Thermodynamic Work vs Mechanical Work

In summary, thermodynamic work refers to the energy transfer associated with a system's change in state, often involving heat exchange and pressure-volume changes, while mechanical work involves the energy transfer resulting from forces acting over distances, such as pushing or lifting objects. Both forms of work are essential in understanding energy transformations in physical systems, but they differ in their contexts and applications.
  • #1
neil123
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Homework Statement
In a heat engine experiment, which showcases the Ericsson Cycle, the thermodynamic work and the mechanical work is computed. Should these values be equal?
Relevant Equations
Thermodynamic Work = Wisothermal + Wisobaric
Mechanical Work = mg(hf - hi)
At first I thought these values should be equal because I thought thermodynamic work is simply the work done as the gas compresses or expands. That is, the work done as the gas expands ( and the mass gets lifted up) should be equal to the work done against gravity as the mass goes up, and the work done as the gas compresses (and the mass also does down) should also be equal to the work done by gravity as the mass goes down.

However, some sources say that they shouldn't be equal because they are defined differently and that thermodynamic work is work done by a thermodynamic process (that can cause a mechanical work), while mechanical work is the work done by a force.

I don't really understand this. Isn't the reason the mass moves in the first place due to the expansion/compression of the gas? So why wouldn't the work done as the gas expands be equal to the work
 
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  • #2
neil123 said:
At first I thought these values should be equal because I thought thermodynamic work is simply the work done as the gas compresses or expands.
You need to be more specific. The work done by what on what?
 
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  • #3
Work is work. It is the integral of the applied force over a displacement.

In thermodynamics, you have to very carefully define what you are calling your system, and where your system ends and its surrounding begins. This choice is entirely up to you.

You also have to specify what is doing work on what (e.g.,, surroundings on system or system on surrounding),

You can provide a specific example, and we can help you work it through.
 
  • #4
As the economizer of that machine cannot return all the thermal energy that it contains into the intake air (heat transfer depends on temperature differential), some of the input heat from an external source will be wasted into the atmosphere.

The mechanical work is measured output useful mechanical energy, which magnitude will always be less than input thermal energy.

Please, see:
https://en.wikipedia.org/wiki/Erics...ith_Carnot,_Diesel,_Otto,_and_Stirling_cycles
 

FAQ: Thermodynamic Work vs Mechanical Work

What is the difference between thermodynamic work and mechanical work?

Thermodynamic work refers to the energy transfer associated with the macroscopic movement of boundaries in a thermodynamic system, such as expansion or compression of gases. Mechanical work, on the other hand, is the energy transferred when a force is applied to an object causing it to move. While all thermodynamic work can be considered a form of mechanical work, not all mechanical work is thermodynamic work, as mechanical work can involve various forms of energy transfer not necessarily related to thermal processes.

How is thermodynamic work calculated?

Thermodynamic work is often calculated using the equation W = -∫PdV, where W is the work done by the system, P is the pressure, and dV is the change in volume. The negative sign indicates that work is done by the system when it expands against an external pressure. For processes at constant pressure, this simplifies to W = -PΔV.

Can you give an example of thermodynamic work?

An example of thermodynamic work is the expansion of a gas in a piston. When the gas inside the piston expands, it exerts pressure on the piston, causing it to move outward. The work done by the gas on the piston is considered thermodynamic work, as it involves energy transfer due to a change in volume of the gas.

What are the units of thermodynamic work?

The units of thermodynamic work are typically expressed in joules (J) in the International System of Units (SI). Other units that may be encountered include liter-atmospheres (L·atm) in specific contexts, particularly in chemistry, where it is used to describe work done by gases during expansion or compression.

How do thermodynamic work and heat transfer relate to each other?

Thermodynamic work and heat transfer are both forms of energy transfer, but they occur under different conditions. Work is energy transferred due to macroscopic forces acting over a distance, while heat transfer is energy transferred due to a temperature difference between systems. In a thermodynamic process, the first law of thermodynamics states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system (ΔU = Q - W), linking the two concepts closely.

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