# I Is the human body a heat engine?

1. Jul 1, 2018

### Taulant Sholla

If this is a valid enough definition of a heat engine:

All heat engines transfer heat energy from higher to lower temperatures, and do work in the process.

... then is the human body a heat engine?

2. Jul 1, 2018

3. Jul 1, 2018

4. Jul 1, 2018

### Staff: Mentor

I suppose it could be said the body as a whole is a heat engine. Not a very good one though.

5. Jul 1, 2018

### Taulant Sholla

The article abstract Dale posted does seem to pretty clearly indicate that mitochondria are very much engines -- unless I'm over reading.

6. Jul 1, 2018

### sophiecentaur

I would say not. Heat may be produced as a byproduct of contraction but the energy from the glucose is transferred directly via a complex process involving ATP (see this link) into motion and not due to glucose being oxidised and heating a medium to produce expansion (as in a Carnot Cycle etc. The efficiency of conversion from chemical to mechanical is far more efficient than that of a heat engine working between 'biological' temperature differences.
The basic statement is true but the inverse is not.

7. Jul 1, 2018

### Taulant Sholla

Very interesting... don't, though, most heat engines involve a chemical process somewhere in their cycle, e.g. combustion of a gas-air mixture?

8. Jul 1, 2018

### sophiecentaur

Yes but their efficiency of energy transfer depends upon the temperature difference; that's thermodynamics for you. Not every energy transfer is thermodynamic - take an electric motor for instance; that is not a heat engine 95% efficiency and no inherent hot source and cold sink with a temperature difference.
1. Avoid trying to allow your understanding to be driven by over simplified classifications and definitions.
2. Definitions do not always work both ways.

9. Jul 1, 2018

### Staff: Mentor

The human body, at 37 C nominal temperature, does not require a source hotter than 37 C or an environment less than 37 C to function. So with that in mind, the human body is not a simple heat engine.

It would be more accurate to say that the human body converts energy to other forms of energy. Energy and temperature are not the same thing.

10. Jul 1, 2018

### Taulant Sholla

Helpful, thanks! Maybe could have done your without #1 and #2 postscript...

11. Jul 1, 2018

### Taulant Sholla

This is helpful, thank you. I read the mitochondria article abstract again, and it does not seem to show they're heat engines.

12. Jul 1, 2018

### Staff: Mentor

Note that devices that generate the heat internally (internal combustion) still qualify as heat engines.

13. Jul 7, 2018

### Monsterboy

Heat engines require a heat source and a sink right ? and the source has to have a higher temperature than the sink for the heat to make the engine work.

Human body does depend on an energy source i.e chemical energy in the food carbohydrates (?) but can it considered as a heat source ? one might ask the same question with respect to gas or diesel or petrol in IC engines but they undergo combustion which results in heat ( the source) which is converted to mechanical energy and some of the heat is lost to the sink (the atmosphere). What if the sink has the same temperature as the source ? The engine will produce no work correct ?

But the human body will produce mechanical work even if the outside temperature is equal to or greater than the internal temperature correct ?

I think human bodies are more like chemical batteries which produce mechanical work. We store energy in the form of ATP molecules and then release the energy to do work and some of this energy will be lost as heat just like we observe chemical batteries heating up.

14. Jul 7, 2018

### Staff: Mentor

The mitochondria operate at 50 deg C. I don’t think that the human body will produce any work if the cells are at 50 deg C, it will be dead.

I think that the way to look at it is whether or not the body’s efficiency is limited by Carnot’s theorem. I don’t know the answer to that.

15. Jul 7, 2018

### Staff: Mentor

Right. The chemical reaction is basically combustion, but my knowledge of biology isn't good enough to know how exactly the energy is converted to work. Perhaps the heat is just a waste byproduct like it is for electric motors.
It's more than just temperature (moisture; latent heat), but no, you will die if you can't reject body heat.
Could be. I'm not clear on the mechanism.

16. Jul 7, 2018

### Monsterboy

Like russ, I am not sure what exactly goes on in biological systems, as I understand it the mitochondria produce ATP molecules using glucose(?) and some other stuff, can this be considered as "energy input" into the mitochondria ? can the output of mitochondria be considered as mechanical work or is it like a chemical processing plant converting one set of chemicals to another ? Is heat just a byproduct of chemical reactions inside the mitochondria or is heat required to produce ATP ?

Looking at the thread title and post #1 I tried to approximate the whole human body as a single engine but for me looked more like a battery than an engine because it looked like heat is more of a byproduct than as source of energy for our bodies. I am not sure.

Last edited: Jul 7, 2018
17. Jul 7, 2018

### Monsterboy

Yea, imagine I have a battery to run some devices and it's normal operating temperature is 37 deg Celsius, now I want to increase the output that might lead to a temperature of 50 deg C, that might ruin the battery due some internal breakdown because of the excess heat, but this is not caused due to the temperature difference with respect to the surroundings, although higher surrounding temperatures can accelerate the breakdown due to lack of heat transfer.

Or

I can operate the battery normally but increase the surrounding temperature to 50 or 60 deg Celsius this can also lead to breakdown even if the battery is operating normally.

In case of a heat engine, say a Carnot engine, the engine simply stops functioning because efficiency becomes zero.

$\eta = 1 - \frac { T_2} {T_1}$ is equal to zero if $T_2 = T_1$. No need of any engine breakdown as this is a mathematical result.

So, which of the above describes a human body in a better way ? The battery or the heat engine ?

Last edited: Jul 7, 2018
18. Jul 7, 2018

### Staff: Mentor

I think what we are lacking is a general definition of a heat engine.

19. Jul 7, 2018

### sophiecentaur

A heat engine does work as a result of heat transfer from a hot source to a cold sink. ?

That's as general as I can imagine and things like muscles and electric motors do not fulfil the requirement.

20. Jul 8, 2018

### Monsterboy

A heat engine is simply a device that converts heat energy to mechanical work.

In order to do so the heat needs to flow.
Since heat can only flow if there is a temperature difference, we need a high temperature heat source and a sink at a lower temperature, when the heat tries cross over to the lower temperature sink, we convert some of it to mechanical work.

21. Jul 8, 2018

### Staff: Mentor

Would photovoltaics count as heat engines by those definitions?

22. Jul 9, 2018

### Monsterboy

Light is used to generate a potential difference by knocking off electrons from atoms which leads to an electric current i.e Input radiation energy is converted to electrical energy. Since radiation is also a form of heat transfer, we might consider that as "heat input" and if you consider movement of electron and holes as mechanical work done, then a photo-voltaic is a heat engine.

https://www.researchgate.net/public...y-entropy_analysis_of_photovoltaic_conversion
https://www2.pvlighthouse.com.au/re...V Principle/Solar cells as a heat engine.aspx

23. Jul 9, 2018

### sophiecentaur

If em radiation is assumed to have a thermal source then 'perhaps'. But isn't there an accepted distinction between thermodynamics and QM? Also, PVs don't do conventional work.
Here we are again, discussing classification. More train spotting and stamp collecting.