We're going to talk about the second law of thermodynamics here. Scientists use a word called entropy to describe the degree of freedom (randomness) in a system. Remember, there are two words in thermodynamics: entropy, which talks about randomness, and enthalpy, which is a measure of the heat energy in a system. Big difference.
Heat flows from hot areas to cold, not the other way. If its energy is to flow from cold to hot, it needs additional energy. Heat is also conserved when energy transfer occurs. That conservation means that when you look at the energy of both systems at the beginning of the reaction and at the end, the total energy amounts are equal. Energy has moved from one area to another, but the total remains the same.
The second law also considers the entropy of a system. Entropy is a measure of the amount of disorder (chaos) in a system. A good rule of thumb is the more disorder you have, the more energy you have.You might hear the term reversibility. Scientists use the term reversibility to describe systems that are in equilibrium with themselves and the environment around them. When a system is in equilibrium, any change that occurs in one direction is balanced by an equal change in the opposite direction. Reversibility means that effects can be reversed. This implies that the system is isolated (nothing is interfering, nothing entering or leaving). Overall, their effect and change on the system are zero. So you've got a system at equilibrium. Look closely and you'll find certain qualities. You'll find that in these systems the heat transfer is due to temperature differences. You'll also discover that wild changes do not happen in an isolated system. To get big changes, you need energy. When you're at equilibrium, there is no gain or loss of energy. Lastly, you'll see that there is no friction involved in the system. If friction occurred, heat would be created and work would be needed to overcome the friction. That work would take energy out of the system.
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LaunchPad: The Heat is On! (NASA-eClips Video)