2 questions: Why only two modes of energy transfer and what exactly is temperature?

[Secret]

Questions highlighted for tl:dr

Q1.
In my phy chem text
Elements of Physical Chemistry (By Peter Atkins), there's the following statement

Energy can be exchanged between a closed system and its surroundings by doing work or by the process called 'heating'

And then in wikipedia, about the first law

The first law of thermodynamics is a version of the law of conservation of energy, specialized for thermodynamical systems. It is often formulated by stating that the change in the internal energy of a closed system is equal to the amount of heat supplied to the system, minus the amount of work done by the system on its surroundings.

From Atkins and my lecture notes (cannot post here else it will constitute an academic misconduct) learnt that a closed system is a system that can exchange energy with the surroundings but not matter

However throughout my 1st year thermodynamics to 2nd year phy chem, I am still confused on why the only ways to transfer energy between a closed system and its surroundings is Work and Heat. What rules out a third type of energy transfer that is neither heat nor work?

Q2.
Another equally confusing quantity to me is Temperature

Temperature, T is an intensive property (a property that does not depend on the amount of substance in the sample, Foundations 0.4) that is used to define the state of a system and determines the direction in which energy flows as heat

Heat, q, is energy in transit as a result of a temperature difference.

So if according to my understanding of the above, temperature is just a flag on two systems S1 and S2 that are in thermal contact and that the larger one will determine where heat will flow from (e.g. if Temperature of S1>S2, then a net amount of heat flows from S1->S2), why is the value of temperature important? What's the difference between these two cases if e.g.

Temperature for
S1 = 273K S2 = 200K

and

S1 = 273 S2 = 100K?

because in both cases heat flows from S1 to S2, so what's the difference?

In kinetic theory temperature basically reflects the average kinetic energy of the particles in the system, but then the notation of entropy complicated things further and then I got very confused

 

[eram]

by including energy conservation and specific heat capacities, you can find the equilibrium temperature of the system.

 

[James R]

I am still confused on why the only ways to transfer energy between a closed system and its surroundings is Work and Heat. What rules out a third type of energy transfer that is neither heat nor work?

Work is energy transfer caused by applying forces to the system. Heat is energy transfer caused by temperature differences between the system and another system that it is brought in contact with.

Can you think of any other ways to transfer energy to a system, apart from the ones mentioned?

So if according to my understanding of the above, temperature is just a flag on two systems S1 and S2 that are in thermal contact and that the larger one will determine where heat will flow from (e.g. if Temperature of S1>S2, then a net amount of heat flows from S1->S2), why is the value of temperature important?

Often, the temperature difference will affect the rate of heat flow between the two systems.

In kinetic theory temperature basically reflects the average kinetic energy of the particles in the system, but then the notation of entropy complicated things further and then I got very confused

Temperature as an average kinetic energy is a good way to think about it. Entropy is a separate concept.

 

[eram]

Often, the temperature difference will affect the rate of heat flow between the two systems.

Newton's Law of Cooling.

 

[origin]

As James said temperature is the average KE of a substance. One additional comment is that it is the vibrational energy - temperature in not the velocity of the material of course. The vibraitonal KE energy of the material is also the translational KE of the material, in other words the vibrational energy can be transfered.

So using this idea it is easy to see that heat is nothing more than the transfer of this vibrational energy from one substance to another. It also follows that the heat is transfered from hot to cold. The vibrations are transfered from a material with a large amount of vibration to a low vibrational material. Absolute zero is the temperature at which there is no translational vibrations and 273 K is the the arbitrary number that represents the amount of vibrational energy that is necessary to break the hydrogen bonding of H2O ice into liquid water (at atmoshoeric pressure).

These represent 2 of the 3 forms of heat transfer - conduction and convection. Both of these types of hear transfer require that there is contact of the solid, liquid or gases. The third type of heat transfer is radiation. This is simply a decrease in the vibrational energy of the substance by the emission of EM radiation. This is typically infrared emission from the electrons of the material. The infrared can be absorbed by material and the vibrational energy of that material can increase or heat up.

