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Conductors and Insulators
11 questions By Tony R. Kuphaldt
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Question 7 of 11
What disadvantages might there be to using undersized wire in an electrical power distribution system, where electricity is transmitted from generators (sources) to “loads” (devices requiring power) far away? What might happen if the wires we choose for this application were to have insufficient cross-sectional area?
Reveal answerAn analogy: what would happen if the water pipes in a municipal water supply system were sized too small?
Notes:The effects of undersized wiring are multiple: not only is there inefficiency in the system when conductors are too small, but there are safety hazards as well!
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Question 8 of 11
What disadvantages might there be to using oversized wire in an electrical power distribution system, where electricity is transmitted from generators (sources) to “loads” (devices requiring power) far away? What might be bad about using wires with more than sufficient cross-sectional area for the purpose?
Reveal answerAn analogy: what would be bad about using oversized water pipes in a municipal water supply system? Nothing, perhaps, once the system was operational. However, what about the cost of building the system?
Notes:The bad effects of oversized wiring are more difficult to discern than the bad effects of undersized wiring. With oversized wiring, there is little electrical inefficiency, but this doesn’t mean the system is inefficient in other ways! The cost of the wiring itself, of course, is obvious, but there are other costs as well. Brainstorm ideas with your students about the cost-inefficiencies of oversized wiring in an electrical system.
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Question 9 of 11
What does the breakdown voltage (also known as dielectric strength) rating for an electrically insulating substance mean?
Reveal answerThe breakdown voltage, or dielectric strength, rating of an insulator is the maximum amount of applied voltage it may withstand per unit thickness without conducting appreciable current.
Notes:This question provides an opportunity for “qualitative” analysis: does the voltage rating of an insulator increase or decrease as its thickness is increased, given a constant dielectric strength?
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?