⇒ The kinetic theory model of solids, liquids and gases assumes that particles are incompressible spheres
⇒ Solids have a close-packed, regular particle structure - the particles vibrate about fixed points
⇒ Liquids have a close-packed, random, irregular particle structure - the particles are free to move
⇒ Gases have a widely spaced, irregular particle structure - the particles are free to move
⇒ Thermal energy can be transferred from somewhere hot (at a high temperature) to somewhere cooler (at a lower temperature) by the processes of conduction, convection, radiation, and evaporation
⇒ Developing engines required an understanding of heat energy - this became known as thermodynamics
⇒ Thermodynamics deals with the macroscopic (large-scale) behaviour of a system, but it is complemented by the kinetic theory of matter, which deals with the microscopic, particle-scale behaviour of matter
⇒ Internal energy, U, is one of the most fundamental properties of thermodynamics - internal energy is the sum (sometimes called an ensemble in thermodynamics) of the randomly distributed kinetic energies and potential energies of the particles in a body
⇒ Consider a glass of water:
⇒ The kinetic energies of the particles depend on their temperature, and the potential energies depend on any intermolecular forces between the particles
⇒ The modern version of this law is stated as follows: the increase in internal energy of a system is equal to the heat added to the system minus the work done by the system
⇒ In terms of symbols, this can be written:
⇒ Where ΔU is the increae in internal energy of the system (usually a gas), ΔQ is the thermal energy added to the system, and ΔW is the work done by the system
⇒ When a gas expands, it exerts a force on the surroundings, causing them to move - the gas does work on the surroundings
⇒ We can use the first law of thermodynamics to determine the work done, ΔW, by an expanding gas at constant temperature (called an isothermal change)
⇒ Consider a gas enclosed in a cylinder by a frictionless piston
⇒ The gas of volume W exerts a pressure p on the walls of the cylinder. This in turn exerts a force on the frictionless piston of area A, where F = pA
⇒ This causes an increase in the volume, ΔV
⇒ The gas does work, and so ΔW is positive
⇒ The force on the piston moves it through a distance, Δx, such that: