# How to tell the difference between reversible and irreversible process

1. Apr 1, 2014

### 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?

2. Apr 1, 2014

### Andrew Mason

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

3. Apr 2, 2014

### PhyIsOhSoHard

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?

4. Apr 2, 2014

### Andrew Mason

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

5. Apr 2, 2014

### PhyIsOhSoHard

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

6. Apr 2, 2014

### Andrew Mason

The direction of heat flow can be reversed. Connecting a heat pump would cause heat to flow the other direction. But that would require more than just an infinitesimal change in conditions. So, your guess is correct, it is an irreversible process.

AM