[Petrochemicals]

The spectrum is irrelevant. As long as the source and detector are coupled and isolated from the rest of the universe, heat always and only flows from a hotter body to a cooler one.
 

Is the thermopile powered, thus allowing you to deduce the electron flow, or is it passive, thus meaning you do not know the temperature of it?

[Eternal Student]

Hi.
   

The spectrum is irrelevant.

    Not if the total power emitted by the fly doesn't change at all with its temperature.   Suppose it always emits  1 W of radiation regardless of the temperature of the fly.
    There's no substance I know that does this exactly but that's why we were hypothesising.   There could be physical and chemical changes in the outer surface of the fly depending on temperature (perhaps just changing the colour of the radiating surface from black to white and keeping total power of emissions almost constant at any temperature).
- - - - - - - - -
    Anyway, I'm not going to keep standing in the way.   Keeping everything simple, the fly and thermopile can be assumed to be in thermal equilibrium.   The forum doesn't need to confuse people with complications.

Is the thermopile powered, thus allowing you to deduce the electron flow, or is it passive, thus meaning you do not know the temperature of it?

   I'm not Alancalverd but the idea seemed to be that the thermopile was powered, or somehow heated, initially to raise it to a particular temperature.   Then it is switched off and can even be disconnected from any battery or circuit.
   For the second part of the experiment you just connect a Volt meter to the thermopile.   That's the basic idea of an idealised thermopile, it's a thing that doesn't need powering by a battery, it just generates a voltage entirely due to the temperature it has.   You can measure that just by connecting a volt meter.  An ideal volt meter has an infinite resistance, so we can imagine that (almost) no electrons need to flow for that measurement.

Best Wishes.

[Bored chemist]

I can't help wondering about the original question.
"Is there a limit to how hot things can get?".
I wonder if the answer is "As hot as they were".

 (about 14 billion years ago)

[chiralSPO]

I can't help wondering about the original question.
"Is there a limit to how hot things can get?".
I wonder if the answer is "As hot as they were".

 (about 14 billion years ago)

I believe this is correct.

[alancalverd]

Is the thermopile powered, thus allowing you to deduce the electron flow, or is it passive, thus meaning you do not know the temperature of it?

A thermopile is a series of thermocouples. If you know the temperature of one set of junctions then the voltage across the  others depends on their temperature difference - no external power involved. But if you break the circuit and inject some current you can raise the temperature of the assembly by ohmic heating. Come to think of it, I'd probably use an auxiliary heater, even simpler.

[alancalverd]

Not if the total power emitted by the fly doesn't change at all with its temperature.   Suppose it always emits  1 W of radiation regardless of the temperature of the fly.

Then you have discovered an insect that does not obey Stefan's Law, and may therefore be an alternative explanation to the Big Bang. The universe was created by a mathematical housefly!

The forum doesn't need to confuse people with complications.

That's half the fun!

[Eternal Student]

Hi.

Then you have discovered an insect that does not obey Stefan's Law,...

    I'm not sure about Steafn's law.   I'm familiar with the Stefan-Boltzmann law but I've only seen that proven for Black bodies.  There are some things that are not black bodies.

   Wikipedia uses this notation for the Stefan-Boltzmann law:
              j* = total power radiated (per unit surface area) =   ε.σ.T⁴ .     
with σ = constant;    T = temperature in kelvin     but  noteably  ε = emissivity   with  0 ≤ ε ≤ 1   and the following comment....

In the still more general (and realistic) case, the emissivity depends on the wavelength, ε = ε (λ).

   I'm not sure what wavelength they were talking about,  I guess it's the peak wavelength of the whole spectrum of emissions.  Anyway, if it is that then something approximating Wien's law implies  λ_peak ~ 1/T.   Hence, ε = ε(λ(T) )  =   a function of Temperature in disguise.
    So, all I'm asking is that  ε(T)  ~  1/T⁴    over a small range of T,   then the power radiated does lose all of it's dependence on T for that range of temperatures.

Best Wishes.

[alancalverd]

In the words of any maths-for-physics lecturer: "just integrate". Time was we'd do it with a capacitor and a sweep generator, or a chart plotter, scissors and a spring balance, but I guess today's youth would use a mobile phone app, look up the answer on Google, or swoon and complain about cultural appropriation. Fact is that a good thermopile does the integration for you.

