Cause of Pressure exerted by a gas // Kinetic Theory of Gases скачать в хорошем качестве

Cause of Pressure exerted by a gas // Kinetic Theory of Gases

Chapter: Kinetic Theory of Gases : What is the cause of Pressure exerted by a gas ... The rapid motion and collisions of molecules with the walls of the container causes pressure (force on a unit area). Pressure is proportional to the number of molecular collisions and the force of the collisions in a particular area. The more collisions of gas molecules with the walls, the higher the pressure. Pressure is a stress. It is a scalar given by the magnitude of the force per unit area. In a gas, it is the force per unit area exerted by the change of momentum of the molecules impinging on the surface. We know from Newton's second law that a net resultant force will cause a change of momentum in a body, and that the rate of change of momentum is equal to the applied force. It is a vector relationship, so that even if the magnitude of the momentum is unchanged, a change in the direction of motion requires a resultant force. The impact of a gas molecule on a solid surface is an elastic impact so that its momentum magnitude and energy are conserved. However, because its direction of motion changes on impact, a resultant force must have been exerted by the solid surface on the gas molecule. Conversely, an equal but opposite force was exerted by the gas molecule on the solid surface. If we consider a very small area of the surface of the solid, so that over a short time interval \Delta t very few molecules collide with the solid over this area, then the force exerted by the gas molecules will vary sharply with time. When we consider a sufficiently large area, so that the number of collisions on the surface during the interval \Delta t is large, then the average force that acts on the solid surface by the molecules is constant. It is this force, acting on the surface of the solid, that we call the force due to pressure.