# Thermal inertia vs effusivity, what is the difference?

• carter7gindenv
In summary, the conversation discusses the confusion surrounding the concepts of thermal inertia and effusivity, and the difficulty in finding clear information about them. While thermal inertia is similar to mechanical inertia and refers to a material's resistance to change in temperature, effusivity is a measure of a material's ability to transfer heat. The conversation also mentions the importance of understanding thermal diffusivity when discussing thermal equilibrium. The speaker expresses frustration with the lack of information available on these concepts and requests help in understanding them.
carter7gindenv
Hello, I'm slowly getting crazy about this stuff. I'm trying to understand what are thermal inertia/effusivity and information have been incoherent so far. The best example of that is seen on wikipedia where the only mention of thermal inertia is in the article
https://en.m.wikipedia.org/wiki/Volumetric_heat_capacity
However in the very section of thermal inertia the article refers to thermal effusivity with the same equation.

I've tried my university physics section but the books talks only about thermal conductivity ( not even thermal diffusivity ).
So far I understood that thermal inertia is akin to mechanical inertia. It is how the material is opposed to change its temperature. I understand that thermal effusivity is how the material exchange heat with its environment. However I don't understand what are their respective equations and why oh god why is it so hard to find information about those concepts.

So here am I humbly asking for helps.

ps: I'm not sure that I posted in the right section : /Edit: two scientific article talking about either effusivity or inertia with the same formula
https://www.hou.usra.edu/meetings/lpsc2015/pdf/2914.pdfhttps://iopscience.iop.org/article/10.1088/0143-0807/24/4/353/meta

Last edited:
Effusivity is a quantitative measure of a material's ability to transfer heat.

Thermal inertia (as I hear it used) addresses the time required for a material to reach thermal equilibrium (longer time = higher thermal inertia) - it tends to be used qualitatively. 'High thermal inertia' could be due to a large quantity of heat being required (specific heat) or poor conductivity, or both.

For what I've read it's thermal diffusivity which describe the time need for an object to reach thermal equilibrium?

carter7gindenv said:
For what I've read it's thermal diffusivity which describe the time need for an object to reach thermal equilibrium?
This is incorrect.

carter7gindenv said:
Summary: Difference between thermal inertia and effusivity

Hello, I'm slowly getting crazy about this stuff. I'm trying to understand what are thermal inertia/effusivity and information have been incoherent so far. The best example of that is seen on wikipedia where the only mention of thermal inertia is in the article
https://en.m.wikipedia.org/wiki/Volumetric_heat_capacity
However in the very section of thermal inertia the article refers to thermal effusivity with the same equation.

I've tried my university physics section but the books talks only about thermal conductivity ( not even thermal diffusivity ).
So far I understood that thermal inertia is akin to mechanical inertia. It is how the material is opposed to change its temperature. I understand that thermal effusivity is how the material exchange heat with its environment. However I don't understand what are their respective equations and why oh god why is it so hard to find information about those concepts.

So here am I humbly asking for helps.

ps: I'm not sure that I posted in the right section : /Edit: two scientific article talking about either effusivity or inertia with the same formula
https://www.hou.usra.edu/meetings/lpsc2015/pdf/2914.pdfhttps://iopscience.iop.org/article/10.1088/0143-0807/24/4/353/meta
Your problem is that you are learning this from crappy sources. See Transport Phenomena by Bird, Stewart, and Lightfoot to get a solid fundamental understanding.

anorlunda

## 1. What is thermal inertia and how is it different from effusivity?

Thermal inertia is the property of a material that describes how resistant it is to changes in temperature. It is a measure of how much energy is required to raise the temperature of a material by a certain amount. Effusivity, on the other hand, is a measure of how quickly a material can transfer heat. In simpler terms, thermal inertia is a measure of a material's ability to store heat, while effusivity is a measure of its ability to conduct heat.

## 2. How are thermal inertia and effusivity related?

Thermal inertia and effusivity are inversely related. This means that materials with high thermal inertia have low effusivity, and vice versa. This is because materials with high thermal inertia are better at storing heat, so they have a lower rate of heat transfer. Materials with low thermal inertia, on the other hand, have high effusivity, meaning they can quickly transfer heat.

## 3. How are thermal inertia and effusivity measured?

Thermal inertia is typically measured by calculating the specific heat capacity and density of a material. This information can then be used to determine how much energy is required to raise the temperature of the material by a certain amount. Effusivity is measured by using a device called a thermal effusivity meter, which measures the rate of heat transfer through a material.

## 4. What are some real-world applications of thermal inertia and effusivity?

Thermal inertia and effusivity are important properties to consider in many industries, such as construction, manufacturing, and energy production. For example, materials with high thermal inertia can be used to insulate buildings and reduce energy costs, while materials with high effusivity are used in cooking and heating applications where quick heat transfer is desired.

## 5. Can thermal inertia and effusivity be changed?

Yes, thermal inertia and effusivity can be altered by changing the properties of a material. For example, the addition of insulation can increase the thermal inertia of a building, while changing the composition or surface treatment of a material can affect its effusivity. Additionally, the temperature and pressure of a material can also impact its thermal inertia and effusivity.

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