# Translations of Encyclopedia about Physics

## Mechanics of Liquids and Gases

Solid objects, liquids and gases differ in the bonds they have between their particles.

In solid objects, these bonds are very strong, where the particles within the object are not able to move and are only able to vibrate around their central position.

With liquids, these bonds are looser, for which reason these substances are fluid. Even so, they still have a firmer structure than do gases. The bond of particles in liquids is called cohesion. This creates surface tension on the water which, for example, pulls small amounts of water into round shapes - drops. The surface of these drops are then flattened due to the force of gravity. With gases, the movement of particles is greater than the bonding forces working on them, for which reason they fill all enclosed areas they happen to be located in.

When adding or taking away energy (heating or cooling), a change in its state may take place.

The attractive force between a liquid and the wall of some container is called adhesive force or adhesion, which also refers, in general, to the force that exists between the molecules of a liquid and a solid substance. The joint effect of cohesive and adhesive forces creates the capillary effect.

Capillary effect means that a liquid which behaves adhesively or cohesively is drawn towards porous substances, the most obvious examples of which are sugar cubes or a mushroom.

An object submersed in a liquid is subjected by a force called buoyancy. According to Archimed’s law, the buoyancy force applied to an object is equal to the lift of the liquid pressing onto the object. Because both gravitational and buoyancy forces are applied to the object, if both forces are equal, the object floats. The pressure in liquids increases with increasing depth. Also, the higher the consistency or density of the water, the higher is its pressure. In this case, the pressure is applied in all directions, which means that the pressure on a sunken object is not applied only on the bottom end of the object but rather from all sides.

In gases, particles move constantly in an unorganised manner, although they gain a homogeneous density if contained within some vessel. Gases are also able to combine together, a phenomenon referred to as diffusion.

When applying heat to gases, the particles within them increase in speed and the collision of the particles on the walls of a vessel create pressure within the closed vessel.

The relation between the volume of a gas (V), its pressure (P) and its temperature (T) can be explained by the equation of state for ideal gases: P.V = K..T, where K is a constant which relates to the type of gas.

This condition is covered under three laws formulated long ago:

Boyle’s - Mariotte’s Law

The volume of a given amount of gas at a constant temperature is indirectly proportional to its pressure, and the product of both quantities remains constant.

Relation Between Quantities - Temperature

With a constant temperature, the pressure of a certain amount of gas increases directly proportionally to its temperature.

Gay-Lussac’s Law

The volume of a certain amount of gas is, under constant temperature, directly proportional to its temperature.

Our weather creates areas of high and low pressure, which in turn creates wind currents. In this way, our weather can serve as a graphical example for the mechanics of gases.

The characteristics of gases and liquids are important to know for many areas, such as for the braking mechanisms of automobiles, or for hot air balloons.

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