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Conductors and Insulators
11 questions By Tony R. Kuphaldt
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Question 10 of 11
How much voltage can a 50 μm thick piece of mica withstand before breaking down?
Reveal answerApproximately 1500 volts, or 1.5 kV assuming a dielectric strength of 31.5 MV/m for mica.
Notes:Be sure to ask your students where they obtained the dielectric strength rating for mica. Different sources of information may give different figures, which will lead to different answers to this question.
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Question 11 of 11
Design a circuit to test the breakdown voltage of a material.
Reveal answerI won’t provide you with a ready-made schematic diagram for such a circuit, but I will give you a hint. If a substance were to be subjected to a voltage in excess of its dielectric strength, what effect(s) would indicate breakdown?
Notes:Designs for testing dielectric strength may range from the crude to the sophisticated, but the real point of this question is to challenge students to reason from the effects of high-voltage breakdown to a means of detecting when that takes place, and measuring the voltage needed for it to happen.
For 10.
I used the second entry for Mica, phlogopite, at https://www.engineeringtoolbox.com/dielectric-strength-electrical-insulator-d_2185.html
with a dielectric strength of 31.5 E6 V/m.
My calculation using dimensional analysis: 50 mm x (10 E-3 m/1 mm) x (31.5 MV/m) = 1.58 MV
This is way off from 1 kV - what went wrong?
For 11.
Could you just take a battery, two wires, a small and measured piece of the insulator, and connect one of the wires to one end of the material and then the other wire to the other end, and bracket the voltage from ammeter readings until a small enough range is found for the voltage at which the insulator becomes a conductor? Taking account of the size of the material, of course.
As for the measurement of V/m - is the measured distance for m from one end of the circuit to the other, rather than any particular length of the piece of material?