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Why does air need to be compressed in order to increase temperature?
When you have a gas, like air, you have molecules that are only interacting with each other when they bump into each other. They simply float around, minding their own business.
However, when you compress them, you make their volume smaller, the molecules are going to bump into each other more frequently. This will increase the energy inside the volume, as each collision will bring energy into play. More enegery means more heat: as a matter of fact, heat is a measure of the total energy of molecular motion in a gas! So more motion = more heat.
Temperature is a measure of the average energy of molecular motion in a substance. So, more motion, more temperature!