Physics

Sound

Solids

In solids, the particles are closely packed [see Figure 22(a)], There is a strong force of attraction between the particles of a solid which holds them together in fixed positions. Thus, the positions of particles in a solid are fixed. The particles of a solid only vibrate about their fixed positions. They cannot move from one position to another. If a solid is heated, its particles start to vibrate faster. The spaces (or distances) between the particles of a solid are very, very small. The particles of a solid have the minimum kinetic energy. Due to this, solids have the most orderly arrangement of particles.
We will now explain the properties of solids. A solid has a fixed shape because the particles of a solid are closely packed and their positions are fixed due to the strong forces of attraction between them. A solid has a fixed volume because the spaces between its particles are fixed. A solid cannot be compressed much because its particles are already very closely packed and there are hardly any spaces between them.

A solid has a high density because its particles are very dose together. A solid does not fill its container completely because its particles are held tightly by strong interparticle forces and hence cannot leave their positions to fill the whole container. Similarly, a solid does not flow (like a liquid does) because its particles are held very strongly and unable to leave their fixed positions.
We have just learnt that a solid has a fixed shape. Now, a rubber band changes its shape on stretching,then why do we call it a solid ? This can be explained as follows : A rubber band is considered a solid because it changes shape under the action of force (which stretches it), and when the force is removed the rubber band regains its original shape.like all other solids 'if rubber band is fetched with a large force,it breaks.
We will now discuss the case of salt and sugar.When sult and sugar are put in jars of different shapes,they take the shape of the jars, indicating that they do not have a fixed shape. So, if salt and sugar do not appear to have a fixed shape, then why do we call them solids ? Actually, this problem arises because salt and sugar have very tiny crystals. Salt and sugar are considered to be solids because the shapes of individual crystals of salt and sugar remain fixed, even when they are put in jars of different shapes.
We have also learnt that solids cannot be compressed much. Now, a sponge can be compressed easily,so why do we call sponge a solid ? This can be explained as follows : A sponge is considered to be a solid because its compressibility is due to the presence of minute pores in it which are filled with air. When we press the sponge, air is expelled from its holes making it highly compressible.

Notes

Sound - Notes

1. 3. Humidity of Air