Crossed Electric and Magnetic Fields

Questions about the Bose Electromagnetic machine

(2.1) The Bose electromagnetic fatigue testing machine in room 114 Upson-I applies load and displacements to a test specimen via the application of a variable magnetic field to permanent magnets aligned along the top load shaft. A simplified model of the internal structure is shown in the attachment. The specimen is held by the top and bottom load shafts. The copper electric coil is held in place via epoxy (thermoset) adhesive connections with the surrounding machine structure (not shown). The motion of the metallic permanent magnets through the magnetic field and the flow of current through the electric coil cause energy to be dissipated as heat.

Based on your knowledge of the potential effects of heat on the magnetic, electrical and mechanical properties of materials, summarize potential problems that could arise in this system (e.g. potential sources of error in the test results or potential mechanisms of failure in the machine itself) if the heat is not properly dissipated.

(2.2) The National Electrical Code specifies, among other things, values for the minimum allowable cross-sectional area of conductors for a given voltage and maximum allowable total length of circuits depending on the diameter of conductor used. Based on your knowledge of the motion of charge carriers, the origins of electrical resistivity, and the typical micro-structure of metallic conductors,

(a) explain why there is a minimum allowable cross-sectional area for a given applied voltage, and

(b) why there is a maximum allowable total length of electrical circuit for a given conductor diameter.

Would you expect these limits to be the same for copper and aluminum wires?

What about two copper wires with different grain sizes, work hardening histories or levels of impurities? Explain.

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