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Heat is perceived as something which produces a sensation of warmth. The sensation will, of course, be stronger in higher temperatures. There cannot be any heat transfer between two objects that are at the same temperature. The amount of heat transferred depends on the temperature difference and the conductivity of the path between the two objects; the direction of heat flow is always toward the cooler object. Earlier philosophers considered heat as a substance that was passed from hot objects to colder ones. This heat fluid was known as phlogiston (meaning flammable in Greek), which was thought to have a mass and the ability to flow in and out of objects during burning; it was considered to be the soul of matter. The concept was refined after Lavoisier an eighteenth century French chemist, showed that mass is a conserved quantity and does not change when a substance undergoes a chemical reaction. The new substance was called caloric, a mass-less fluid thought to flow from hot to cold objects.

It was not until the middle of the nineteenth century that Thompson and Joule showed that this theory is also wrong; heat is not a substance, but rather a manifestation of motion at the molecular level (kinetic theory). For example, when we rub our hands against each other both hands get warmer, even though initially they were at the same cooler temperatures. If the cause of the heat were a fluid, then it would have flowed from a (hotter) body with more energy to another with less energy (colder). Instead, the hands are heated because the kinetic energy of motion (rubbing) has been converted to heat in a process called “friction”.

The reason that the concept of heat as a fluid so well received for so many centuries was that it met all the human notions of how a fluid should behave. It could pass from one body to another, similarly to the flow of water in a river or a pipe. When a hot object comes into contact with a colder one, the hot object cools and the cold one gets hotter as if some substance were passing from one to the other. Heat is also capable of running a steam engine in the same manner that water could turn a waterwheel.

Sensible Heat

While energy is a property possessed by particles of matter, heat is the transfer of this energy between the particles. It is common sense that heat flows from an object at a higher temperature to one at a lower temperature. Heat , like work, is a transfer of thermal energy rather than the flow of a substance.

See Main Article Sensible heat

Latent Heat

Latent heat is amount of heat being absorbed or released during phase change. The heat required to melt a solid substance of unit mass is defined as the latent heat of fusion. The heat required to vaporize a liquid substance of unit mass is defined as the latent heat of vaporization.

See Main Article Latent heat

Phase Change

When a process involves a change in phase, the heat absorbed or released can be expressed as the product of the mass quantity and the latent heat of the material.

See Main Article Phase change