The Laws of Thermodynamics - Assignment Example

Name: Course: College: Course Tutor: Date: 28th March 2008 1.) Define the four laws of thermodynamics using words, diagrams and equations where appropriate. (8 marks) Thermodynamics is a field which studies the way energy behaves in natural systems, from this there are several Laws that explain some of the fundamental truths that are existent in our universe. Zeroth law of thermodynamics- this law states that once two thermodynamic systems are in equilibrium with a third one, then they are all in equilibrium to each other. 1st Law of thermodynamics- also the law of conservation of energy this law states that energy can undergo change from one state to another but energy can neither be created nor destroyed. This means that the energy found in the universe remains constant only changing from one state to another. 2nd Law of thermodynamics- states that “if there is absolutely no energy that leaves a system during energy exchanges then the potential energy of the state will be less that of the initial state.” This simply implies that energy systems have the tendency of increasing their entropy. 3rd Law of thermodynamics- this law sates that if all kinetic energy (thermal motion) of molecules was removed then there would occur a state of absolute zero. This state resultant temperature is Zero Kelvin. 2) What is entropy? This is a measure of disarray or randomness of energy on a matter system. When ice melts, water molecules move faster due to the change of the state to being more free and flexible. In this case entropy is higher than the way it was when it was in ice form. 3) Calculate ΔS for the following reaction, using the information in a Table of Thermochemical Data, and state whether entropy increases (becomes more random) or decreases (becomes less random)? Based on entropy changes, do you predict a spontaneous reaction? 2 NO(g) + O2(g) →N2O4(g) (6 marks) 4) These questions test your understanding of temperature measurements and temperature scales. i) What is absolute zero on the Kelvin, this is a state of matter where the resultant temperature is Zero (0) in Kelvin, -273.15 in Celsius, -459.67 in Fahrenheit and zero (0) in Rankine. ii) The boiling point of water if 100°C what is this in Kelvin? It is 373.1339 Kelvin’s iii) The temperature of a system rises by 30°C during a heating process. Express this rise in temperature in Kelvin. Since 1 Kelvin is equivalent to – 272.15 then 30°C is equivalent to 303.15 Kelvin. iv) The temperature of a system rises by 60°F during a heating process. Express this rise in temperature in R, K and °C. (4 marks) 60°F this is equivalent to: 15.55556 °C 288.70556K ([°R] = [K] × 9⁄5)=519.67R 5) How is work related to equilibrium? (1 mark) 6) Give examples of equilibrium state, steady state and uniform. (3 marks) Equilibrium state- for an object to be at equilibrium, it has to be at rest (not moving) or moving at uniform velocity. An example is that of a book at rest on a table. Uniform equilibrium The above figure shows uniform equilibrium Steady Equilibrium This figure is for a steady state of equilibrium 7) State whether the following are open or closed systems, give reasons for your answer. i) Rechargeable battery, Closed system, this is because the battery is relatively self contained. ii) Household refrigerator, Open system, this is because the refrigerator has a relation or has an interaction with its environment. iii) Radiator. (6 marks) 8) What is the difference between a gas, a liquid and a solid? (2 marks) Gas molecules are very loose and gas never has a specific volume or shape. Liquid have a definite volume but not shape and its molecules are relatively close to one another Solids have their molecules very close to one another and have both definite volume and shape 9) What does thermodynamics tell us with regards to heat transfer? (1 mark) Thermodynamics tells us that heat can not be transferred from colder to hotter bodies. 10) Explain the difference between internal energy (u) and enthalpy (h). (1 mark) Internal energy (h) refers to the energy of molecules which is internal to the very substance while enthalpy is the measure the content of heat in a chemical or physical system. 11) The mass flow rate is 4kg/s, the heat of combustion for C3H8 is 46450kJ/kg. Determine the heat release rate. (2 marks) 12) What is Fourier’s Law? It is also known as the law of heat conduction and it states that “the heat transfer total rate through a material is proportional to the negative gradient to the temperature and to the areas at right angles, to that gradient which through it, the heat is flowing.” What is thermal conductivity? Thermal conductivity is one characteristic of heat of a material that shows materials ability to conduct heat. Compare the values of thermal conductivity of metals, insulating materials and gases. Thermal conductivity in most metals tracks electrical conductivity due to the moving valence electrons transfer, this does not only apply in electric current but also in conducting heat energy thus metals have a relatively high thermal conductivity. The functionality of many insulating materials is by having gas-filled pockets which prevent large-scale convection they therefore have a low thermal conductivity. Characteristically gases have a high thermal conductivity. 