Heat- Work interconversion

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In summary, the conversation discusses the conversion of energy from work to heat and vice versa. It is understood that work can be completely converted to heat, but heat cannot be completely converted to work. However, a case is presented where all of the heat supplied to a gas is obtained as work output during its isothermal expansion, contradicting the fact that heat cannot be completely converted to work. The conversation also brings up the applicability of this reasoning to ideal gases and reversible processes. Ultimately, the second law of thermodynamics states that heat cannot be completely converted to work due to the principles of entropy.
  • #1
Ritz_physics
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It is understood that energy supplied in the form of work can be completely converted to heat.
However energy transferred in the form of heat cannot be completely obtained from the system as work.

But, consider this case: A Q amount of heat is supplied isothermally to a gas confined in a piston-cylinder device. By the energy balance equation:
ΔU = Q - W
Since ΔU = Cv.ΔT (Cv= specific heat at constant volume, ΔT=0), ΔU = 0.
Hence we get Q= W. This means all of the heat supplied to the gas was obtained as work output from the gas during its isothermal expansion. This comes as a contradiction to the fact that heat cannot be completely converted to work.
Please clarify the doubt.
 
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  • #2
Your reasoning is applicable only to ideal gases, which actually do not exist in nature.
 
  • #3
Infinitum said:
Your reasoning is applicable only to ideal gases, which actually do not exist in nature.

But reversible processes also do not exist in nature, even then Kelvin-Planck statement of the second law is applicable to reversible cycles.
 
  • #4
The way you wrote those equations and presented those equations doesn't prove that you have showed a way that heat can be turned into work.
All you really wrote was that if the change in potential energy is 0, then the change in heat is equal to the change in work -- according to the equation.
However, an exception was created to that equation which is now called the second law of thermodynamics. This law, in essence, states that heat cannot be changed completely into work, based on the principles of entropy.
 
  • #5


I can clarify this doubt by explaining the concept of efficiency. In thermodynamics, efficiency is defined as the ratio of the output energy to the input energy. In the case of heat and work interconversion, the efficiency is always less than 100% due to the second law of thermodynamics.

In the scenario described, the gas is undergoing an isothermal expansion, meaning that its temperature remains constant. This results in a change in internal energy (ΔU) of zero. However, this does not mean that all the heat supplied (Q) was converted into work (W). In fact, some of the heat is still lost to the surroundings, resulting in a lower efficiency.

The equation ΔU = Q - W only applies to closed systems, where there is no exchange of energy with the surroundings. In reality, there will always be some energy lost to the surroundings, which decreases the efficiency.

Moreover, the specific heat at constant volume (Cv) is a constant for a given substance, but it may vary for different substances. So, while in this case, ΔU = CvΔT = 0, it may not be the case for all substances.

In conclusion, while it is true that work can be completely converted to heat, the reverse is not possible due to the limitations of efficiency and the second law of thermodynamics. The scenario described is only an idealized case and does not reflect the real-world processes.
 

1. What is the difference between heat and work?

Heat is the transfer of thermal energy from a hotter object to a colder object, while work is the transfer of energy that results in a change in motion or position of an object.

2. Can heat be converted into work?

Yes, heat can be converted into work through the process of heat engines, which use the temperature difference between a hot and cold reservoir to produce work.

3. What is the first law of thermodynamics?

The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed, only transferred or converted from one form to another.

4. How does the second law of thermodynamics relate to heat-work interconversion?

The second law of thermodynamics states that in any energy conversion, some energy will be lost as heat. This means that not all of the energy from heat can be converted into work, as some of it will always be lost in the process.

5. What is the efficiency of a heat engine?

The efficiency of a heat engine is the ratio of the work output to the heat input. It is always less than 100% due to the second law of thermodynamics, which dictates that some heat will be lost in the conversion process.

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