# Illustration Joule Heating: Resistors Heat Up to Different Extents

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Joule heating (also called ohmic heating) is caused by collisions of the charge carriers of the electric current $$I$$ with the atoms. A constant voltage $$U$$ is applied to two different resistors $$R_1$$ and $$R_2$$.
• The resistance $$R_1$$ is made of aluminum (Al) and is smaller than $$R_2$$
• The resistor $$R_2$$ is made of plumbium (Pb), i.e. lead.

The constant current $$I$$ passing through the two resistors heats the larger resistor $$R_2$$ more than $$R_1$$. The larger resistor $$R_2$$ glows, while $$R_1$$ hardly heats up compared to $$R_2$$.