Contents

Rate of heat flow

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:

${\displaystyle {\frac {Q}{\Delta t}}=-kA{\frac {\Delta T}{\Delta x}}}$

where

• ${\displaystyle Q}$ is the net heat (energy) transfer,
• ${\displaystyle \Delta t}$ is the time taken,
• ${\displaystyle \Delta T}$ is the difference in temperature between the cold and hot sides,
• ${\displaystyle \Delta x}$ is the thickness of the material conducting heat (distance between hot and cold sides),
• ${\displaystyle k}$ is the thermal conductivity, and
• ${\displaystyle A}$ is the surface area of the surface emitting heat.

If a piece of material whose cross-sectional area is ${\displaystyle A}$ and thickness is ${\displaystyle \Delta x}$ with a temperature difference ${\displaystyle \Delta T}$ between its faces is observed, heat flows between the two faces in a direction perpendicular to the faces. The time rate of heat flow, ${\displaystyle {\frac {Q}{\Delta t}}}$, for small ${\displaystyle Q}$ and small ${\displaystyle \Delta t}$, is proportional to ${\displaystyle A\times {\frac {\Delta T}{\Delta x}}}$. In the limit of infinitesimal thickness ${\displaystyle \Delta x}$, with temperature difference ${\displaystyle \Delta T}$, this becomes ${\displaystyle H=-kA({\frac {\Delta T}{\Delta x}})}$, where ${\displaystyle H=({\frac {Q}{\Delta t}})}$ is the time rate of heat flow through the area ${\displaystyle A}$, ${\displaystyle {\frac {\Delta T}{\Delta x}}}$ is the temperature gradient across the material, and ${\displaystyle k}$, the proportionality constant, is the thermal conductivity of the material.^[2] People often use ${\displaystyle k}$, ${\displaystyle \lambda }$, or the Greek letter ${\displaystyle \kappa }$ to represent this constant.^{[citation needed]} 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.^[3]

• Heat transfer coefficient
• Heat transfer
• Thermal conduction
• Thermal conductivity
• Heat flux
• Watt
• Flux

This thermodynamics-related article is a stub. You can help Wikipedia by expanding it.