So how did we get from ionising half of the universe (the answer to the OP) to analysing the emission spectrum of a fly?

Jim Al-Khalili has a series on "size" on BBC4. His continuity advisers should study this thread!
 

[Petrochemicals]

I'm not Alancalverd but the idea seemed to be that the thermopile was powered, or somehow heated, initially to raise it to a particular temperature.   Then it is switched off and can even be disconnected from any battery or circuit.
   For the second part of the experiment you just connect a Volt meter to the thermopile.   That's the basic idea of an idealised thermopile, it's a thing that doesn't need powering by a battery, it just generates a voltage entirely due to the temperature it has.   You can measure that just by connecting a volt meter.  An ideal volt meter has an infinite resistance, so we can imagine that (almost) no electrons need to flow for that measurement.

A thermopile is a series of thermocouples. If you know the temperature of one set of junctions then the voltage across the  others depends on their temperature difference - no external power involved. But if you break the circuit and inject some current you can raise the temperature of the assembly by ohmic heating. Come to think of it, I'd probably use an auxiliary heater, even simpler.

The theory of operation is then to raise the thermocouple temperature seperatley to the same and as yet unknown temperature of the object body. Connect them into one system by pure luck and register a zero voltage.

Firstly how do you know the temperature of the thermocouple if you are not withdrawing heat from it. Perhaps with another thermocouple? It would be classed as a single system

Secondly even though there is a remote possibility you may achieve equilibrium by pure chance is this really credible!

[alancalverd]

At some point in your schooldays you should have been introduced to zero-current potentiometric measurements, the Wheatstone bridge, or some other classic null device. If not, I  can only recommend that you review a basic physics text. All we are doing here is a heat-flow null using rate of change to indicate the null point.

Here's a basic aircraft instrument panel. When the dial on the lower right shows zero  rate of change you are neither climbing nor descending so your lift vector equals your weight. 

It is true that some physics students (and some pilots) achieve a null balance by pure chance, but most of us do it by successive approximation.

Photon coupling with mirrors is not luck.

[Petrochemicals]

At some point in your schooldays you should have been introduced to zero-current potentiometric measurements, the Wheatstone bridge, or some other classic null device. If not, I  can only recommend that you review a basic physics text. All we are doing here is a heat-flow null using rate of change to indicate the null point.

Here's a basic aircraft instrument panel. When the dial on the lower right shows zero  rate of change you are neither climbing nor descending so your lift vector equals your weight. 

It is true that some physics students (and some pilots) achieve a null balance by pure chance, but most of us do it by successive approximation.

Photon coupling with mirrors is not luck.

But the thermo couple is still not measured, unless you use a thermometer of a kind.

[alancalverd]

You would do well to review the basic physics of thermoelectricity. Standard thermocouples and thermopiles have a known temperature coefficient of voltage. If you buy a cheapish digital multimeter it will probably come with a Type K thermocouple and thermistor compensation block that you just plug in to the meter and measure temperatures to better than ±0.1K.

Come on, PC, this is very simple, robust engineering hardware. The guy who repaired my cooker had one in his bag.

[Bored chemist]

unless you use a thermometer of a kind.

What do you think "a thermometer" means?

[Petrochemicals]

You would do well to review the basic physics of thermoelectricity. Standard thermocouples and thermopiles have a known temperature coefficient of voltage. If you buy a cheapish digital multimeter it will probably come with a Type K thermocouple and thermistor compensation block that you just plug in to the meter and measure temperatures to better than ±0.1K.

Come on, PC, this is very simple, robust engineering hardware. The guy who repaired my cooker had one in his bag.

Coefficients spake greatly of powered monitoring thus allowing as you say ohmic heating. Also if you never see a change how can you be sure that your measurement is correct. You need to alter the objects temperature.

[alancalverd]

So you always withdraw money whilst checking your bank account, because you don't believe numbers that don't change?

You do have a small point. If the vertical speed indicator doesn't fluctuate a bit, there may be a problem. Happened to a couple of chaps flying a glider in Scotland in the 1970s. They flew into falling snow but weren't panicked because the air was smooth, the wings were level, and the top right instrument (altimeter) showed a constant 1200 feet. After a few minutes it occurred to them that the airspeed indicator (top left) was reading zero, which is not good. Turned out they had  grazed the top of a hill and come gently to rest in a snowdrift.  Houseflies have more sense than to take to the air in a snowstorm.