Momentum and Energy


A hot object has thermal energy. How can it transfer this energy?

What happened to the increased thermal energy of the eraser and your hand, or to the block and the table? A short time later all will have cooled. They have transferred their thermal energy to the cooler surroundings. Energy transfer that results from a difference in temperature is called heat. Heat always flows from the hotter (energy sources) to the cooler objects (energy receivers).

Thermal energy can be transferred in three ways: conduction, convection, and radiation. Conduction occurs when two objects are in contact, like when you put your hand in hot water. Convection is the motion of a fluid, usually air or water. The fluid is heated by the hotter object, then moves until it contacts a colder object where it heats that object. You can think of convection as two instances of conduction. Radiation is infrared waves that are emitted by hotter objects and absorbed by colder ones. You can feel radiation if you bring your hands near a hot electric burner on a stove. The sun heats Earth by radiation.

What happens to the thermal energy in the surroundings? They get warmer, so their thermal energy increases. Then they transfer this energy to a colder object. Earth is warmer than the space around it, and so it radiates energy into space. When that energy reaches another planet or star, those objects are warmed, and so on through the entire universe.


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