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As stated by @Mark Eichenlaub in this link heat is the energy transferred between two systems by means aside from work. But then when you pull the brakes in your car, the tires, the road everything becomes hotter aka they gain thermal energy. Here, the car is doing work on the road.But the road doesn't gain 100% energy of the work done.Some of the energy is 'lost' to the environment as heat. But if some thermal energy is lost to the environment, then there must be some other amount of thermal energy transferred to the roads, bcz otherwise they wouldn't gain thermal energy . My assumption is this thermal energy is gained via the actual mechanical work done by the car on the road. So, some amount of energy of the total work done by the car is used to actually move the car forward on the road. But there is some other amount of energy that is used to 'heat up' the road. Note that this is all my assumption. I'm not 100℅ sure about this. And So, this is essentially my question.

Can thermal(putting extra stress on the word 'thermal' bcz the internal energy aka sum of all energies of a system that isn't due to an external force is obviously going to increase when work is done on the system, but I am asking explicitly about the thermal energy) energy change by means other than heat( let's say when mechanical work is done on the system)

ACRafi
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  • What do you mean by "thermal energy"? It seems like you are referring to internal energy, but I'm not sure because I'm having trouble following your presentation. – Bob D Jan 15 '22 at 20:51
  • @BobD I don't think that there is general agreement on the definition of internal energy. For example, it might include energy stored in chemical bonds. So one shouldn't think of it as synonymous with thermal energy (except for the case of the ideal gas). – garyp Jan 15 '22 at 23:49
  • @garyp I believe there is agreement on the definition of internal energy. It includes all forms of kinetic and potential energy at the molecular level, including the potential energy stored in chemical bonds. I subscribe to the view of Mark Zemansky, noted author of Heat and Thermodynamics that it is the term “thermal energy” that is vague and ill defined and should be avoided. – Bob D Jan 16 '22 at 02:37
  • In his paper “the use and misuse of the work “heat” in physics teaching, he points out three errors commonly made in teaching introductory physics and chemistry, which I quote from his paper “(1) referring to the “heat in a body”, (2) using heat as a verb and (3) combining heat and internal energy into one undefined concept “thermal energy”, which on one page means heat and on the next means internal energy”. – Bob D Jan 16 '22 at 02:37
  • You can find his paper here: https://dokumen.tips/documents/the-use-and-misuse-of-the-word-heat-in-physics-teaching.html – Bob D Jan 16 '22 at 02:38
  • @BobD out of curiosity, what term does he prefer for the thermalized portion of internal energy, I.e. the portion of internal energy that is not contained in macroscopic degrees of freedom. – Dale Jan 16 '22 at 13:04
  • @Dale You would have to ask him, but unfortunately he is deceased. But he would probably ask you exactly what do you mean by the "thermalized" portion of internal energy? See my most recent comment to the OP below. – Bob D Jan 17 '22 at 02:38
  • @BobD By thermalized I mean the portion of internal energy that is not contained in macroscopic degrees of freedom. For example, suppose that the system included a capacitor and a resistor in a thermally insulated block of material. We would include the energy dissipated into the material by the resistor but not the energy in the capacitor. If he doesn’t have a term for that, then I think his opposition to the term “thermal energy” is impractical – Dale Jan 17 '22 at 03:12
  • @Dale Sorry but I don't understand the applicability of your capacitor resistor example. I think he is concerned about the term "thermal" inferring that a portion of the internal energy can be identified as being heat, given that Oxford defines thermal (as an adjective) as "relating to heat". – Bob D Jan 17 '22 at 03:37
  • @Dale I think I just realized what you were saying regarding the capacitor/resistor. If the capacitor/resistor in your adiabatic (insulated) system discharged through the resistor, that would increase the temperature and thus internal energy of the resistor. That is not "heating" the resistor. That is electrical work done on the resistor. Heat would occur if there was a transfer of energy from the resistor (whose temperature became elevated) to a lower temperature surroundings. But if your resistor/capacitor system is insulated from the surroundings such a transfer would not occur. – Bob D Jan 17 '22 at 03:56
  • @BobD yes, that is exactly the point. It is not heat but the temperature has raised due to the electrical work within the system. The internal energy has just shifted. Electrical energy in the capacitor has become what I would call thermal energy in the material. So what would Zemansky call that portion of the internal energy if not thermal energy? If there is no alternative then it seems impractical. – Dale Jan 17 '22 at 04:04
  • @Dale I think we are (almost) on the same page, except that I see no need to say that the electrical energy that was in the capacitor has become "thermal energy". It suffices to say that the internal electrical potential energy of the capacitor has become internal molecular kinetic energy of the resistor. I (and I think Zemansky) would say that labeling it "thermal energy" only tends to confuse it with heat. In any case, I don't know what time it is where your are, but here it's 11:30 PM so I'm going to call it a night. TBC. Bob – Bob D Jan 17 '22 at 04:28
  • @BobD thanks. I don’t think that I will adopt that terminology. The energy associated with temperature includes lots of non-kinetic energy too. All of the vibrational, bending, torsion and other such modes include potential energy, as well as electronic orbital excitations and other EM degrees of freedom. Listing all of those forms is too cumbersome and neglecting any seems incomplete. I think I will risk the confusion of calling it thermal energy. – Dale Jan 17 '22 at 04:55

