How Does Heat Transfer Impact Our Sensation of Temperature?

[rudransh verma]

We know heat is the motion of molecules of a gas for example. As temperature increases this motion increases and the gas expands. We know this gas would be hot if we touch it. So I want to ask how does the fast motion of molecules somehow translates to the heat that we feel. Why does it feel hot? Does the fast kinetic energy of the particles converts into heat energy on collision with our hands and we feel that heat?
What does it do to our skin. Does the particles of our hand gain that energy and try to fly apart from our body and that causes pain?

 

[jbriggs444]

We know heat is the motion of molecules of a gas for example. As temperature increases this motion increases and the gas expands. We know this gas would be hot if we touch it. So I want to ask how does the fast motion of molecules somehow translates to the heat that we feel. Why does it feel hot? Does the fast kinetic energy of the particles converts into heat energy on collision with our hands and we feel that heat?
What does it do to our skin. Does the particles of our hand gain that energy and try to fly apart from our body and that causes pain?

Increasing heat means a greater chance that any given reactive pair of molecules will have enough activation energy to actually react. One rough rule of thumb is that a 10 degree Celsius increase in temperature is enough to double reaction rate. [Quick Google result here]

Our bodies are fairly well tuned to operate in a narrow temperature range where the various biochemical reactions proceed at well controlled rates. Go outside those ranges and the body performs poorly. We've evolved as warm-blooded creatures to prevent our bodies (or at least our core) from getting too hot or too cold.

Part of keeping the body from getting too hot or too cold is to prevent parts of the body getting way way to hot or way way too cold. That is what pain sensors are for. Get one of these sensors way out of temperature range and it will start transmitting. That is what you feel.

It hurts when you do that so that you stop doing that.

 

[TeethWhitener]

I'll let others comment on the precise definitions of heat and temperature and how they relate to kinetic energy of molecules, but these:

So I want to ask how does the fast motion of molecules somehow translates to the heat that we feel. Why does it feel hot? Does the fast kinetic energy of the particles converts into heat energy on collision with our hands and we feel that heat?
What does it do to our skin. Does the particles of our hand gain that energy and try to fly apart from our body and that causes pain?

are some great questions. We actually don't really have a clear answer for how we perceive changes in temperature. The wikipedia article on thermoreception has some good information, but in a nutshell, what seems to happen is that nerve endings contain ion pumps (proteins that shuttle various ions into and out of nerve cells) which operate with a temperature-dependent efficiency. So for instance, some pumps shut off completely at cold temperatures, while others have a variety of behaviors. When we touch something hot (or cold), the temperature change will cause these changes in ion pump behavior which then get transmitted up through the nerve cells as action potentials and are then interpreted by the brain.

 

[rudransh verma]

I'll let others comment on the precise definitions of heat and temperature and how they relate to kinetic energy of molecules, but these:

are some great questions. We actually don't really have a clear answer for how we perceive changes in temperature. The wikipedia article on thermoreception has some good information, but in a nutshell, what seems to happen is that nerve endings contain ion pumps (proteins that shuttle various ions into and out of nerve cells) which operate with a temperature-dependent efficiency. So for instance, some pumps shut off completely at cold temperatures, while others have a variety of behaviors. When we touch something hot (or cold), the temperature change will cause these changes in ion pump behavior which then get transmitted up through the nerve cells as action potentials and are then interpreted by the brain.

It says when we compress a gas slowly we increase it’s temperature because the particles on collision with the moving part gain kinetic energy and so the temperature of the gas rises. It said the atoms become hotter. I didn’t get that . What exactly it means when we say atoms became hotter?

 

[Hornbein]

It says when we compress a gas slowly we increase it’s temperature because the particles on collision with the moving part gain kinetic energy and so the temperature of the gas rises. It said the atoms become hotter. I didn’t get that . What exactly it means when we say atoms became hotter?

It is confusing. An individual atom can't get hot. It isn't a property of an atom, its a property of a collection of atoms. A collection of atoms is hot if there is a lot of random motion relative to each other. Orderly motion like rotation doesn't count as heat. If the atoms aren't moving relative to one another then they are at absolute zero which is something like -273 C. That's as cold as you can get.

When a hot object touches a cooler object then its atoms randomly knock into the others and causes them to move randomly faster at expense of their own motion. Eventually the two things will tend to be at the same temperature.

 

[jbriggs444]

It says when we compress a gas slowly we increase it’s temperature because the particles on collision with the moving part gain kinetic energy and so the temperature of the gas rises. It said the atoms become hotter. I didn’t get that . What exactly it means when we say atoms became hotter?

