The key difference between heat flow and heat flux is that heat flow refers to the exchange of thermal energy between physical systems, whereas heat flux refers to energy flow between physical systems per unit of area per unit of time.
The terms heat flow and heat flux are important concepts in physical chemistry regarding the behaviour and interchange of thermal energy between physical systems.
CONTENTS
1. Overview and Key Difference
2. What is Heat Flow
3. What is Heat Flux
4. Side by Side Comparison – Heat Flow vs Heat Flux in Tabular Form
5. Summary
What is Heat Flow?
Heat flow or heat transfer is the generation, use, conversion, and exchange of thermal energy between physical systems. We can classify this concept into various mechanisms as thermal conduction, thermal convection, thermal radiation and heat flow through phase changes. These mechanisms have different characteristic features, and they often occur simultaneously in the same system. Let us understand some of these mechanisms in detail.
Heat conduction is the most common type of heat flow that involves a direct microscopic interchange of the kinetic energy of particles via the boundary between two systems. It is also called diffusion. In this type of heat flow, when a body is in a different temperature from another body or from the surrounding, heat flows from high temperature to low temperature until a thermal equilibrium occurs.
Heat convection is the other common type of heat flow where a bulk flow of a fluid carries heat along with the mass flow of the fluid. Sometimes, the flow of the fluid occurs due to an external process or due to buoyancy forces that are caused due to the thermal energy expansion of the fluid.
Figure 01: Thermal Convection in the Earth’s Mantle
Thermal radiation, on the other hand, is a type of heat flow that occurs through a vacuum or any transparent medium. This transfer of energy occurs by means of photons in EMR waves that are governed by the same law.
What is Heat Flux?
Heat flux is a flow of energy per unit of area per unit of time. This term is also given as thermal flux, heat flux density, heat flow density, and heat flow rate intensity. We can use the SI units for the measurement of the heat flux; Watts per square meter (W/m2). This property has both magnitude and direction. Therefore, we can name it as a vector quantity.
Figure 02: Heat Flux as a Vector Quantity
We can use Fourier’s law for most solids in usual conditions where heat is transported mainly by conduction and heat flux. There are few ways of measuring the heat flux, but the most common but impractical method is the measurement of temperature difference over a piece of material with a known thermal conductivity.
What is the Difference Between Heat Flow and Heat Flux?
The terms heat flow and heat flux are important concepts in physical chemistry regarding the behaviour and interchange of thermal energy between physical systems. The key difference between heat flow and heat flux is that heat flow refers to the mass flow of the fluid, whereas heat flux refers to the flow of energy per unit of area per unit of time.
Below is a summary of the difference between heat flow and heat flux in tabular form.
Summary – Heat Flow vs Heat Flux
Heat flow and heat flux are related terms in physical chemistry. The key difference between heat flow and heat flux is that heat flow refers to the generation, use, conversion and exchange of thermal energy between physical systems, whereas heat flux refers to the flow of energy per unit of area per unit of time.
Reference:
1. “Heat Flux.” Science Direct, Available here.
Image Courtesy:
1. “Convection-snapshot” – original uploader was Harroschmeling at German Wikipedia. – Transferred from de.wikipedia to Commons. (CC BY-SA 3.0) via Commons Wikimedia
2. “Heatflux” By Hukseflux at English Wikipedia (CC BY 3.0) via Commons Wikimedia