First law of thermodynamics, q=w for a reversible isothermal process

In summary, the first law of thermodynamics states that the energy absorbed in a reversible isothermal process is used to do work. However, according to the second law, there cannot be 100% efficiency in a heat engine, meaning there must be some heat output in order to remove the entropy brought in by the heat input. This results in a work output to heat input ratio that is less than 0.
  • #1
jarman007
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according to first law of thermodynamics,q=w for a reversible isothermal process.this means all the energy absorbed is being used to to do the work ,but according to second law of thermodynamics,there cannot be 100% effeciency.please tell where i am going wrong
 
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  • #2


More precisely, the first law says that the sum of the heat input ([itex]q_\mathrm{in}[/itex]) and the work input ([itex]w_\mathrm{in}[/itex]) equals the sum of the heat output ([itex]q_\mathrm{out}[/itex]) and the work output ([itex]w_\mathrm{out}[/itex]). The second law says that in a heat engine (where [itex]w_\mathrm{in}=0[/itex] and [itex]w_\mathrm{out}/q_\mathrm{in}[/itex] is a measure of efficiency), there must be some [itex]q_\mathrm{out}[/itex],* so [itex]w_\mathrm{out}/q_\mathrm{in}<0[/itex]. Does this answer your question?

*(To remove the entropy brought in by the heat input; entropy can't decrease and isn't transferred by work.)
 

Related to First law of thermodynamics, q=w for a reversible isothermal process

1. What is the first law of thermodynamics?

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

2. What does q=w mean in regards to the first law of thermodynamics?

In regards to the first law of thermodynamics, q=w means that the amount of heat (q) transferred to a system is equal to the work (w) done by the system. This is known as the principle of energy conservation.

3. What is a reversible isothermal process?

A reversible isothermal process is a thermodynamic process in which the temperature of a system remains constant throughout the process. This means that the system is in thermal equilibrium with its surroundings and there is no change in internal energy.

4. How does the first law of thermodynamics apply to a reversible isothermal process?

In a reversible isothermal process, the first law of thermodynamics states that the change in internal energy of the system is equal to the heat added to the system minus the work done by the system. This can be represented by the equation q=w.

5. What are the implications of the first law of thermodynamics for energy use and conservation?

The first law of thermodynamics has important implications for energy use and conservation. It reminds us that energy cannot be created or destroyed, so it is important to use energy efficiently and to find ways to convert it from one form to another without wasting it. This is crucial for maintaining a sustainable and environmentally responsible approach to energy use.

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