[like @Hornbein said], Strictly speaking, the atoms do not become hotter. Temperature is a property of a large collection of atoms, not of a single atom by itself. At an [over-]simplified level, temperature in a gas is determined by the average kinetic energy of the molecules in the gas in a frame of reference where the gas as a whole is motionless.

One or two levels of abstraction back from this, temperature is based on how heat energy flows from one object to another. We observe that heat moves from hot objects to cold. We define our temperature scale to reflect this.

Energy is conserved (first law of thermodynamics). Entropy increases (second law). If we define temperature in terms of the marginal rate of increase of energy with respect to entropy then entropy will increase when heat flows from high temperature to low. For ideal gases, it turns out that this marginal ratio corresponds with the average kinetic energy per particle in the center of momentum frame.

 

[rudransh verma]

@Hornbein @jbriggs444 I didn’t say an atom get hot. I said atoms get hotter. So what does this mean?

 

[Hornbein]

@Hornbein @jbriggs444 I didn’t say an atom get hot. I said atoms get hotter. So what does this mean?

I've explained as best I can.

 

[jbriggs444]

@Hornbein @jbriggs444 I didn’t say an atom get hot. I said atoms get hotter. So what does this mean?

The gas gets hotter. This means that, on average, its atoms are moving faster relative to one another.

If we put our fingers in such a gas, the more-frequent and more-energetic collisions of the gas atoms with our finger atoms means that our finger atoms, on average, gain kinetic energy. Which means that our finger is getting hotter.

 

[jim mcnamara]

In theory, the lowest temperature- meaning no atoms/molecules "moving" is absolute zero. Gasses can become liquids or even solids at extreme temperatures, example ammonia ( NH₃ )

A. Boiling point: -28.01°F (-33.34°C) liquid->gas
B. Melting point: -107.9°F (-77.73°C) solid->liquid

Below temperature B means the gas is now a solid. And is solid all the way down to absolute zero.

So what do you think temperature means in this case?

 

[rudransh verma]

So if we sit in a cloud of gas for long enough we will get evaporated.

 

[jbriggs444]

So if we sit in a cloud of gas for long enough we will get evaporated.

It takes a very long time for bone to sublimate. Usually one sits down for lunch and a drink first.

If you are dining on turkey, please realize that after 3 to 4 hours in a 325 degree oven, a 16 pound turkey has not yet evaporated away into nothingness. It will remain moist and flavorful.

 

[rudransh verma]

The gas gets hotter. This means that, on average, its atoms are moving faster relative to one another.

If we put our fingers in such a gas, the more-frequent and more-energetic collisions of the gas atoms with our finger atoms means that our finger atoms, on average, gain kinetic energy. Which means that our finger is getting hotter.

That is what I thought would be going on. You can see my question.

 

[jbriggs444]

That is what I thought would be going on. You can see my question.

Actually no. If the answers you have received so far are not on-point, I do not understand what you are asking.

 

[rudransh verma]

Actually no. If the answers you have received so far are not on-point, I do not understand what you are asking.

I am asking why does our hand gets burned on touching something hot? You said it why. Are you not sure ?
Also @TeethWhitener told how do we feel pain on touching something hot.
There are two things . One is how we feel pain and second is how skin gets damaged which you answered.

 

[jbriggs444]

I am asking why does our hand gets burned on touching something hot? You said it why. Are you not sure ?
Also @TeethWhitener told how do we feel pain on touching something hot.
There are two things . One is how we feel pain and second is how skin gets damaged which you answered.

So you are asking about the pathophysiology of thermal burns?

 

[rudransh verma]

So you are asking about the pathophysiology of thermal burns?

I am asking about what causes burns and the pain associated with it.

 

[TeethWhitener]

This is actually two questions, as @jbriggs444 indicated: burn physiology and pain perception (nociception). As far as nociception goes, at the end of the day, afferent (sensory) neuron endings are peppered with ion channels that open or close in response to some stimulus. For burn-related pain, various thermally responsive (and likely chemically responsive) ion channels associated with thermal damage are activated and interpreted as burn pain in the brain.

 

[sophiecentaur]

In theory, the lowest temperature- meaning no atoms/molecules "moving" is absolute zero. Gasses can become liquids or even solids at extreme temperatures, example ammonia ( NH₃ )

A. Boiling point: -28.01°F (-33.34°C) liquid->gas
B. Melting point: -107.9°F (-77.73°C) solid->liquid

Below temperature B means the gas is now a solid. And is solid all the way down to absolute zero.

