# Can mechanical work change thermal energy?

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)

• 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. Jan 15, 2022 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). Jan 15, 2022 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. Jan 16, 2022 at 2:37
• 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”. Jan 16, 2022 at 2:37
• You can find his paper here: dokumen.tips/documents/… Jan 16, 2022 at 2:38

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.

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.

• 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? Jan 16, 2022 at 9:45
• @AbdullahChowdhuryRafi I would not say “always”. Maybe “often” or even “usually”.
– Dale
Jan 16, 2022 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? Jan 16, 2022 at 17:13
• That sounds like another good question to ask
– Dale
Jan 16, 2022 at 17:15
• Should I ask it separately then? Jan 16, 2022 at 17:21

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.

• by thermal energy I mean the total microscopic kinetic energy of the molecules of the system Jan 16, 2022 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? Jan 16, 2022 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. Jan 16, 2022 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: dokumen.tips/documents/… Jan 16, 2022 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. Jan 16, 2022 at 18:02