# Hot objects

## Main Question or Discussion Point

G'day
This question may sound silly but believe it or not i cant seem to find the answer on the internet, What makes a hot object hot?

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Andy Resnick
One object is hotter than another when it's temperature is larger.

One object is hotter than another when it's temperature is larger.
And what makes its temperature larger?

Andy Resnick
That's a totally different question.

I'm not trying to be obtuse- honestly. "temperature" should be viewed as a property of matter similar to 'mass'. "What makes one object heavier than another?" "it has more mass".

"temperature" is a way to quantify what we mean by 'hot' and 'cold'. How we assign a single, specific, number to a given object is not trivial to understand- at the most basic level, we require the object be in a state of 'equilibrium', which then opens up the question of how to define equilibrium (how long do you want to wait?) and the meaning of 'thermodynamic state'.

Here's an example of how 'temperature' can be ambiguous: blackbody radiation is a specific state of electromagnetic radiation that can be identified as being at thermodynamic equilibrium and assigned a unique temperature. What about laser light? What is the temperature of laser light? AFAIK, there is no satisfactory answer:

http://dx.doi.org/10.1119/1.1603268

How the temperature of an object changes when a process occurs is mostly straightforward (i.e. heat -> specific heat -> temperature change), but again that assumes equilibrium conditions (and a few other assumptions).

"temperature" should be viewed as a property of matter similar to 'mass'. "What makes one object heavier than another?" "it has more mass".
?

Extensive properties of a body, such as volume or mass or kinetic energy are additive; The value for the whole body is equal to the sum of the values for the individual parts.

Intensive properties of a body, such as pressure or temperature or density are not additive; The value for the whole body is the same as the value for any part for constant systems; Intensive properties may also vary throughout a body.

This is a very important distinction. In particular the temperature of a body is not a measure of the heat energy in a body. The polar ice cap contains much more energy than my body, and yet my body is at a higher temperature.

Andy Resnick
I'm not sure what you are asking me: nothing you said contradicts what I said. Clearly, a body can have hotter parts and colder parts, but then we do not assign "a" temperature to the body.

Hello Andy

Sorry to have to disagree but temperature is not similar to mass.

If you add a 1Kg weight to a 10 Kg stack of weights the result is 11 Kg.

If you do this in an ordinary gym the temperature of the stack of weights does not alter.

Andy Resnick
Obviously mass is not the same thing as temperature. Here's an analogy: sound and light. Sound and light have many shared properties (diffraction, interference, etc), but some essential differences as well.

That is sense that I mean temperature and mass are similar: they are both properties of matter, both scalar fields, and the origin of both is currently unknown.

IF one were to raise the "temperature" of a 1Kg weight to 10^2 of an identical weight, would they weigh the same on a calibrated scale?

And less relevantly, but equally interesting:

If you hadn't heard of him, Feynman was one of the great physicists of the 20th century, and won a Nobel Prize for his work on quantum mechanics, so a reliable source!

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