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From Wikipedia

Latent heat

Latent heat is the heat released or absorbed by a chemical substance or a thermodynamic system during a change of state that occurs without a change in temperature, meaning a phase transition such as the melting of ice or the boiling of water. The term was introduced around 1750 by Joseph Black as derived from the Latin latere, to lie hidden.

In meteorology, latent heat flux is the flux of heat from the Earth's surface to the atmosphere that is associated with evaporation or transpiration of water at the surface and subsequent condensation of water vapor in the troposphere. It is an important component of Earth's surface energy budget. Latent heat flux is commonly measured with the Bowen ratio technique, or by eddy covariance.

Usage

Two of the more common forms of latent heat (or enthalpies or energies) encountered are latent heat of fusion (melting) and latent heat of vaporization (boiling). These names describe the direction of energy flow when changing from one phase to the next: from solid to liquid, and to gas.

In both cases, the change is endothermic, meaning that the system absorbs energy on going from solid to liquid to gas. The change is exothermic (the process releases energy) for the opposite direction. For example, in the atmosphere, when a molecule of water evaporates from the surface of any body of water, energy is transported by the water molecule into a lower temperature air parcel that contains less water vapor than its surroundings. Because energy is needed to overcome the molecular forces of attraction between water particles, the process of transition from a parcel of water to a parcel of vapor requires the input of energy causing a drop in temperature in its surroundings. If the water vapor condenses back to a liquid or solid phase onto a surface, the latent energy absorbed during evaporation is released as sensible heat onto the surface. The large value of the enthalpy of condensation of water vapor is the reason that steam is a far more effective heating medium than boiling water, and is more hazardous.

The terms sensible heat and latent heat are not special forms of energy, instead they characterize the same form of energy, heat, in terms of their effect on a material or a thermodynamic system. Heat is thermal energy in the process of transfer between a system and its surroundings or between two systems with a different temperature.

Both sensible and latent heats are observed in many processes while transporting energy in nature. Latent heat is associated with the phase changes of atmospheric water vapor, mostly vaporization and condensation, whereas sensible heat is energy transferred that affects the temperature of the atmosphere.

History

The term latent heat was introduced around 1750 by Joseph Black, and is derived from the Latin latere, meaning to lie hidden. In 1847, James Prescott Joule characterized latent energy as the energy of interaction in a given configuration of particles, i.e. a form of potential energy, and the sensible heat as an energy that was indicated by the thermometer, relating the latter to thermal energy.

Specific latent heat

A specific latent heat (L) expresses the amount of energy in form of heat (Q) required to completely effect a phase change of a unit of mass (m), usually , of a substance as an intensive property:

L = \frac {Q}{m}

Intensive properties are material characteristics and are not dependent on the size or extend of the sample. Commonly quoted and tabulated in the literature are the specific latent heat of fusion and the specific latent heat of vaporization for many substances.

From this definition, the latent heat for a given mass of a substance is calculated by

Q = {m} {L}

where:

Q is the amount of energy released or absorbed during the change of phase of the substance (in kJ or in BTU),
m is the mass of the substance (in kg or in lb), and
L is the specific latent heat for a particular substance (kJ-kgm−1 or in BTU-lbm−1), either Lf for fusion, or Lv for vaporization.

Table of latent heats

The following table shows the latent heats and change of phase temperatures of some common fluids and gases.

Latent heat for water

The latent heat of condensation of water in the temperature range from −40 °C to 40 °C is approximated by the following empirical cubic function:

L_{water}(T)=-0.0000614342 T^3+0.00158927 T^2-2.36418 T+2500.79

with a determination coefficient of R^2=0.999988, where T is in °C.



From Encyclopedia

heating

heating means of making a building comfortably warm relative to a colder outside temperature. Old, primitive methods of heating a building or a room within it include the open fire, the fireplace, and the stove . In ancient Rome a heating system, called a hypocaust, warmed a building by passing hot gases from a furnace through enclosed passages under the floors and behind the walls before releasing them outside. The principal modern systems that are used to heat a building are classified as warm air, hot water, steam, or electricity. In the warm-air system air, heated in a furnace, rises through warm-air ducts and enters the rooms through outlets, while cooler air in the rooms passes into return ducts that lead back to the furnace. The air circulates through the system by convection, i.e., the tendency of a fluid such as air to rise when warm and sink when cool. In newer buildings the circulation is assisted by a fan. The hot-water system has a boiler for heating the water that is sent through connecting pipes to radiators and convectors, the latter devices being metal enclosures containing hot-water pipes surrounded by metal fins. The circulation is maintained by pumps or, in older buildings, by convection. In the steam-heating system, steam generated in a boiler is circulated by its own pressure (sometimes aided by a vacuum pump) through radiators. There are many kinds of electric heating systems. In one type current is sent through wires into electric resistors that are contained in convectors in rooms. The resistors convert the current into heat. In a radiant panel heating system a room is warmed by heat emitted from wall, floor, or ceiling panels. They are warmed by the circulation of warm air, hot water, or steam or by an electric current in resistors within or behind the panels. Experiments are being made to utilize solar energy for heating buildings. In many large buildings, such as theaters, public libraries, and municipal buildings, the heating, ventilating, and air-conditioning units are combined in one system. In district heating, heat is distributed from a heating plant to buildings in a section (usually commercial) of a city. Bibliography: See F. Porges, Handbook of Heating, Ventilating, and Air Conditioning (1982).


From Yahoo Answers

Question:(1) 42kJ (2) 21 kJ (3) 34 kJ (4) 17 kJ An 80.0 gram sample of water at 10.0 C absorbs 1680 Joules of heat energy. What is the final temperature of the water? (1) 50.0 C (2) 15.0 C (3) 5.00 C (4) 4.00 C At the same temperature and pressure, 1.0 liter of CO(g) and 1.0 liter of CO2(g) have (1) equal masses and the same number of molecules (2) different masses and a different number of molecules (3) equal volume and the same number of molecules (4) different volumes and a different number of molecules

Answers:For the first two, use the equation q=mc(T2-T1). n this equation, q is the heat absorbed or lost, m is the mass of the water sample, c is the specific heat of water (4.184 J/gC) and T2 and T1 are the final and initial temperatures of the water. In both problems, you can just plug in what you have and solve for the one you don't have. In the last one, the answer is (3) equal volumes and the same number of molecules.

Question:Or is it the other way around? I will choose best answer.

Answers:yep!

Question:A)c6h12o6 B)o2 C)ATP D)h2o E)co2

Answers:ATP

Question:A physical change is one which a material changes from one state to another. Ex) Ice melting, solid to liquid. Can I get an example that results in heat release?

Answers:Going from gas to liquid or liquid to solid involves a heat release (Think of it this way- you have to put in energy to go the other way!)