 

[origin]

So if according to my understanding of the above, temperature is just a flag on two systems S1 and S2 that are in thermal contact and that the larger one will determine where heat will flow from (e.g. if Temperature of S1>S2, then a net amount of heat flows from S1->S2), why is the value of temperature important? What's the difference between these two cases if e.g.

Temperature for
S1 = 273K S2 = 200K

and

S1 = 273 S2 = 100K?

because in both cases heat flows from S1 to S2, so what's the difference?

Keeping in mind the vibrational aspect of temperature it is easy to see that if the differences in the vibrational energy is greater there will be a more rapid transfer of the vibrations. It also matters how efficiently the material can transfer the vibrations. For instance I can easily sit in a sauna that is 175 degress F for 15 minutes, if I jumped into a pool of water at 175 degrees survival time would be measured in seconds. The is because the are MANY more molucules in the water per unit area than there are in air.

The equation for heat transfer is

$$ Q = c_pm(\Delta T) $$

So the bigger the delta T the larger the heat flow.
The higher the heat capacity the larger the heat flow (water vs gas)
The larger the mass flow (where m is the mass flow) the higher the heat flow.

Sticking with the sauna example if you blow on your skin in a sauna (before you sweat too much) you will almost be able to burn yourself due to the increase in the mass flow. This is because there will be more molecules per unit area per unit time to transfer the heat from the air to your skin.

 

[eram]

Sticking with the sauna example if you blow on your skin in a sauna (before you sweat too much) you will almost be able to burn yourself due to the increase in the mass flow.

I'm a little fearful, but I feel like trying that. How bad does it hurt?

 

[origin]

I'm a little fearful, but I feel like trying that. How bad does it hurt?

It takes a concerted effort to make it hurt. The effect is immedately detectable though, it a simple and effective experiment to test the principle.

 

[eram]

It takes a concerted effort to make it hurt. The effect is immedately detectable though, it a simple and effective experiment to test the principle.

Blowing on your hand makes it feel cooler. I guess this is because the surrounding air is usually colder than your body, whereas in the sauna it's hotter.

 

[exchemist]

Questions highlighted for tl:dr

Q1.
In my phy chem text
Elements of Physical Chemistry (By Peter Atkins), there's the following statement



And then in wikipedia, about the first law


From Atkins and my lecture notes (cannot post here else it will constitute an academic misconduct) learnt that a closed system is a system that can exchange energy with the surroundings but not matter

However throughout my 1st year thermodynamics to 2nd year phy chem, I am still confused on why the only ways to transfer energy between a closed system and its surroundings is Work and Heat. What rules out a third type of energy transfer that is neither heat nor work?

Q2.
Another equally confusing quantity to me is Temperature



So if according to my understanding of the above, temperature is just a flag on two systems S1 and S2 that are in thermal contact and that the larger one will determine where heat will flow from (e.g. if Temperature of S1>S2, then a net amount of heat flows from S1->S2), why is the value of temperature important? What's the difference between these two cases if e.g.

Temperature for
S1 = 273K S2 = 200K

and

S1 = 273 S2 = 100K?

because in both cases heat flows from S1 to S2, so what's the difference?

In kinetic theory temperature basically reflects the average kinetic energy of the particles in the system, but then the notation of entropy complicated things further and then I got very confused

Secret, let me have a go at your Q1 and Q2:

Q1) Good point. But then, what forms of energy besides heat and work could flow between a closed system and its surroundings, given that no matter exchange is permitted? One might perhaps say Light, but then light is converted to heat when it is absorbed and a hot system can radiate heat by black body radiation, which is light (i.e electromagnetic radiation, whether visible or otherwise). I'm not sure I can think of any other form of energy exchange. Can you?

