heat - History of Heat, Notation, Thermodynamics

The transfer of energy from one object to another, due solely to their difference in temperatures; symbol Q, units J (joule). The quantity of heat sometimes ascribed to an object or process is the total amount of energy transferred in this way.

Generally, heat is a form of energy transfer associated with the different motions of atoms, molecules and other particles that comprise matter when it is hot and when it is cold. High temperature bodies, which often result in high heat transfer, can be created by chemical reactions (such as burning), nuclear reactions (such as fusion taking place inside the Sun), electromagnetic dissipation (as in electric stoves), or mechanical dissipation (such as friction). Heat can be transferred between objects by radiation, conduction and convection. Temperature, defined as the measure of an object to spontaneously give up energy, is used as a measure of the internal energy or enthalpy, that is the level of elementary motion giving rise to heat transfer. Heat can only be transferred between objects, or areas within an object, with different temperatures (as given by the zeroth law of thermodynamics), and then, in the absence of work, only in the direction of the colder body (as per the second law of thermodynamics).

History of Heat

The first to have put forward a semblance of a theory on heat was the Greek philosopher Heraclitus who lived around 500 BC in the city of Ephesus in Ionia, Asia Minor.

The hypothesis that heat is a form of motion was proposed initially in the 12th century.

In 1761, Scottish chemist Joseph Black discovered that ice absorbs heat without changing temperature when melting. Between 1759 and 1763 he evolved that theory of "latent heat" on which his scientific fame chiefly rests, and also showed that different substances have different specific heats.

In this direction, the ability to be able to use heat transfer to generate work allowed the invention and development of the steam engine by people such as Thomas Newcomen and James Watt. In addition, in 1797 a cannon manufacturer Sir Benjamin Thompson, Count Rumford, demonstrated through the use of friction it was possible to convert work to heat.

Several theories on the nature of heat were developed. In the 17th century, Johann Becher proposed that heat was associated with an undetectable material called phlogiston that was driven out of a substance when it was burnt. He proposed instead the caloric theory which saw heat as a type of weightless, invisible fluid that moved when out of equilibrium. He set forth the importance of heat transfer: "production of motive power is due not to an actual consumption of caloric, but to its transportation form a warm body to a cold body, i.e. The internal energy of a substance is then the sum of the kinetic energy associated with each molecule, and heat transfer occurs from regions with energetic molecules, and so high internal energy, to those with less energetic molecules, and so lower internal energy.

The work of Joule and Mayer demonstrated that heat and work were interchangeable, and led to the statement of the principle of the conservation of energy by Hermann von Helmholtz in 1847.

== Overview ==an over view of heat and its history Under the First Law of Thermodynamics, heat (and work) are processes that change the internal energy of a substance or object. Heat is the transfer of energy over the boundary of a system owing to a temperature gradient. The SI unit for heat is the joule (as it is a form of energy), though the British Thermal Unit is still occasionally used in the United States.

Heat is a process quantity, as opposed to being a state quantity, and is to thermal energy as work is to mechanical energy. Heat flows between regions that are not in thermal equilibrium with each other; The amount of energy transferred is the amount of heat exchanged. It is a common misconception to confuse heat with internal energy: heat is related to the change in internal energy and the work performed by the system. The term heat is used to describe the flow of energy, while the term internal energy is used to describe the energy itself. The concept that warm objects "contain heat" is not uncommon, but hot is nearly always used as a relative term (an object is hot compared with its surroundings or those of the person using the term) so that high temperature is directly associated with high heat transfer.

The amount of heat that has to be transferred to or from an object when its temperature varies by one degree is called heat capacity. Heat capacity is specific to each and every object or substance. When referred to a quantity unit (such as mass or moles), the heat exchanged per degree is termed specific heat, and depends primarily on the composition and physical state (phase) of an object. Fuels generate predictable amounts of heat when burned; this heat is known as heating value and is expressed per unit of quantity. Upon changing from one phase to another, pure substances can exchange heat without their temperature suffering any change. The amount of heat exchanged during a phase change is known as latent heat and depends primarily on the substance and the initial and final phase.

Notation

The total amount of energy transferred through heat transfer is conventionally abbreviated as Q. The conventional sign convention is that when a body releases heat into its surroundings, Q < 0 (-); Heat transfer rate, or heat flow per unit time, is denoted by:

. Heat flux is defined as rate of heat transfer per unit cross-sectional area, and is denoted q, resulting in units of watts per metre squared. Slightly different notation conventions can be used, which may denote heat flux as, for example, .

Thermodynamics

Heat is related to the internal energy U of the system and work W done by the system by the first law of thermodynamics: