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Heat exchange reflects and drives changes in energy state between two objects—or more generally systems—in thermal contact due to a difference in temperature. Heat flows from a system at higher temperature to one at lower temperature until both systems are at the same temperature. Systems at the same temperature are said to be in thermal equilibrium.

The term "heat" is sometimes used, incorrectly, to refer to a form of energy that a system contains. Heat is a form of energy-in-transit; it is not energy-in-residence. The energy contained in a system (exclusive of energy depending on external factors) is called internal energy and, unlike heat, is a property of a system like the volume or mass.

The first law of thermodynamics states that the internal energy of a system can change only if "energy" flows into or out of the system. This flow, or energy-in-transit, appears as heat or as work (or a combination), and the change in internal energy is equal to the total of heat and work appearing during the change. After the change, however, the system contains neither heat nor work; it contains internal energy.

Units of heat are units of energy. One classical unit, the calorie, was defined as the amount of energy required to raise the temperature of one gram of water one degree Celsius. A more precise definition recognizes that this energy depends slightly on the temperature of the water, so the interval was specified as 14.5–15.5°C (58.1–59.9°F). The dietary Calorie (capital C) is a kilocalorie (1000

water 4.18
iron 0.45
mercury 0.14
ethyl alcohol 2.46

calories). The energy available from the metabolism of a given amount of food is commonly given in Calories.

In the International System of Units (SI or extended metric system) the joule is the unit of energy. Although based on mechanical rather than thermal considerations, the joule is now the preferred energy unit for both mechanical and thermal applications. The joule is about 1/4 of a calorie and now formally defines the calorie. One calorie is by definition exactly 4.184 joules, although the practical difference between this definition and the original one is negligible.

The specific heat capacity, or specific heat, is the heat required to raise the temperature of one gram of substance one degree Celsius. The specific heats of a few substances in joules per gram per degree Celsius are listed above.

For example, to raise the temperature of equal amounts of all four of these substances, the water would require considerably more heat than the others (over 9 times as much as the iron, for example, because 4.18 divided by 0.45 is 9.3). Or if you added the same amount of heat to equal amounts of all four of these substances, the temperature of the water would rise least. In short, it is more difficult to change the temperature of water than most other substances. This is one of the main reasons coastal climates usually have smaller seasonal temperature variations than inland climates. Because of its relatively high specific heat, water is a good thermal moderator.

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