IB Physics Topic 3 – Introduction to Thermo physics Mr. Jean

28/10/2015 Thermal Physics

14-1 Heat As Energy Transfer We often speak of heat as though it were a material that flows from one object to another; it is not. Rather, it is a form of energy. Unit of heat: calorie (cal) 1 cal is the amount of heat necessary to raise the temperature of 1 g of water by 1 Celsius degree. Don’t be fooled – the calories on our food labels are really kilocalories (kcal or Calories), the heat necessary to raise 1 kg of water by 1 Celsius degree.

14-1 Heat As Energy Transfer If heat is a form of energy, it ought to be possible to equate it to other forms. The experiment below found the mechanical equivalent of heat by using the falling weight to heat the water:

14-1 Heat As Energy Transfer Definition of heat: Heat is energy transferred from one object to another because of a difference in temperature. Remember that the temperature of a gas is a measure of the kinetic energy of its molecules.

14-2 Internal Energy The sum total of all the energy of all the molecules in a substance is its internal (or thermal) energy. Temperature: measures molecules’ average kinetic energy Internal energy: total energy of all molecules Heat: transfer of energy due to difference in temperature

28/10/2015 Heat flows from hot to cold Net energy flow stops when their temperatures are the same i.e. They are in thermal equilibrium T c = T k T k = T c +273

28/10/2015 The Mole and Molar mass One mole of carbon contains 6x10 23 atoms One mole of green atoms contains 6x10 23 atoms One mole of “anything” contains 6x10 23 (the Avogadro constant N A ) number of atoms (or molecules) Definition of the mole: the amount of substance containing as many elementary particles as there are in 12 g of Carbon-12 Molar mass = mass of one mole

Temperature, Internal Energy and Thermal energy (Heat) Temperature: A measure of the average random kinetic energy per molecule. The internal energy of a substance is the sum of the molecular kinetic and potential energies. Kinetic energy arises from the translational and rotational motions Potential energy arises from the forces between the molecules The term heat represents energy transfer due to a temperature difference resulting in an increase in the kinetic energy of the molecules.

Thermal (HEAT) Capacity Heat capacity =  Q /  T in JK -1 ◦ Q = the change in thermal energy in joules ◦ T = the change in temperature in Kelvin Defined as the amount of energy needed to change the temperature of a body by unit temperature. Applies to a specific BODY Specific Heat Capacity Specific Heat Capacity =  Q / (m  T) in J kg -1 K - where m is the MASS of the material Heat Capacity = m x Specific Heat Capacity

14-2 Internal Energy Internal energy of an ideal (atomic) gas: But since we know the average kinetic energy in terms of the temperature, we can write: (14-1)

Microscopic Characteristics

14-2 Internal Energy If the gas is molecular rather than atomic, rotational and vibrational kinetic energy needs to be taken into account as well.

14-3 Specific Heat The amount of heat required to change the temperature of a material is proportional to the mass and to the temperature change: (14-2) The specific heat, c, is characteristic of the material. Some values are listed at left.

14-3 Specific Heat Specific heats of gases are more complicated, and are generally measured at constant pressure (c P ) or constant volume (c V ). Some sample values:

14-4 Calorimetry – Solving Problems Closed system: no mass enters or leaves, but energy may be exchanged Open system: mass may transfer as well Isolated system: closed system where no energy in any form is transferred For an isolated system, Energy out of one part = energy into another part Or: heat lost = heat gained

14-4 Calorimetry – Solving Problems The instrument to the left is a calorimeter, which makes quantitative measurements of heat exchange. A sample is heated to a well-measured high temperature, plunged into the water, and the equilibrium temperature measured. This gives the specific heat of the sample.

14-4 Calorimetry – Solving Problems Another type of calorimeter is called a bomb calorimeter; it measures the thermal energy released when a substance burns. This is the way the Caloric content of foods is measured.

28/10/2015 Heating ice Temp/ O C Time/s This flat line shows where energy is being used to break bonds – this has to be done during melting. The amount of energy needed to turn 1kg of a solid into a liquid is called the Specific Latent Heat of Fusion L. This flat line shows where energy is being used to break the temporary bonds for boiling. The amount of energy needed to turn 1kg of a liquid into a gas is called the Specific Latent Heat of Vaporisation L.

14-5 Latent Heat Energy is required for a material to change phase, even though its temperature is not changing.

14-5 Latent Heat Heat of fusion, L F : heat required to change 1.0 kg of material from solid to liquid Heat of vaporization, L V : heat required to change 1.0 kg of material from liquid to vapor

14-5 Latent Heat The total heat required for a phase change depends on the total mass and the latent heat: (14-3) Problem Solving: Calorimetry 1. Is the system isolated? Are all significant sources of energy transfer known or calculable? 2. Apply conservation of energy. 3. If no phase changes occur, the heat transferred will depend on the mass, specific heat, and temperature change.

14-5 Latent Heat 4. If there are, or may be, phase changes, terms that depend on the mass and the latent heat may also be present. Determine or estimate what phase the final system will be in. 5. Make sure that each term is in the right place and that all the temperature changes are positive. 6. There is only one final temperature when the system reaches equilibrium. 7. Solve.

14-5 Latent Heat The latent heat of vaporization is relevant for evaporation as well as boiling. The heat of vaporization of water rises slightly as the temperature decreases. On a molecular level, the heat added during a change of state does not go to increasing the kinetic energy of individual molecules, but rather to break the close bonds between them so the next phase can occur.

28/10/2015 Latent heat ( changes in potential energy) Energy = mass x specific latent heat

28/10/2015 Evaporation and Boiling Boiling occurs at a fixed temperature Evaporation occurs at any temperature Boiling happens throughout the body of the liquid Evaporation only happens at the surface of the liquid. The average kinetic energy of the remaining particles drops The temperature of the remaining liquid is lowered

28/10/2015 Assumptions of the kinetic theory of gases 1.Molecules behave as if they were hard, smooth, elastic spheres. (i.e. the collisions are perfectly elastic) 2.Molecules are in continuous rapid, random motion. 3.The average kinetic energy of the molecules is proportional to the absolute temperature of the gas. 4.The molecules do not exert any appreciable attraction on each other. 5.The volume of the molecules is infinitesimal when compared with the volume of the gas. 6.The time spent in collisions is small compared with the time between collisions. Because the collisions are perfectly elastic there is no loss of KE as a result of the collisions

Pressure = Force / Area Pressure can be explained by the collisions with the sides of the container If the temperature increases, the average KE of the particles increases The increase in velocity of the particles leads to a greater rate of collisions and hence the pressure of the gas increases as the collisions with the side have increased. When the volume of a gas decreases collisions are more frequent with the sides of the container leading to an increase in pressure and/or temperature.