Thermal Physics

• Temperature is a measure of how hot or cold something is.
• The light scattered from the smoke particles can be seen.
• Smoke particles move in a random manner with random speeds in random directions.
• The motion of the smoke particles keeps changing, they jiggle.
• Larger smoke particles move more slowly on average and their path lengths are shorter.
• At higher temperatures, the particles have a higher average speed.
• Smoke particles are undergoing collisions with air molecules.
• These collisions tend to average out with large particles.
• The air molecules must also be moving with random speeds in random directions between collisions.
• Raising the temperature increases the average kinetic energy of the air molecules.
• A gas is made up of molecules moving randomly.
• In order to understand the nature of heat and temperature, it is necessary to appreciate the fact that matter consists of moving particles (atoms or molecules) which can interact more or less strongly with one another.
• The motion of the particles is increased by raising the temperature.
• Conversely, the motion of the particles is reduced by lowering the temperature, until, at the absolute zero (0 K), the motion of the particles ceases altogether.
• Because the particles are in motion, they will have kinetic energy.
• The particles will not all have the same energy, and the energy of the particles is constantly changing as they undergo changes in speed.
• For a given sample of matter, we can only talk about the average kinetic energy of the particles.
• Temperature is a measure of the average kinetic energy of the particles.
• Heat is the energy that will flow from one object to another when they are at different temperatures.
• In the absence of frictional forces, when two bodies have the same temperature, no heat is transferred between them when they are brought into contact.
• If they are not at the same temperature, heat will flow from the hotter to the colder body, never the other way round!
• An object may acquire heat by mechanical means: friction between surfaces, mechanical shock, compression (in gases), stirring (in liquids), an object experiencing a fall.
• James Prescott Joule determined the amount of work required to raise the temperature of 1 kg of water by 1 ºC - the so-called mechanical equivalent of heat.
• The mechanical equivalent of heat is the work done in raising the temperature of 1 kg of water by 1 ºC (or 1K).
• Heat is measured in joules, J.
• Before the widespread adoption of the SI system, heat was measured in units called calories.
• The Specific Heat Capacity of a material is the heat energy required to raise the temperature of 1 kg of the material by 10C.
• The concept of an ideal gas is embodied in the kinetic theory, i.e., an ideal gas will behave as expected by the basic assumptions of the Kinetic Theory.
• Real gases are those in the world around us - they differ from ideal gases in some very important ways:
• In real gases, the molecules' dimensions are not negligible compared to the total volume in which they move.
• The size of the molecules becomes increasingly significant as the average distance between them is reduced.
• Collisions which are undergone by the molecules in real gases are not perfectly elastic, and the molecules of real gases do have interactions between them, either attractive or repulsive forces.
• These molecular interactions become all the more important as the average kinetic energy of the molecules decreases and/or the distance between them is reduced, such as occurs with low temperatures and/or high pressures.
• Helium, under moderate conditions of temperature and pressure, behaves quite closely as if it were an ideal gas.
• For the purpose of calculations, gases are assumed to conform to ideal behaviour.
• The volume of a gas depends both on the pressure and temperature at which it is measured.
• Boyle's law states that at constant temperature, the volume occupied by a fixed amount of gas is inversely proportional to its pressure.
• Boyle's law may be stated as follows: "At constant temperature, the volume occupied by a fixed amount of gas is inversely proportional to its pressure."
• Experimental results from a typical Boyle's law experiment show that as the pressure on the sample increases, its volume decreases.
• The data in the table is plotted on a graph, which is a hyperbola.