Thermodynamics-Conceptual problem

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Homework Help Overview

The discussion revolves around the thermodynamic principles related to the expansion of an object when heated. Participants explore whether work is done by the object during this process and if the heat added equals the increase in internal energy, considering various states of matter such as solids, liquids, and gases.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants question how to define work done during expansion, especially in the context of different materials. Some express uncertainty about whether the object in question is a gas, solid, or liquid. Others discuss specific scenarios, such as the expansion of a metal plate and the implications of heating in a vacuum versus in air.

Discussion Status

The discussion is active, with participants providing insights into the conditions under which work is done. There are multiple interpretations being explored, particularly regarding the assumptions about the state of the object and the nature of the work involved in expansion.

Contextual Notes

Some participants note the importance of sign conventions in thermodynamic equations and the need to clarify the assumptions about the object's state, as well as the conditions under which work is performed.

Tanya Sharma
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Homework Statement



When we heat an object,it expands.Is work done by the object in this process? Is heat given to the object equal to the increase in its internal energy ?

Homework Equations





The Attempt at a Solution



Had it been a gas ,the work done would be pΔv,but I am unclear how do we see the work done in general while an object expands.I am assuming,the object may be a solid or a liquid.

I am unsure whether the problem assumes the object is a gas.

Kindly help me in clearing the doubt.
 
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Tanya Sharma said:

Homework Statement



When we heat an object,it expands.Is work done by the object in this process? Is heat given to the object equal to the increase in its internal energy ?

When we heat an object, it is not sure that expands. Think about ice. You heat,. it becomes water, and occupies less volume.

Or think about gas heated at constant volume. You can realize a set-up when the volume does not change.

Tanya Sharma said:
Had it been a gas ,the work done would be pΔv,but I am unclear how do we see the work done in general while an object expands.I am assuming,the object may be a solid or a liquid.

I am unsure whether the problem assumes the object is a gas.

Kindly help me in clearing the doubt.

If the object exerts force on its surroundings and that force causes some displacement then work is done by the object.

ehild
 
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Tanya Sharma said:

Homework Statement



When we heat an object,it expands.Is work done by the object in this process? Is heat given to the object equal to the increase in its internal energy ?

Generally use First Law Q = U + W, for these types of problems. Do follow the correct sign conventions carefully
Tanya Sharma;4434808 Had it been a gas said:
The general formula for Work Done by gas or solid is ∫P*dv.
 
ehild said:
If the object exerts force on its surroundings and that force causes some displacement then work is done by the object.
ehild

Suppose the object is a round metal plate which expands on heating.The radius increases from R1 to R2.

Do we say that the plate exerts a force on the surroundings ? Will the work done be given by pΔv ?
 
No work in vacuum. In air, the surface of the expanding disk exerts force on the air, compressing it. So it does work on the air. If the expansion is so slow that the pressure of the air stays constant during the expansion, the disk does pdV work on the surrounding air.

If there is a box touching the disk and you heat the disk, there will be a normal force between the box and disk. Supposing, the disk is fixed at the centre, the box is shifted by distance R2-R1. The work done by the disk on the box is the integral of the normal force between R1 and R2.

ehild
 
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ehild said:
No work in vacuum. In air, the surface of the expanding disk exerts force on the air, compressing it. So it does work on the air. If the expansion is so slow that the pressure of the air stays constant during the expansion, the disk does pdV work on the surrounding air.

If there is a box touching the disk and you heat the disk, there will be a normal force between the box and disk. Supposing, the disk is fixed at the centre, the box is shifted by distance R2-R1. The work done by the disk on the box is the integral of the normal force between R1 and R2.

ehild

Very nice...Thanks a lot :biggrin:
 

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