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Electrical extension cords

2. It is generally dangerous to tuck electrical extension cords away behind furniture, under carpets, or anywhere else that they might cause a fire. It's the same reason why electric heating elements on stoves or in room heaters operate. What principle is involved and how does it operate in each of these situations?

3. Extension cords which are designed to carry heavier loads are generally larger in diameter than those which are not. Is this just a matter of adding more insulation, or is there a physical principle at work causing the larger diameter? Explain.

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Answer to question 2:

The passage of electric current involves transformation of electric energy into heat.
The power of heating = heat energy released per unit time = R I^2 where I is the current and R is the resistance of the wire in question.

Stoves and heating elements use this effect for good cause.

However the warming of electrical wires intended for other use is an unwanted side effect which, in case something goes wrong, can cause fires.
By "something goes wrong" we usually ...