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

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• IB PhysicsTopic 3 Introduction to Thermo physics

Mr. Jean

• *Thermal Physics

• 14-1 Heat As Energy TransferWe 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.Dont 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 TransferIf 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 TransferDefinition 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 EnergyThe sum total of all the energy of all the molecules in a substance is its internal (or thermal) energy.Temperature: measures molecules average kinetic energyInternal energy: total energy of all moleculesHeat: transfer of energy due to difference in temperature

• *Heat flows from hot to coldNet energy flow stops when their temperatures are the samei.e. They are in thermal equilibriumTc = Tk - 273 Tk = Tc +273

• *The Mole and Molar massOne mole of carboncontains 6x1023 atomsOne mole of green atoms contains 6x1023 atomsOne mole of anything contains 6x1023 (the Avogadro constant NA) 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-12Molar 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 motionsPotential energy arises from the forces between the moleculesThe term heat represents energy transfer due to a temperature difference resulting in an increase in the kinetic energy of the molecules.

• Thermal (HEAT) CapacityHeat capacity = Q / T in JK-1

Q = the change in thermal energy in joulesT = 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 BODYSpecific Heat CapacitySpecific Heat Capacity = Q / (mT) in J kg -1 K-where m is the MASS of the materialHeat Capacity = m x Specific Heat Capacity

• 14-2 Internal EnergyInternal 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

Characteristics

Solid

Liquid

Gas

KE

Vibrational

Vibrational

Rotational

Some Translational

Mostly Translational

Higher Rotational

Higher Vibrational

PE

High

Higher

Highest

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

• 14-3 Specific HeatThe 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 HeatSpecific heats of gases are more complicated, and are generally measured at constant pressure (cP) or constant volume (cV).Some sample values:

• 14-4 Calorimetry Solving ProblemsClosed system: no mass enters or leaves, but energy may be exchangedOpen system: mass may transfer as wellIsolated system: closed system where no energy in any form is transferredFor an isolated system,Energy out of one part = energy into another partOr: heat lost = heat gained

• 14-4 Calorimetry Solving ProblemsThe 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 ProblemsAnother 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.

• *Heating iceTemp/OCTime/s

150100500-50

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

• 14-5 Latent HeatHeat of fusion, LF: heat required to change 1.0 kg of material from solid to liquidHeat of vaporization, LV: heat required to change 1.0kg of material from liquid to vapor

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

• 14-5 Latent Heat4. 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 HeatThe 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.

• *Latent heat (changes in potential energy)Energy = mass x specific latent heat

• *Evaporation and BoilingBoiling occurs at a fixed temperatureEvaporation occurs at any temperatureBoiling happens throughout the body of the liquidEvaporation only happens at the surface of the liquid.The average kinetic energy of the remaining particles dropsThe temperature of the remaining liquid is lowered

• *Assumptions of the kinetic theory of gasesMolecules behave as if they were hard, smooth, elastic spheres. (i.e. the collisions are perfectly elastic)Molecules are in continuous rapid, random motion.The average kinetic energy of the molecules is proportional to the absolute temperature of the gas.The molecules do not exert any appreciable attraction on each other.The volume of the molecules is infinitesimal when compared with the volume of the gas.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 / AreaPressure can be explained by the collisions with the sides of the containerIf the temperature increases, the average KE of the particles increasesThe 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.