2 Answers2

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Can thermal energy change by means other than heat( let's say when mechanical work is done on the system)

Absolutely. This is the subject of Joule's famous 1850 paper entitled "The Mechanical Equivalent of Heat". With careful measurements of the work done on a system and careful measurements of the temperature of the system he was able to show that mechanical work was proportional to heat.

It turns out that this is a general principle. For example, not only mechanical work but electrical work can also change thermal energy. As far as I know, all forms of work can be used to produce a change in thermal energy.

However, I am a little concerned about your example:

when you pull the brakes in your car, the tires, the road everything becomes hotter aka they gain thermal energy. Here, the car is doing work on the road. But the road doesn't gain 100% energy of the work done.

A car does very little work on a road while braking, ideally it does no work on the road. The work is done on brake shoes and pads, and almost all of the energy is dissipated into the material of the brakes themselves. Due to deformation of the road there is some work done on the road itself, but it is very small. The tires do get warm and do transfer heat to the road, but that is also a very small percentage compared to the work done in the brakes themselves.

Dale
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  • So when work is done on a system, some of that work 'is' always transferred as heat aka a portion of the work is lost in increasing the thermal energy of the system? – ACRafi Jan 16 '22 at 09:45
  • @AbdullahChowdhuryRafi I would not say “always”. Maybe “often” or even “usually”. – Dale Jan 16 '22 at 12:16
  • Suppose you drag a piece of sandpaper along the surface of a wall(let's keep it simple). Here the sandpaper is doing work on the wall. And so a portion of that wotk is transferred to the wall as heat thus making it a little bit hotter. But the wall is not the only thing that becomes hotter aka gains thermal energy in this situation. The sandpaper also becomes hotter. Why does this happen? The sandpaper's the one that's doing the work and so heat is being transferred from it to the wall. Shouldn’t this make it(sandpaper) lose thermal energy through heat thus actually make it colder? – ACRafi Jan 16 '22 at 17:13
  • That sounds like another good question to ask – Dale Jan 16 '22 at 17:15
  • Should I ask it separately then? – ACRafi Jan 16 '22 at 17:21
  • Check @Bob D's answer of this question in his comments section. – ACRafi Jan 17 '22 at 01:12
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Can thermal....energy change by means other than heat( let's say when mechanical work is done on the system)

If by "thermal energy" you are referring to internal energy, then yes internal energy can change by means other than heat. It can change by the same amount by doing an equal amount of work, since the change in internal energy is related to heat and work by the first law:

$$\Delta U=Q-W$$

where $Q$ is is positive if heat transfers to the system and $W$ is positive if work is done by the system.

For example, in the case of an ideal gas where internal energy depends only on temperature, you can increase the temperature of the gas with heat with no work, or by doing work (compressing the gas) with no heat (compressing it adiabatically). The end result is the same increase in temperature and the same increase in internal energy.

But then when you pull the brakes in your car, the tires, the road everything becomes hotter aka they gain thermal energy.

In the case of disc brakes, when you step on the brakes the main things that get hot (experience a substantial increase in temperature) are the brake pads (which are stationary) and the rotors (which are attached to and rotate with the wheels) due to kinetic friction between them.

In effect, the majority of the loss of macroscopic kinetic energy of the vehicle (kinetic energy of the vehicle as a whole when it decelerates) is converted to microscopic kinetic energy (an increase in the molecular kinetic energy of the brake materials which results in an increase in temperature), and thus the internal kinetic energy of the brake materials.

