What you should know

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

Introduction

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

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

Thermodynamics

Thermodynamics

Consider a glass of water:

  • The water particles have two types of energy associated with their movement (the faster they move, or vibrate, or rotate, the higher their kinetic energy) and potential energy associated with any forces or interactions between the particles (such as any electrostatic attraction or repulsion)

The kinetic energies of the particles depend on their temperature, and the potential energies depend on any intermolecular forces between the particles

  • For ideal gases, in which there are no intermolecular forces, the internal energy is dependent on only the kinetic energies

The first law of thermodynamics

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:

Thermodynamics

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

Work done by an expanding gas

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

Thermodynamics

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

  • We assume that ΔV is very small and that the force moves the piston at a slow but steady rate such that the external force exerted on the piston is equal to the force exerted by the pressure p of the gas in the culinder
  • This effectively makes the pressure exerted by the gas constant during the expansion

The gas does work, and so ΔW is positive

The force on the piston moves it through a distance, Δx, such that:

Thermodynamics