I have a dilemma, which has been nagging at me for a week. So first off, can anyone verify my definitions are correct? Heat is the flow of energy, and that flow is caused by the collisions of the atoms in a system. The collisions cause a transfer of KE, hence heat will flow from a substance that has higher KE to a substance with lower KE, in other words, heat diffuses from higher temp to lower temp due to this microscopic view. The internal energy of a system is the total KE and PE of the atoms. And in an adiabatic process, the internal energy change is only determined by the work being done on or by the system. So if my previous definitions are correct, this is where my confusion comes in. How is an adiabatic process possible? Isn't the ability to do work essentially the transfer of energy? In the microscopic view, the atoms in an adiabatic process does work on its surrounding by expanding against it, so the atoms collides and transfers its KE. And by earlier definition, heat is this type of transfer, so isn't there also a loss of heat? My textbooks says, "... is defined as one with no heat transfer into or out of a system; Q = 0. ....When a system expands adiabatically, W is positive (the system does work on its surroundings), so ∆U is negative and the internal energy decreases. When a system is compressed adiabatically, W is negative (work is done on the system by its surroundings) and U increases. In many (but not all) systems an increase of internal energy is accompanied by a rise in temperature, and a decrease in internal energy by a drop in temperature." Isn't a drop in internal energy/temperature a loss of heat?