The actual slowing down of the vehicle during braking is the result of the counter-clockwise torque that the brakes apply to the wheels. The force of that torque acting forward on the ground is opposed by the equal and opposite static friction force that the ground exerts backwards on the vehicle per Newton's third law. This backwards force, neglecting air resistance, is the only external force acting backwards on the vehicle and is thus responsible for decelerating the vehicle.

Now (ignoring rolling resistance), there are no frictional losses at the contact surfaces between the ground and the tire as long as the maximum possible static friction force is not exceeded so that the vehicle skids on the ground. So as long as the vehicle doesn't skid, the only transfer of energy in the form of heat to the ground would be in the form of heat conduction from the brake materials to the ground.

Eventually, the elevated temperatures of the brake materials relative to their environment (other materials in the vehicle and its environment including the ground) will result in energy transfer in the form of heat to the environment.

Hope this helps.

Bob D
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  • by thermal energy I mean the total microscopic kinetic energy of the molecules of the system – ACRafi Jan 16 '22 at 11:34
  • And I'll ask you the same question as @Dave, so when a system does work on another system, does some portion of that work get 'lost' as heat transferred to the system that the work is done on,which raises the temperature of that system which increases the thermal energy(by my understanding, the total microscopic kinetic energy) which ultimately increases the internal energy of the system? – ACRafi Jan 16 '22 at 11:40
  • @ACRafi You are free, of course, to refer to the kinetic portion of the internal energy as "thermal energy" if you wish, but I don't recommend it. I subscribe to the view of Mark Zemansky, noted author of Heat and Thermodynamics that the term “thermal energy” is vague and ill defined and should be avoided. – Bob D Jan 16 '22 at 18:01
  • In his paper “the use and misuse of the word “heat” in physics teaching, he points out three errors commonly made in teaching introductory physics and chemistry, which I quote from his paper “(1) referring to the “heat in a body”, (2) using heat as a verb and (3) combining heat and internal energy into one undefined concept “thermal energy”, which on one page means heat and on the next means internal energy”.

    You can find his paper here: https://dokumen.tips/documents/the-use-and-misuse-of-the-word-heat-in-physics-teaching.html

     – Bob D Jan 16 '22 at 18:01
  • You made the following comment to Dale: “Suppose you drag a piece of sandpaper along the surface of a wall(let's keep it simple). Here the sandpaper is doing work on the wall. And so a portion of that work is transferred to the wall as heat thus making it a little bit hotter”. The work is not transferred to the wall as heat. The work done on the wall increases the internal energy of the wall by raising the temperature of the wall. This is not heat. – Bob D Jan 16 '22 at 18:02
  • When the wall’s surface temperature becomes elevated then there is energy transfer to the cooler interior materials of the wall as well to other cooler things in its environment. That is heat. – Bob D Jan 16 '22 at 18:02
  • So work doesn't directly increase the thermal portion of the internal energy of an object, but rather it increases the internal energy overall? – ACRafi Jan 17 '22 at 01:00
  • But how does increase in internal energy increase the temperature of an object? I think you're trying to say that 'will heat be transferred or not' is dependant only on the temperature of the two systems. If there's a difference in temperature there will be heat transfer. And if they are of the same temperature there won't be any heat transfer. I understand this. But I'm asking is this the only way? Can mechanical work directly change the thermal portion of internal energy of a system? – ACRafi Jan 17 '22 at 01:09
  • Let me ask you this, If one were to do mechanical (e.g. friction) work on, or transfer heat to, a mixture of water and ice at zero C causing a phase change from ice to water, what would be the change in the "thermal portion" of the water/ice mixture.? Does it even make any sense to talk about the change in the "thermal portion" of the internal energy of the water/ice mixture given that the temperature of the mixture doesn't change, and yet there is energy transfer in the form of work or heat? – Bob D Jan 17 '22 at 02:31
  • Let me get this straight. In short, you're saying that mechanical work *cannot* change the thermal portion of the internal energy of a system, rather it can be *only* changed if there is a difference in temperature among the two systems? – ACRafi Jan 17 '22 at 11:28
  • I’ve already said mechanical work can change internal energy. I am saying I don’t agree with the idea, or necessity, of labeling the kinetic energy portion of internal energy as “thermal energy” for the reasons I’ve cited and Zemansky gave in his paper. (Did you read it?). You are obviously free to continue to refer to it that way. – Bob D Jan 17 '22 at 11:54