (namely the electromagnet core), it is not being moved regularly and as such does not present a serious risk, nor a reasonable chance of injury.

Results

Other than the single anomaly, highlighted in red, the results of this experiment were very consistent, with at most .6 of a second difference between any two repeats. Overall, this method has proven to give good results in terms of consistency. The main limitations in my experiment were the maximum frame rate of the camera, and the fact that not all frames were recorded, as some were 'dropped' during transmission from camera to computer.  I made sure that I got the best results I could by setting the camera to it's maximum frame rate of 30 frames per second.

The results are not what one would expect. My background research suggested that an increase in conductivity would result in an increase in the induced eddy currents. Hence, a decrease in resistance would have the same effect, with conductivity being equivalent to (1/resistance). Following are typical values for the resistance of the metals I tested, in ascending order:

Thus one would expect Copper to be effected most by the induced eddy currents, when it was in fact quite the opposite. One possible explanation for this would be to do with the fact that the metals have very different densities. Because of the differences in densities between the metals, it was impossible to keep constant both the volume and the mass. Having opted to keep the volume the same, there was large variation between the masses of the metals. Looking now at a table comparing the densities of the materials (on the next page), we see the vast difference between the density of aluminium and that of the other metals. It seems that the large percentage decrease in aluminium was simply due to its lower mass, and therefore decreased momentum. In fact, the results seem to follow exactly the trend represented by the density – that is, the greater the density, the greater the momentum, and thus the less effective the eddy current dampening.

Conclusion

All in all, the experiment did not achieve its goal. Whilst the method did yield good results, the factors I was trying to measure were simply too small when compared to others over which I had little control.

I did not seek any help throughout the duration of my coursework.

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