# How to tell the difference between reversible and irreversible process

PhyIsOhSoHard
Imagine a rigid block of steel at 100 degrees celsius which is inserted into water at 0 degrees celsius. They then both get an equilibrium temperature.

How can I tell whether this is a reversible or irreversible process? What is the argumentation?

Homework Helper
Imagine a rigid block of steel at 100 degrees celsius which is inserted into water at 0 degrees celsius. They then both get an equilibrium temperature.

How can I tell whether this is a reversible or irreversible process? What is the argumentation?
If it is reversible, the direction of the process can be reversed by an infinitesimal change in conditions. In the example you have given, half way through the process could you, by changing the conditions by the tiniest amount, make the heat flow back from the water into the steel block so that each reaches their initial states?

AM

PhyIsOhSoHard
If it is reversible, the direction of the process can be reversed by an infinitesimal change in conditions. In the example you have given, half way through the process could you, by changing the conditions by the tiniest amount, make the heat flow back from the water into the steel block so that each reaches their initial states?

AM

My guess is no because the steel block cannot spontaneously go back to 100 degrees celsius after it has been cooled so it is an irreversible process?

Homework Helper
My guess is no because the steel block cannot spontaneously go back to 100 degrees celsius after it has been cooled so it is an irreversible process?
Why is it a guess? Just apply the principle stated in my previous post: can you reverse the direction of the process (heat flow from block to water) by an infinitesimal change of the initial conditions? The only conditions given are the temperatures of the block and water. If you changed those temperatures by the tiniest amount would the process (heat flow from block to water) reverse its direction?

AM

PhyIsOhSoHard
Why is it a guess? Just apply the principle stated in my previous post: can you reverse the direction of the process (heat flow from block to water) by an infinitesimal change of the initial conditions? The only conditions given are the temperatures of the block and water. If you changed those temperatures by the tiniest amount would the process (heat flow from block to water) reverse its direction?

AM

No you can not reverse the process so it is an irreversible process. Was that correct?