Why Do Solids Expand When Heated?

[Amith2006]

Sir,
Why does solid expand on heating?
The answer to this question given in my book is “The potential energy curve is asymmetric about the equilibrium distance between neighbouring atoms. Is it right?
I think solid expands due to increase in kinetic energy of atoms.

 

[Gokul43201]

Sir,
Why does solid expand on heating?
The answer to this question given in my book is “The potential energy curve is asymmetric about the equilibrium distance between neighbouring atoms. Is it right?

Yes it is. The most eggregious flaw of the harmonic approximation to the treatment of solids is that it leads to zero thermal coefficient of expansion. Without anharmonicity (or asymmetry, or deviation from purely quadratic interaction terms), you can't solve this problem.

I think solid expands due to increase in kinetic energy of atoms.

The classical oscillator turning points are separated by distances smaller than typical interatomic spacings. In the harmonic approximation, it's possible to make atoms oscillate much faster (irrespective of phase differences) about their mean positions, without having to change the distance between these mean positions. Alternatively, one can see that with a symmetric potential, no matter what the energy (and hence turning points), the mean position is always that of the minimum.

Now there are a few solids with virtually zero thermal expansion coefficient. These are solids where the interaction between atoms is almost exactly parabolic over a significant range of energies.

Edit : Look here for a picture - http://www.mse.uiuc.edu/info/mse182/t79.html

 

[kyle8921]

This is because when solids are heated, the atoms and molecules within the solid gain kinetic energy, causing them to vibrate more rapidly. As a result, the atoms and molecules move further apart, leading to an increase in volume and expansion of the solid.

However, the answer given in your book is also correct. The potential energy curve refers to the relationship between the distance between atoms and the potential energy of the solid. When a solid is heated, the atoms gain kinetic energy and move further apart, causing the potential energy curve to shift. This shift in the curve results in an increase in the equilibrium distance between atoms, leading to expansion of the solid.

In summary, both explanations are correct and can be used to understand why solids expand when heated. It is important to consider both the increase in kinetic energy and the shifting of the potential energy curve when explaining this phenomenon.