How does my space heater get so hot but it’s 20AGW power cable stays cool?

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This is one of those space heaters that makes a coil get orange hot and a fan blows air across them.

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Those coils get hot because they have a high resistance value. The cord between the wall and the heater has a low resistance value. So it creates heat by having resistance. And uses the fan to keep those coils from melting.

On a micro level heat is atoms moving about.

Think of the cord like a hallow tube with ball bearings (electrons) being shot through it. When the balls collide with the inner walls of the tube, it moves the tube a little but not that much. You can say that the tube is low resistance since it doesn’t do much to slow down the balls.

Think of the heating coil like a forest of pegs with the same ball ball bearings being shot through it. As the balls collide with the pegs, they cause them to move about. Since heat on the atomic scale is atoms moving about, this heats them up. The heating coil and the peg forest have high resistance since they slow down what flows through it.