When a positive anode and a negative cathode are attached at either end of a conductor, electrons flow from the negative end toward the positive end as the charges approach a state of equilibrium. The current diminishes as the difference between the charge levels grows smaller, unless the charges are replenished via chemical action, as in a battery, or mechanical action as in an electrical generator.
It is a historical accident that current is said to flow from the negative to the positive pole. There is nothing inherently negative about an electron. Benjamin Franklin believed current flowed in the direction opposite to our present notion, and his nomenclature has been retained.
The medium through which the current flows is never a perfect conductor, nor is there such a thing as a perfect insulator. Any wire that is not a superconductor will have a certain finite resistance, measured in ohms. This will vary depending on the type of material, its temperature, strain and physical dimensions. The reciprocal of resistance is conductance, in SI notation measured in Siemens.
Early electrical experimenters and telegraph system engineers found that electrical insulation was necessary to prevent conductors from shorting against each other or grounding out. When electrical power and light emerged as a principle application, ampacity and its relation to temperature rise became the focus. It was quickly found that excessive heat over a period of time degraded insulation. Today a principal concern in the National Electrical Code and other standards is to coordinate maximum current flow, wire size and insulation type to prevent electrical fire in premises wiring systems.
What differentiates a conductor from an insulator is the existence of mobile charge carriers, free electrons or charged ions. Even the best (highest resistance, physically robust) insulators contain a small quantity of charge carriers, which permit some current flow.
The application of an excessive voltage across an insulator (duration also being a factor) can cause a sudden, sometimes catastrophic event that causes a conductive path through the insulator. The resulting current flow can damage electrical equipment. When atmospheric air is the insulator between earth and thunderstorm clouds, the result is also seen in the sky as lightning.
As we all know, at our non-quantum scale, energy, electrical or otherwise, never disappears. It just changes form. Depending on circuit parameters, a certain amount of electrical energy, rather than passing through a conductor without loss, converts to heat, measurable as I2R, where I is current, originally labeled intensity, and measured in amps; and R is resistance, measured in ohms.
Engineers speak of the I2R loss in conductors, circuits or devices. Because I is squared, it is by far the more significant contributor.
Copper is entirely suitable as a conductor in most electrical applications, residential, commercial and industrial. The only drawback is that commodity prices sometimes drive the cost of this product off the chart for large diameter/length configurations.
For an electrical service, high ampacity feeder or distribution line, especially a long underground or aerial installation, aluminum is the realistic alternative. It has higher resistivity than copper, but that is figured into the scenario when the installation complies with the National Electrical Code, which generally requires the next size larger for aluminum.
There remains the problem of terminations. When using copper, the termination process is simply a matter of laying the stripped end in the lug and torquing to the correct specification. In contrast, aluminum conductors demand special techniques to avoid electrical fire hazard.
You can torque an aluminum electrical connection to the specified amount, but eventually it will loosen up. The reason: The aluminum shrinks slightly, retreating from the mating metal surface, because of an extremely small accumulating amount of corrosion which is a consequence of normal moisture in the air. The resulting spatial separation boosts the resistance in the connection so there is a greater amount of I2R heat that must dissipate. This temperature sets the stage for ever more corrosion and heat, which eventually translates to flashes of light, an audible frying sound and the possibility of electrical fire which may escape the termination enclosure.
Fortunately, there is a simple procedure that is certain to mitigate this hazard. It employs a small container of corrosion inhibitor that is good for a great many jobs.
Electrical corrosion inhibitor, suitable for aluminum-to-aluminum and aluminum-to-copper connections, is applied to the stripped end of the aluminum conductor and to the mating surface of the lug. It is important to use a product that is approved for the application, and to follow the manufacturer’s instructions which are part of the listing. Specifically, the stripped conductor end must be inspected to ensure that there are no bits of insulation that did not get removed. These would make hot spots.
Also, the stripped aluminum end is to be wire brushed to make a good surface for the inhibitor. The entire procedure takes less than a minute. But the installer must scrupulously perform it on each and every aluminum connection to avoid the possibility of an electrical fire years later.
Another issue to be aware of is the galvanic corrosion that may arise when joining aluminum wire to copper wire. Aluminum and copper are dissimilar metals. Corrosion can occur in the presence of an electrolyte (salt air, for example) and these connections can become unstable over time.
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