Also, Thermodynamics is the science of heat and work. As soon as other forms of energy are included, you have a scrambled situation involving other branches of science in various ways, which cloud the thermodynamic picture until you unscramble their effects. But all will be manifest in heat in the end, so it will be a case of working through how they would affect the heat balance - and then applying thermodynamics to the result.

Q2) I think of Temperature as being to Heat what Pressure is to a fluid, or what Voltage is to electric charge. A temperature difference tells you not only the direction of heat flow but also the strength of the tendency to flow, just as pressure difference does with a fluid and voltage difference does with electric charge.

Entropy (dS = dQ/T) is another discussion, I think.

 

[wellwisher]

For entropy to increase it needs to gain/absorb energy. While the second law states, that the entropy of the universe has to increase. What these two conditions together imply, is that heat needs to flow from higher to lower temperature; direction of entropy increase. In the case of the closed system, we can still get heat loss from a closed system; radiational cooling. This can lead to convection outside the closed system with the convection a form of entropy as the heat the moves from hot to cold. Entropy is not released but will increase outside via the heat middleman.

Work is force times distance and has the units of joules. Work is a form of energy. There is a relationship between entropy and work. In a system that is increasing entropy, the entropy increase will decrease by the amount of work; energy conservation. If I took gas and ignited it, the gas will explode and heat the gas molecules to high entropy. If I do this in a closed system to get work (piston) the entropy of the final gases will decrease by the amount of work I generate. The exhaust will come out differently, in terms of its entropy; gases will be cooler and lower pressure, in proportion to the amount of energy I extracted via doing work.

In open systems, we can also have mass transfer across the boundary. The mass will have heat capacity so it can drag heat and energy across the boundary. The mass can also contain entropy and can bring that across the boundary. Say the seal is worn in our piston so there is pressure leaking during combustion. The work will go down as fumes/smoke appears with the balance of free energy (heat and entropy).

The reason closed systems are used is because open system can get very complicated. For example, say the leaking gas also contains some the gasoline. The system will lose potential energy via lost heat of combustion. If does not explode, the exhaust gases will appear too low in temperature; work seems too high. The extra energy would be accounted for by the chemical bonds within the unburnt octane.

 

[origin]

Blowing on your hand makes it feel cooler. I guess this is because the surrounding air is usually colder than your body, whereas in the sauna it's hotter.

Exactly right (we can ignore evaporation if your skin is not wet or sweaty). The equation still holds of course in this case you skin is warmer than the air so the delta T is negative implying a heat flow out of your skin, so there is cooling.

 

[eram]

Exactly right (we can ignore evaporation if your skin is not wet or sweaty). The equation still holds of course in this case you skin is warmer than the air so the delta T is negative implying a heat flow out of your skin, so there is cooling.

A curious thing is that our skin senses the temperature change and not the temperature itself. You've probably done this experiment where you leave each hand in hot/cold water and then plunge them into lukewarm water.

One hand will sense the water to be warm while the other will sense it to be cold.
It makes one wonder about the mechanisms behind our perceptions of heat, pressure and pain.

 

[Motor Daddy]

A curious thing is that our skin senses the temperature change and not the temperature itself. You've probably done this experiment where you leave each hand in hot/cold water and then plunge them into lukewarm water.

One hand will sense the water to be warm while the other will sense it to be cold.
It makes one wonder about the mechanisms behind our perceptions of heat, pressure and pain.

Interesting. Say for instance that the hand did in fact sense the temperatures and not the change. You stick your hand in the cold water and the heat starts traveling from your hand into the water. You stick your other hand into warm water (higher temperature than your hand) and the heat travels from the water to your hand. So simultaneously one hand is experiencing a change in temp in the positive direction, and the other hand is experiencing a change in temp in the negative direction. A time elapses and the temperature of the hands are changed accordingly. Now the hands are at different temperatures. The hands are placed in water that is the average temperature of the two hands. Now the hands experience a change in direction in temperature change. They are each trying to come to an equilibrium temperature in that water, but one hand is increasing temperature and the other hand is decreasing temperature, even though they are submerged in the same water.