Q&A: How Does Work & Energy Relate to Solids, Liquids & Gases?

[Sarah0001]

Homework Statement

"If the volume of the substance does not change, no work is done on or by the substance"

Relevant Equations

ΔU = W+ Q
work=−PΔV

I can understand how this applies to gases as in that scenario work done = pressure * change in volume and if change in volume is zero then W=0
so change in internal energy = Q. But I do not understand how this applies generally to all types of substances, say solid, liquid and gas.

 

[Charles Link]

It applies simply to solids, liquids and gases that e.g. in a container or otherwise fixed position where their volume may change, in the presence of an external atmosphere.
It does not apply to a mechanical arm connected to a motor that is able to do work on an object to set it in motion.
Perhaps others can help to answer to this question, because I don't know that I have answered it satisfactorily. @Chestermiller might you have an input here?

 

[Chestermiller]

I can understand how this applies to gases as in that scenario work done = pressure * change in volume and if change in volume is zero then W=0
so change in internal energy = Q. But I do not understand how this applies generally to all types of substances, say solid, liquid and gas.

You don't understand the statement because it is obviously incorrect. Any loading that involves the deformation of a solid, liquid, or gas automatically involves doing work on or by the substance. Examples of this are stirring, shearing between parallel plates, forcing through a pipe, etc., ad infinitum. It does not only apply to volume changes. In addition to this, even without deformation, the substance can still experience a change in kinetic energy and/or kinetic energy, which is the result of doing work on or by the substance.

 

[Charles Link]

You don't understand the statement because it is obviously incorrect. Any loading that involves the deformation of a solid, liquid, or gas automatically involves doing work on or by the substance. Examples of this are stirring, shearing between parallel plates, forcing through a pipe, etc., ad infinitum. It does not only apply to volume changes. In addition to this, even without deformation, the substance can still experience a change in kinetic energy and/or kinetic energy, which is the result of doing work on or by the substance.

Thank you very much @Chestermiller I didn't know the complete answer, but that's exactly the kind of input I was looking for. Many thanks!