[alancalverd]

So we have established that the microwave thermometer can measure the temperature of a liquid or matrix that is being heated by microwaves. HY's question was how to measure the temperature of the air in the oven.

Quote from: alancalverd on Today at 15:32:39

Bit of a sensitive point as, in a former incarnation, I was asked to review the draft European Directive on Physical Hazards which would have prevented the construction of any oven that could accommodate a human

Bull?

Sadly, no. The draft (compiled at your expense) contained a lot of figurative testes and bovine excrement including a ban on interventional use of MRI systems, which was the burden of my evidence to the Parliamentary scrutiny committee (at your expense). The draft was eventually rejected, to be replaced by something a little less absurd but even more expensive.

[hamdani yusuf (OP)]

The engineer who lives inside my head wants to ask why you are interested in the air temperature inside an oven. Most people are more interested in the wall temperature, which determines the thermal radiative heating of whatever you want to cook, or the power input to the microwave or fan heater.

Here's a tweet from @engineers_feed
"Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands."

My intention when starting this thread is to find out the precise definition of temperature. We know that a system can have many forms of energy. They are often classified as kinetic and potential energy. Which category does temperature fall into?
If we put energy into a system, sometimes the system's temperature increases, sometimes it doesn't. What makes the difference?

[hamdani yusuf (OP)]

Not really.
https://www.amazon.co.uk/Brannan-Microwave-Thermometer/dp/B005RDUSAW
Though it wouldn't achieve much to measure air temperature in a microwave.

To measure food temperature, a non-contact infrared thermometer is safer and more convenience to use.

[hamdani yusuf (OP)]

Which is just as well. If it's an alcohol-in-glass instrument, it's quite likely to boil and shatter in a microwave, even if the ambient air is cold.

It might happen if you cook the thermometer alone, without being inserted into the food.

[hamdani yusuf (OP)]

I think more people are actually interested in how quickly stuff cooks.

Agree.
Other things we usually care are:
Safety. We don't want harmful side effects, such as eye damage, hot splash, electrocution, carbon monoxide, NOx, radioactivity, etc.
Homogeneity of the heating. We don't want the food to be burnt into charcoal at one part while still being raw at other parts.
Energy bill. How much energy really goes to heating the food instead of being wasted away heating the container or exhaust air, or deforming the food.

[Bored chemist]

Not really.
https://www.amazon.co.uk/Brannan-Microwave-Thermometer/dp/B005RDUSAW
Though it wouldn't achieve much to measure air temperature in a microwave.

To measure food temperature, a non-contact infrared thermometer is safer and more convenience to use.

That's a great way of getting food poisoning.
Please don't make assertions like that.

If you really don't understand that food has to be cooked through, but an IR thermometer only measures surface temperature, then don't try giving advice.

[Spring Theory]

When describing what temperature is, it is easier to think of a single hydrogen atom.  The closest you can get to absolute zero is the ground state where the electron orbits the proton in the 1S orbital.  This electron is traveling at 1/137 the speed of light and thus a contributor to kinetic energy.

Absorbing a photon increases energy (and velocity) which results in more kinetic energy of the atom.  As more and more photons are absorbed in longer and longer wavelengths, the total energy rises as well as the velocities at the outer orbitals.  So think of the temperature of just one atom as the number (or concentration) of photons absorbed by the atom. Of course the longest wavelength photons absorbed are in the infrared range. These are also typically the first to be released by the atom.

If you combine many atoms together, the outer orbital electrons will interact with each other to transfer photons back and forth as well as momentum.  The more the photons, the more the atoms bounce back and forth and the more kinetic energy of the group of atoms. This is the statistical link of temperature to kinetic energy.

Simplifying the description, you can define temperature as a scalar field (coordinates of position and time) where the temperature at each point (at the macro scale) is based on the concentration of photons (including the ones absorbed) at that point.

[Bored chemist]

This electron is traveling at 1/137 the speed of light

No it isn't.
The uncertainty principle says you can't know exactly what its speed is.

[Spring Theory]

This electron is traveling at 1/137 the speed of light

No it isn't.
The uncertainty principle says you can't know exactly what its speed is.

To be more exact, I have determined the velocity of the electron at the 1s orbital to be 2.1876912636431E+06 m/s.

Based on the uncertainty principle, you cannot disprove that.

[hamdani yusuf (OP)]

That's a great way of getting food poisoning.
Please don't make assertions like that.

What makes it great?

If you want to know the inner temperature of the food, just cut it in half before measuring it using infrared thermometer. Why make it complicated?

[hamdani yusuf (OP)]

This electron is traveling at 1/137 the speed of light

No it isn't.
The uncertainty principle says you can't know exactly what its speed is.

That's not what the uncertainty principle says.

[hamdani yusuf (OP)]

The closest you can get to absolute zero is the ground state where the electron orbits the proton in the 1S orbital. 

