MadSci Network: Physics

Re: VRD--argument about heat vs infra-red radiation

Date: Tue Oct 10 16:59:16 2000
Posted By: Gareth Evans, Senior Research Associate
Area of science: Physics
ID: 970872765.Ph

Gordon, I hope you and your friend enjoy a good technical argument. It’s a great way to learn as long as you stay friends !

Your question covers a lot of ground but reflects a good attitude to science in that we must all agree what we mean about all the terms we use, particularly when the more general use of words is a little different. There are several true statements in what you say to explain your position but there are some fuzzy areas too.

Let’s start where you did with heat and energy. Is heat the same as energy ? If you had said “Is heat a form of energy ?” it would have been easy to say “ yes”. Heat is a form of energy but you recognised that there are other forms so the two terms are not exactly the same. Energy has a broader definition namely “the ability of a body to do work.” It takes a bit of imagination to see how a hot body can do more work than a cold one but it can be done !

So how should we define "heat" and distinguish it from other forms of energy ? A good way is by a behavioural observation, in particular it is “that form of energy which is transmitted from a hot body to a cold body”. The method of transmission can, as you mention, be convection, conduction or by radiation. By “radiation” here, we mean the process of radiating. What is the hot body radiating ? Answer: Radiation ! This time we mean the form of energy which is radiative energy in the form of electromagnetic waves.

This form of radiation can be what we call infra-red radiation but it can be radiation in other parts of the electromagnetic spectrum. You are right in saying that if a body is hot enough a significant amount of the radiation is in the visible part of the spectrum and the body glows. The colour we sense depends on the distribution of energy amongst the various wavelengths of the spectrum. “White hot” implies that the energy distribution is approximately centred about the middle of the visible range of 400 to 700 nm.

What we normally call a hot body does not glow with visible light but radiates longer wavelength radiation including IR We can feel this radiation and often say we can feel the heat. What we really mean is that we can feel the consequnce of the body having heat ( thermal energy ) namely the emission of IR radiation which we absorb. That process of absorbing the radiation results in a local increase in heat content of the parts of our bodies sensing the IR radiation.

We should also look at this issue at a molecular level. The difference between the molecules and atoms of a hot body and a colder body is that the kinetic energy of the atoms and molecules is higher in a hot body. Heat is a collective kinetic energy of these components of matter. This is very different from IR radiation which is, just like the whole electromagnetic spectrum, a stream of photons with both particle and wave- like nature.

You said “whilst not exactly synonymous, heat and infra-red are the same thing”. Yes they are both forms of energy. No, they are not synonymous. They are different enough for me to tend to side with your friend !! Sorry, but thanks for a great question !

(PS. Try this for a quick analogous answer. In a bucket of coloured balls, a yellow ball is a coloured ball and so is a blue ball but a yellow ball is not a blue ball. )

Current Queue | Current Queue for Physics | Physics archives

Try the links in the MadSci Library for more information on Physics.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2000. All rights reserved.