Give examples of changes in the internal energy of bodies when external forces are working.

Place a small piece of dry cotton wool on the bottom of the cylinder and insert the piston into the cylinder on a rod with a plastic handle. With a sharp blow of the palm on the handle in a darkened room, a flash of light in the cylinder is visible. A flash of cotton wool is due to the fact that when the air is compressed under the influence of an external force, its temperature has increased so much that the cotton wool ignites. The internal energy of the gas has increased due to the work of external forces.

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