What's the temperature corresponding to that state?

[Origin]

To be more exact, I have determined the velocity of the electron at the 1s orbital to be 2.1876912636431E+06 m/s.

How did you determine that.

[Bored chemist]

That's not what the uncertainty principle says.

In this context, it does.

[hamdani yusuf (OP)]

Absorbing a photon increases energy (and velocity) which results in more kinetic energy of the atom.

How does it affect the potential energy?
For comparison, to send a satellite to geostationary orbit, more energy is required, compared to sending it to Low Earth Orbit, although the orbital speed is lower.

[Bored chemist]

What makes it great?

Simplicity.

If you want to know the inner temperature of the food, just cut it in half before measuring it using infrared thermometer. Why make it complicated?

So, it is useless for things like a burger which you want to cook whole.

Why not just accept that your idea was wrong?

[Bored chemist]

The closest you can get to absolute zero is the ground state where the electron orbits the proton in the 1S orbital. 

What's the temperature corresponding to that state?

The temperature of a single atom is still not well defined.

Perhaps I can add another example. The air inside a running microwave oven, radio wave oven, induction cooker, laser oven, may show different temperature measurement value, depending on the thermometer type used.

Temperature is still only meaningful if a system is at equilibrium.

For some systems- far from equilibrium- the temperatures can be different- or even negative.

[Eternal Student]

Hi @Spring Theory ,

    I don't think I've spoken to you before, welcome to the forum.

    You've said a lot of things that are interesting and quite reasonable.   There are also a few things that might be a little bit dangerous or misleading.
    Let's start by establishing the following:
1.   I'm not the definitive expert on this.
2.   As far as I'm concerned the purpose of a forum is to discuss things and ideally move people's undertsanding onwards and upwards.  For example, my own understanding of temperature has had a bit of development since reading some articles about temperature.

     So if I or other people disagree with some of the things you've said, don't worry about it too much.   We need a few more users on this forum, don't disappear.  You should be welcome to join the discussion.   

Best Wishes. 

[Eternal Student]

Hi again.

My intention when starting this thread is to find out the precise definition of temperature.

Then let's spend a moment and bring all the ideas that have already been mentioned in this thread together and up-to-date.  It might also be useful for people joining the discussion who don't want to plough their way through the whole thread.

What is temperature?
   There's been a lot of discussion already.   Historically, there have been different ways of measuring temperature and therefore many different definitions of what temperature is.   The earliest definitions were based on thermometer devices like mercury in glass thermometers.  So temperature was just "that" which you can measure with a thermometer.

     Later there were some empirical observations made on gases that seemed to behave a certain way.   Some gases obeyed Boyles' Law and Charles' Law to a fair amount of accuracy.   This was discovered prior to 1854.   This started to give some reason to believe macroscopic properties of gases like their pressure and volume would be related to the temperature (the temperature determined by a thermometer) and opened the door to the possibility of defining temperature for some systems (like ideal gases) without reference to any thermometer device.

    Thermodynamics and the formalisation of the modern ideal gas law started to be developed around 1854.  After this, a different (let's say "better" and more usefull) understanding of what "temperature" might be and how it should be defined was available.   Using thermodynamics you could define a temperature for various systems and not just ideal gases.  For example, you can declare two systems to be at the same temperature    if and only if    no heat flows between them when they are in thermal contact.   You can have an ideal gas as one of those systems and use the ideal gas law to put a numerical value on that temperature.   You could also obtain definitions for temperature based on the operation of a Carnot engine.
    Thermodynamic approaches to defining temperature have been discussed in this thread.   These remained the most useful and commonly accepted definitions of temperature until May 2019.

    Another area of science, "Kinetic Theory" and more generally microscopic statistical mechanics was also being developed alongside Thermodynamics.   They (the scientific community) have really run away with that idea recently.....

   As of May 2019, the Kelvin temperature scale has been formally re-defined.  This is now an entirely theoretical approach to defining a temperature scale and therefore what "temperature in Kelvin" is understood to be.   
   It's still a bit arbitrary to suggest this definition of temperature is "better" than a thermodynamic approach or an empirical definition of temperature.   Arguably it is "better" if you are trying to connect temperature to microscopic properties of particles, which is what you (Hamdani) seem to be seeking to do.

Best Wishes.

[alancalverd]

My intention when starting this thread is to find out the precise definition of temperature. We know that a system can have many forms of energy. They are often classified as kinetic and potential energy. Which category does temperature fall into?
If we put energy into a system, sometimes the system's temperature increases, sometimes it doesn't. What makes the difference?

Temperature is a measure of the mean kinetic energy of the molecules inside an object. It is not a measure of the kinetic energy of the whole object, or the potential energy of any stresses within it. If you input energy in such a way as to increase the mean kinetic energy of the molecules inside an object, you will increase its temperature. If you do something else, you won't.