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What is Energy Flux?

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Energy exists in a number of different forms like thermal, mechanical, kinetic, potential, nuclear, etc. The unit of energy is the joule (J) or kilojoule (kJ) in SI units. The internal energy of an object is the sum of the kinetic and potential energies of its molecules. This energy is also related to the intermolecular forces that attach the molecules to one another. Although these forces are stronger in solids than in liquids and gases, a system in a gas phase has more internal energy than solids and liquids. This is because when heat energy is supplied to a liquid, it causes the molecules to break away from one another converting the system into a gas. This means more internal energy exists in a system where the molecules are farther apart from one another.

Energy flux is a quantity that measures the rate of transfer of energy per unit area. The word "flux" means change or fluctuation. Heat flux and radiative flux are specific cases of energy flux that involve the rate of transfer of heat and radiation (photons) respectively. Energy flux is a function of time as well as area, and has units of J/m2/s or W/m2. It is sometimes also referred to as flux density. In more technical terms, flux density is defined as the energy travelling over an area perpendicular to the flow of energy divided by time and by area.

Four types of energy fluxes for ideal (flat and smooth) surfaces have been identified so far. They are the net radiation to or from the surface, the direct or sensible heat flux, the indirect or latent heat flux, and the heat flux entering or leaving the system. The direct heat flux occurs at and above the surface of the object and it is a result of the temperature difference between the surface and the atmosphere. The latent heat flux (also known as water vapor flux) occurs as a result of evaporation or condensation at the surface.

When studying energy fluxes at the surface of water, the above quantities are not always measured. Only the sensible and latent energy flux quantities are measured as the significance of net radiation in the measurement of energy at or above the surface of water is not so apparent. There are many applications of measuring energy flux at or above the earth’s surface. These include prediction of surface temperature, estimation of the rate of evaporation, and forecast of icing conditions to avoid accidents on highways.

To understand the concept of energy flux, we need to study the effects of heat, which essentially means the process of energy transfer from one system to another. The first law of thermodynamics states that energy can change its forms, but cannot be created or destroyed. It is also known as the law of conservation of energy. Energy transfer between two bodies occurs as a result of certain phenomena, usually either radiation, convection or conduction.

The first law of thermodynamics allows us to represent the change in energy of a system, which is,

Change in the total energy of the system = Total energy entering the system - Total energy leaving the system

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