Why low temperature copper bar is shorter than higher one?

[alpha2cen]

Everyone knows well high temperature copper is bigger than colder one.

We know this macroscopical phenomena very well.

And there is a parameter related to this phenomena such as thermal expansion factor.

But we don't know well about it at the molecular level.

Why copper is expanded when temperature is increased ?

 

 

[John Cuthber]

"But we don't know well about it at the molecular level."

Yes we do.

 

The simple answer is that at higher temperatures the electrons have more energy. So they can occupy energy levels further away from the nucleus.

That makes the atoms in the copper slightly bigger. Since the number of atoms in the copper bar is the same and the atoms get bigger the bar gets bigger.

[swansont]

Atoms in a lattice are similar to masses connected by springs. When you shake them (i.e. they get hotter), the amplitude of the vibration is larger. The whole ensemble gets bigger.

[confidenceguy]

you can also find more details about it on http://en.wikipedia.org/wiki/Soldering

[alpha2cen]

Are there any possibility about molecular attraction force is reduced?

There are many reason to be done like this.

Is the vibration important role on the solid state energy storage at high temperature?

Which percentage does the vibration energy contribute to?

[John Cuthber]

"Are there any possibility about molecular attraction force is reduced?"

Yes, that's why blacksmiths heat iron bars before hammering them into shape.

 

"Is the vibration important role on the solid state energy storage at high temperature?"

Yes

"Which percentage does the vibration energy contribute to? "

It depends on the system and the maths is pretty horrible.

http://en.wikipedia.org/wiki/Debye_model