The rate of heat flow is the amount of
heat
In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...
that is transferred per unit of time in some material, usually measured in
watt
The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
s (
joules
The joule ( , or ; symbol: J) is the unit of energy in the International System of Units (SI). In terms of SI base units, one joule corresponds to one kilogram- metre squared per second squared One joule is equal to the amount of work don ...
per second). Heat is the flow of
thermal energy
The term "thermal energy" is often used ambiguously in physics and engineering. It can denote several different physical concepts, including:
* Internal energy: The energy contained within a body of matter or radiation, excluding the potential en ...
driven by thermal non-equilibrium, so the term 'heat flow' is a redundancy (i.e. a
pleonasm
Pleonasm (; , ) is redundancy in linguistic expression, such as "black darkness", "burning fire", "the man he said", or "vibrating with motion". It is a manifestation of tautology by traditional rhetorical criteria. Pleonasm may also be used f ...
). Heat must not be confused with stored thermal energy, and moving a hot object from one place to another must not be called heat transfer. However, it is common to say ‘heat flow’ to mean ‘
heat content Heat content may refer to:
* Enthalpy, measure of energy in a thermodynamic system
* Heat of combustion, amount of heat released by combustion of a quantity of substance
* Ocean heat content
Ocean heat content (OHC) or ocean heat uptake (OHU) is ...
’.
The equation of heat flow is given by Fourier's law of heat conduction.
Rate of heat flow = - (heat transfer coefficient) * (area of the body) * (variation of the temperature) / (length of the material)
The formula for the rate of heat flow is:
:
where
*
is the net heat (energy) transfer,
*
is the time taken,
*
is the difference in temperature between the cold and hot sides,
*
is the thickness of the material conducting heat (distance between hot and cold sides),
*
is the
thermal conductivity
The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1.
Heat transfer occurs at a lower rate in materials of low ...
of the material conducting heat, and
*
is the surface area of the surface emitting heat.
If a piece of material whose cross-sectional area is
and thickness is
with a temperature difference
between its faces is observed, heat flows between the two faces in a direction perpendicular to the faces. The time rate of heat flow,
, for small
and small
, is proportional to
. In the limit of infinitesimal thickness
, with temperature difference
, this becomes
, where
is the time rate of heat flow through the area
,
is the ''
temperature gradient
A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature spatial gradient is a vector quantity with Dimensional analysis, ...
'' across the material, and
, the proportionality constant, is the ''
thermal conductivity
The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1.
Heat transfer occurs at a lower rate in materials of low ...
'' of the material. People often use
,
, or the Greek letter
to represent this constant. The minus sign is there because the rate of heat flow is always negative—heat flows from the side at higher temperature to the one at lower temperature, not the other way around.
See also
*
Heat transfer coefficient
In thermodynamics, the heat transfer coefficient or film coefficient, or film effectiveness, is the Proportional (mathematics), proportionality constant between the heat flux and the thermodynamic driving force for the Heat transfer, flow of heat ...
*
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
*
Thermal conduction
Thermal conduction is the diffusion of thermal energy (heat) within one material or between materials in contact. The higher temperature object has molecules with more kinetic energy; collisions between molecules distributes this kinetic energy ...
*
Thermal conductivity
The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1.
Heat transfer occurs at a lower rate in materials of low ...
*
Heat flux
In physics and engineering, heat flux or thermal flux, sometimes also referred to as heat flux density, heat-flow density or heat-flow rate intensity, is a flow of energy per unit area per unit time (physics), time. Its SI units are watts per sq ...
*
Watt
The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
*
Flux
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications in physics. For transport phe ...
References
Thermodynamic properties
Heat flow
Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
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