Thermodynamics is a branch of physics which deals with the energy and work of a system. It was born in the 19th century as scientists were first discovering how to build and operate steam engines. Thermodynamics deals only with the large scale response of a system which we can observe and measure in experiments. Small scale gas interactions are described by the kinetic theory of gases. The methods complement each other; some principles are more easily understood in terms of thermodynamics and some principles are more easily explained by kinetic theory. And since we spoke about thermodynamics sometime back, now we will return to it cause i found more cool stuff on it online, specifically the Kinetic Theory Of Gases.
In a molecular model, small molecules relative to distance apart; Molecules are also in constant random motion; There are frequent collisions between molecules; Ordered motion is superimposed on random motion. Gases can be studied by considering the small scale action of individual molecules or by considering the large scale action of the gas as a whole. We can directly measure, or sense, the large-scale action of the gas. But to study the action of the molecules, we must use a theoretical model. The model, called the kinetic theory of gases, assumes that the molecules are very small relative to the distance between molecules. The molecules are in constant, random motion and frequently collide with each other and with the walls of any container. Similar to my little balls in a jar example in the last post.
In a more simplified scientific explanation from NASA: The individual molecules possess the standard physical properties of mass, momentum, and energy. The density of a gas is simply the sum of the mass of the molecules divided by the volume which the gas occupies. The pressure of a gas is a measure of the linear momentum of the molecules. As the gas molecules collide with the walls of a container, the molecules impart momentum to the walls, producing a force that can be measured. The force divided by the area is defined to be the pressure. The temperature of a gas is a measure of the mean kinetic energy of the gas. The molecules are in constant random motion, and there is an energy (mass x square of the velocity) associated with that motion. The higher the temperature, the greater the motion.
In a solid, the location of the molecules relative to each other remains almost constant. But in a gas, the molecules can move around and interact with each other and with their surroundings in different ways. As mentioned above, there is always a random component of molecular motion. The entire fluid can be made to move as well in an ordered motion (flow). The ordered motion is superimposed, or added to, the normal random motion of the molecules. At the molecular level, there is no distinction between the random component and the ordered component. We measure the pressure produced by the random component as the static pressure. The pressure produced by the ordered motion is called dynamic pressure. And Bernoulli's equation tells us that the sum of the static and dynamic pressure is the total pressure which we can also measure.
To make this even cuter, here is a poem about the wind:
WHO HAS SEEN THE WIND ?
BY: CHRISTINA ROSSETTI
Who has seen the wind?
Neither I nor you:
But when the leaves hang trembling
The wind is passing thro'
Who has seen the wind?
Neither you nor I:
But when the trees bow down their heads
The wind is passing by.
