# Why can't thermal energy be converted completely to motion

• osnarf
In summary, the professor in the thermo class states that thermal energy cannot be completely converted into energy of motion. This is due to the second law of thermodynamics, which explains that there will always be excess heat left over when trying to convert heat into mechanical energy. This is why there are not 100% efficient engines. Even in a hydroelectric dam, not all of the stored energy can be used to drive a turbine due to quantum fluctuations preventing absolute zero temperature.
osnarf
In my thermo class I'm currently taking, the professor constantly says thermal energy cannot be completely converted into energy of motion. Just to be clear, I'm not arguing this point. I'm just wondering why we say that, when earlier in the course it said that thermal energy at the macroscopic level is the kinetic energy of the particles at the microscopic level. It seems to me that heat transfer is just transfer of kinetic energy... so what am I missing?

I think what he means is that it can't be converted entirely into "usable" energy (e.g. to drive an engine). You can't get all the randomly moving particles to all of a sudden move in tandem to drive a machine because of the random nature of the particles. This is the second law of thermodynamics. There are not 100% efficient engines (where all the heat is converted to mechanical energy), there will always be excess heat left over moving into your cold reservoir from your hot reservoir.

Consider a hydroelectric dam. Can all of the stored energy drive a turbine? Why/why not?

I see what you're saying russ, thanks for the response, but matterwave hit it on the head. I was just making sure I wasn't missing something important. Thanks again.

SOLVED

If 100% of the thermal motion of a gas were converted to the kinetic energy of a piston, the gas would be left at absolute zero temperature, which is impossible. We can get very close to absolute zero, but never finally there because of quantum fluctuations.

## 1. Why can't thermal energy be converted completely to motion?

Thermal energy is a type of energy that is produced by the movement of atoms and molecules. This movement is random and chaotic, and it is not possible to direct all of this energy in a single direction to produce motion.

## 2. Can't we use advanced technology to convert all thermal energy to motion?

While technology has advanced significantly, it is still not possible to convert all thermal energy to motion. The second law of thermodynamics states that in any energy conversion process, there will always be some energy that is lost in the form of heat.

## 3. What happens to the energy that is not converted to motion?

The energy that is not converted to motion is released as heat. This heat is often considered as waste energy, but it is an important byproduct of energy conversion processes. It helps to maintain the balance of energy in the universe.

## 4. Is there a limit to how much thermal energy can be converted to motion?

Yes, there is a limit to how much thermal energy can be converted to motion. This is known as the Carnot efficiency, and it is based on the temperature difference between the hot and cold reservoirs in a heat engine. The higher the temperature difference, the higher the Carnot efficiency.

## 5. Are there any real-life examples of converting thermal energy to motion?

Yes, there are many real-life examples of converting thermal energy to motion. Steam engines, for example, use heat energy to produce motion. Similarly, internal combustion engines in cars convert the heat energy from burning fuel into motion. However, these processes are not 100% efficient, and there will always be some energy lost as heat.

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