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the PTC (positive temperatue coefficient) it is a resistance that its value increase with temperature. and as power dissipation in resistance = I*I*R(I2*R) so as current increase temperature increase and so the resisatnce value
Even your basic piece of copper wire has a PTC (Positive temperature coefficient). As the current through the copper increases there are more free ions moving. These free ions impede the current flow and increase the resistance of the wire.