# Change of state and potential energy

1. Jul 24, 2015

### steve oliver

A book said that the PE of molecules increase during melting of ice but shouldnt it be decrease since the distance between molecules increase ?Isnt the force between the molecules determine the PE?

2. Jul 24, 2015

### H Smith 94

You're right about the force $F$ and potential energy $U$ being interlinked. They have the following relation

$$U = -\frac{dF}{dr} \approx -\frac{\Delta F}{\Delta r}$$

See the below figure.

[source]

The "rest" state of the molecules is at $r_0$, where potential energy in minimum. If the distance increases from $r_0$, it can be seen from the above graph that the potential energy will also increase.

3. Jul 24, 2015

### BvU

Well, if the distance decreases from $r_0$, doesn't the potential energy increase also ?

What does the graph represent in the context of melting ice ?

see other threads on melting water, e.g. this one
Wikipedia on ice and on properties of water is good, too.

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