13) Explain the Stefen-Boltzman Law. The law is also known as the Stefan’s law states that “the energy in total of a black material radiates per unit area and in unit time is directly proportional to the fourth body’s absolute zero temperature which is also known as thermodynamic temperature.” What is emissivity? This is the ratio of the energy that is radiated on a particular material to that energy that is radiated by a black body; this should take place at the same temperature. What is a blackbody? A black body is an object that absorbs electromagnetic radiation which falls on it. 14) Explain the Newton’s Law of Cooling. The law states that the surface temperature of a given object changes at a rate proportional to its relative temperature i.e. the temperature within itself and that of the surrounding environment. What is the heat transfer coefficient? This is the proportionality coefficient between the heat flux and the thermodynamic force responsible for the flow of heat. What is the Nusselt number? This is the ratio of the convective heat transfer to conductive heat transfer across a boundary in a heat transfer surface. The following are types of convection: i.) Natural or the free convection ii.) Forced convection 15) Define heat of combustion, The heat of combustion is the amount of heat energy that is released per unit volume or unit mass of a substance at the event of completely burning the substance. Heat release rate and combustion reaction giving appropriate equations Explain the different types of combustion and definitions of the following: Specific heat capacity, Specific heat is the other name for this and it is the measure of the amount f heat energy required to raise the temperature of a unit quantity of any substance and by a certain temperature level. Latent heat, this sis the mount of that a chemical substance absorbs in the event of change of heat i.e. liquid, solid or gas Calorimetry, this is the quantitative science of measuring heat during physical reactions or chemical reaction Combustion temperature, this is the temperature of a substance in the combustion chamber in degree Kelvin Chemical equilibrium, this is the state in which chemical activities of substance in a reaction have no change over a given time. 16) What is the efficiency of an engine that produces 150J of work from 212J of energy? 17) A Styrofoam cup (of negligible heat capacity) contains 150g of water at 10°C. If you add 100g of water at a temperature of 85°C what is the final temperature of the mixture after it has been thoroughly mixed? (2 marks) 19) Define flame? A flame is the emanating light that is visible with our eyes and which is caused by reactions, e.g. combustion. Describe the different types of flames and whether they are laminar or turbulent? Premixed flame e.g. Bunsen flame – Laminar Candle flame – Laminar Non-premixed flame e.g. SI engine flame – Turbulent 20) Define fluid. A fluid is a substance that continually flows under an applied shear stress. Viscosity of a fluid: Viscosity is the measure of the resistance of fluid to flow; it is simply the thickness of a particular liquid. 21) Compressibility or incompressibility of air as a fluid? How about water? Explain your answer. Air is a compressible fluid, from the definition of compressible fluids; they are the matter where a change in pressure causes change in their fluid density. 23) A box is pushed 5m across a room with a force of 30N. Show the work done and energy is used? Work done is the force applied times the distance moved; therefore: Work done W= 5m x 30 N= 150Joules 24) A piece of aluminium siding is 12.45 meters long on a cold winter's day, -18°C. How much longer is it on a very hot summer's day, 37°C? (1 mark) 25) Define boundary layers. Boundary layer is the thin layer of fluid created near the surface where the velocity changes from zero on the surface of the fluid to the free stream value away from the surface. Draw a velocity profile for a fluid in a pipe showing both laminar and turbulent flow. (6 marks) BOUNDARY Free Stream Boundary Layer Unsteady Works Cited Pidwirny, M. "Laws of Thermodynamics." Fundamentals of Physical Geography. 2nd Edition. 2006. retreived on 28th March 2009 from http://www.physicalgeography.net/fundamentals/6e.html Read More