So what do you think temperature means in this case?

I think it's a pity that we don't use the good old fashioned heat terms.
They used Latent Heat for the heat that is absorbed or given off during a change of state and Sensible Heat for the (feel-able) change in temperature when there's no change of state.
Also, they had Super Heat to describe the changes in temperature of the steam; a bit superfluous really but graphic.

 

[Johndarby]

Per the first law of thermodynamics, heat is energy transferred, without transfer of mass, across the boundary of a system solely because of a temperature difference between the system and surroundings. Work is energy transferred, without transfer of mass, across the boundary of a system because of an intensive property difference other than temperature between the system and surroundings. Note: sometimes the name heat is incorrectly used to mean internal energy. It makes no more sense to consider the heat in a body than the work in a body. The first law addresses the change in internal (and gross kinetic and potential) energy of a system due to: heat added to the system, work done by the system, and mass transfer into and out of the system. See a good text on thermodynamics for more details: I especially like an old textbook, Thermodynamics by Obert, for its clarity of definitions. The above definitions of heat and work are from the Obert textbook.

 

[Chestermiller]

In theory, the lowest temperature- meaning no atoms/molecules "moving" is absolute zero. Gasses can become liquids or even solids at extreme temperatures, example ammonia ( NH₃ )

A. Boiling point: -28.01°F (-33.34°C) liquid->gas
B. Melting point: -107.9°F (-77.73°C) solid->liquid

Below temperature B means the gas is now a solid. And is solid all the way down to absolute zero.

So what do you think temperature means in this case?

You can also have vapor below the freezing point if the pressure of the vapor is low enough.

 

[vanhees71]

Per the first law of thermodynamics, heat is energy transferred, without transfer of mass, across the boundary of a system solely because of a temperature difference between the system and surroundings. Work is energy transferred, without transfer of mass, across the boundary of a system because of an intensive property difference other than temperature between the system and surroundings. Note: sometimes the name heat is incorrectly used to mean internal energy. It makes no more sense to consider the heat in a body than the work in a body. The first law addresses the change in internal (and gross kinetic and potential) energy of a system due to: heat added to the system, work done by the system, and mass transfer into and out of the system. See a good text on thermodynamics for more details: I especially like an old textbook, Thermodynamics by Obert, for its clarity of definitions. The above definitions of heat and work are from the Obert textbook.

Another great book is

R. Becker, Theory of Heat, Springer (1967)

 

[Andrew Mason]

Per the first law of thermodynamics, heat is energy transferred, without transfer of mass, across the boundary of a system solely because of a temperature difference between the system and surroundings. Work is energy transferred, without transfer of mass, across the boundary of a system because of an intensive property difference other than temperature between the system and surroundings. Note: sometimes the name heat is incorrectly used to mean internal energy. It makes no more sense to consider the heat in a body than the work in a body. The first law addresses the change in internal (and gross kinetic and potential) energy of a system due to: heat added to the system, work done by the system, and mass transfer into and out of the system. See a good text on thermodynamics for more details: I especially like an old textbook, Thermodynamics by Obert, for its clarity of definitions. The above definitions of heat and work are from the Obert textbook.

That is the correct definition of heat, (the transfer of energy across a boundary due to temperature difference).

Just a few comments that may help (or add to confusion):
1. Temperature is a measure of the average translational kinetic energy of the molecules of a body that is in thermodynamic equilibrium.
2. The transfer of energy via heat can occur by way of direct molecular interaction (collisions) across the boundary or indirectly by exchange of radiation (photons), or both.
3. Although the overall transfer of energy is from higher to lower temperature, individual exchanges at the molecular level may go either way. Heat is the statistically mandated (net) transfer of energy across the boundary.

AM

 

[ikihi]

We know heat is the motion of molecules of a gas for example. As temperature increases this motion increases and the gas expands. We know this gas would be hot if we touch it. So I want to ask how does the fast motion of molecules somehow translates to the heat that we feel. Why does it feel hot? Does the fast kinetic energy of the particles converts into heat energy on collision with our hands and we feel that heat?
What does it do to our skin. Does the particles of our hand gain that energy and try to fly apart from our body and that causes pain?

Heat transfers from hot to cool areas. The sensation of cold is heat leaving our body. The sensation of warmth is heat entering our body. Temperature differences lead to an energy transfer of heat.