PowerPoint Presentation

Wk 7 LectureThermodynamics

Know the Understand the Zeroth Law of thermodynamics.Understand the First Law of thermodynamics.Apply the laws to solving problems.Explain thermodynamics to explain temperature and state changes in gasses2Objectives

The laws of thermodynamics help us to explain the changes that happen to gasses. They were developed during the industrial revolution by Ludwig Boltzmann and his peers. 3Zeroth Law

sheffield.ac.uk/international-collegeThe zeroth law of thermodynamics provides a simple definition of thermodynamic equilibrium.It is known that some property of an object (the pressure of a gas, the length of a metal rod, the electrical conductivity of a wire) can change when the object is heated or cooled.If two of these objects (e.g. two metal rods) are brought into physical contact there is initially a change in the property of both objects (in this case their length) but eventually, the change in the property stops.

4Zeroth Law

sheffield.ac.uk/international-collegeAt this point the objects are said to be in thermal (thermodynamic) equilibrium.Thermodynamic equilibrium leads to the large scale definition of temperature i.e. when two objects are in thermal equilibrium we say they have the same temperature.During the process of reaching thermal equilibrium, heat, which is a form of energy, is transferred between the objects.

5Zeroth Law

sheffield.ac.uk/international-collegeObjects are in thermal contact if heat can flow between them.

6Zeroth LawDefinition of heat:Heat is the energy transferred between objects because of a temperature difference.

sheffield.ac.uk/international-college7Zeroth LawThe zeroth law of thermodynamics:Two systems in thermal equilibrium with a third system are in thermal equilibrium with each other.In other words: All objects at the same temperature are in thermal equilibrium with each other if they are in contact.

sheffield.ac.uk/international-college8Zeroth Law

sheffield.ac.uk/international-college9Internal EnergyKinetic theory assumes that the particles in a body in any state are in constant motion and so possess kinetic energy. The particles also possess potential energy due to the separation between atoms/molecules.The internal energy U, of a body (or system group of bodies) is the sum of the kinetic and potential energies of the particles within it.

sheffield.ac.uk/international-college10Internal EnergyThe internal energy U of a system remains constant unless energy is transferred to or from it.U = energy input energy outputpositive U = energy gained negative U = energy lost

sheffield.ac.uk/international-college11Internal EnergyWhether the body be solid, liquid or a gas the energy supplied increases the kinetic energy and potential energy of the molecules and the temperature rises.However, the energy supplied during boiling or melting simply frees or separates the molecules, increasing their potential energy. Therefore, during boiling and melting, there is no change in the kinetic energy of the molecules so the temperature remains constant.

sheffield.ac.uk/international-college12Internal Energy

sheffield.ac.uk/international-college13Internal EnergyTboilSolidMeltingLiquidEvaporatingGasIncreasing KE and PEIncreasing PEIncreasing KE and PEIncreasing PEIncreasing KETmelt

sheffield.ac.uk/international-college14Internal EnergyThe internal energy i.e. temperature (assuming no change of phase takes place) of a system can be changed in two ways:1.Heating2.Doing work

sheffield.ac.uk/international-college15First Law of ThermodynamicsHeating is energy transfer due to temperature difference.

Energy will be transferred from A to B until the average kinetic energy of the particles in B is the same as that of those in A i.e. until their temperatures are the same.

sheffield.ac.uk/international-college16First Law of ThermodynamicsWork is energy transfer by the action of a force. In both of the above, the work done on the system causes an increase in internal energy and the temperature rises.For example hitting a metal block with a hammer or passing a current through a metal.

sheffield.ac.uk/international-college17First Law of ThermodynamicsAs already seen, the internal energy U of a system can only change by working or heating or both. This is the first law of thermodynamics.Stated mathematically, this law is: U = W + Q

sheffield.ac.uk/international-college18First Law of Thermodynamics

sheffield.ac.uk/international-college19First Law of Thermodynamics

sheffield.ac.uk/international-college20First Law of ThermodynamicsIsothermal change is a change that occurs with no change in temperature. Thus U = 0 An example of this is slowly stretching an elastic band. Work is done on the system by the stretching force but thermal energy is transferred from the system to the surroundings. In this case, W = -Q.

sheffield.ac.uk/international-college21First Law of ThermodynamicsAdiabatic change is one in which work is done quickly so there is no time for energy transfer by heating. Thus Q = 0 An example of this is stretching a rubber band very quickly. Work is done on the system by the stretching force but there is not enough time for thermal energy to be transferred. So Q = 0 and W = U. Thus the temperature of the band increases.

sheffield.ac.uk/international-college22Work and Gasses

sheffield.ac.uk/international-college23Work and Gasses

sheffield.ac.uk/international-college24Work and GassesNote that this equation is only true if pressure P is constant during the change i.e. process is isobaric.This work increases the internal energy of the gas and its temperature rises. An example of this is a bicycle pump getting hot as it is used to pump up a tyre.

sheffield.ac.uk/international-college25Work and GassesThe volume of the gas increases from V1 to V2The work done W is negative since it is done by the gasW = P V and is equal (but opposite in sign) to the area under the PV graph.

If the arrow on the isobar is in the opposite direction then work done is positive since it is done on the gas.

sheffield.ac.uk/international-college26Work and GassesIsothermal change The gas is being compressed.The shaded area is the work done ON the gas.If the arrow on the isotherm is in the opposite direction then the shaded area is the work done BY the gas.

sheffield.ac.uk/international-collegeWork and Gasses

27

Adiabatic change (no heat transfer) Q = 0If the gas is decreasing in volume the shaded area is the positive work done on the gas.The temperature of the gas increases.PT1T2V1V2

sheffield.ac.uk/international-collegeSecond Law of Thermodynamics

28The Second Law of Thermodynamics is simply the stating of a fact that should be obvious. No system is 100% efficient Every time energy is transferred some energy is dissipated from the system.

e.g. In a steam train some heat energy from the fire warms the surroundings rather than the water it is meant to boil.

29Summary

The Laws of ThermodynamicsZerothTwo systems in thermal equilibrium with a third system are in thermal equilibrium with each other.FirstU = W + QSecondNo system is 100% EfficientWork done by gasses

30Preparation for the Tutorial

You need to do the questions for the tutorial. They are called wk7 tutorial (Prep Questions)

All the required questions can be found on MOLE.