Energy Transfer and Thermodynamics - Essay Example

Question Laws of thermodynamics: Zeroth law: This is a law of thermodynamics that s that thermo equilibrium of a system is achieved only whenthe temperature of the system does not change over time. This law also states that given two systems that are at thermo equilibrium with another system then both of them are at thermo equilibrium with each other. First law The first law states that energy cannot be created or destroyed but can be changed from one form of energy into another form of energy, there are a number of form of energy such as electric, heat and light, example we can transform electric energy into light energy using an electric bulb and electric energy is transformed into light and heat energy, therefore it is evident that we cannot destroy energy but we can transform energy into other forms. Law of entropy: This is the second law of thermodynamics that was coined by Clausius and Thomson, entropy is denoted as S and this law states that the entropy of a given system that is not at equilibrium will tend to increase over time, and that at equilibrium maximum entropy is achieved. Third law This law was formulated by Walther Nernst and states that the entropy of a given system is zero when temperatures are at absolute zero. Question 2: Entropy definition: Entropy is the energy of a system that is not available to do work, Water molecules: Water molecules in ice vibrate and do not move freely, however water molecules can freely move when ice melts. Entropy When temperatures are lowered and the water becomes ice, molecules of water can only vibrate and entropy declines, therefore when we melt ice entropy increases. Question 3: Chemical reaction: 2 NO(g) + O2(g) N2O4(g) Using Hess law we can determine the enthalpy change S, this is determined by subtracting the heat of formation of the product minus that of the reactants: In this case reactants are NO heat of formation = 90.29 and O = 0, N2O4 = -35.05 -35.05 -90.29 = -125.34, therefore from the above it si evident that enthalpy declines, there is a spontanueos reaction where the reactant releases energy forming a more stable compound. Question 4: A. absolute zero Absolute zero is 00Kelvin, 273.150Celsius, 459.670Fahrenheit and 00Rankine B. 100C Kelvin Kelvin = Celsius + 273.15 Kelvin = 373.150R C. rise of 30Celsius in Kelvin: Assume increase from 0Celsius to 30Celsius Kelvin = 30 + 273.15 = 303.15 Kelvin Kelvin = 0 + 273.15 = 273.15 =30Kelvin d. rises by 60F in R, K and C Rankine: Assume increase from -60 to 0 F Rankine = Fahrenheit + 459.67 R = -60 + 459.67 = 399.67 R = 0 + 459.67 = 459.67 = 600Rankine Kelvin: Kelvin = (Fahrenheit + 459.67) X (5/9) Kelvin = (-60 + 459.67) X 5/9 = 222.04 Kelvin = (0 + 459.67) X 5/9 = 255.37 = 33.330 Kelvin Celsius: Celsius = (Fahrenheit 32) X (5/9) Celsius = (0 32) X 5/9 = - 17.78 Celsius = (-60 32) X 5/9 = - 51.11 = 33.330C Question.5 Work and equilibrium: Work and equilibrium are related, for example when we consider two systems at equilibrium and some work is done on one of the object and temperature increases then the systems are no longer at equilibrium. Question.6 Equilibrium: hot water in a cup and the cup is at the same temperature as the water Stable state: filling a birth tab with water and yet draining the tab and therefore water will flow in and out and therefore the birth tab system is in a stable state. Uniform state: The universe is said to be uniform according to the second law of thermodynamics Question.7 Rechargeable battery: open, because it exchanges energy and matter with the environment and matter and this is why battery water is required. Household refrigerator: it is closed because it only exchanges energy with the environment but no matter is exchanged. Radiator: open water is added to the system after period of time. Question.8 Difference between a gas, a liquid and a solid In this we use water to explain the difference between gas, liquid and solid, for a solid which is ice molecules vibrate and do not move freely and that ice has a distinct shape, when we melt ice we get liquid water and the molecules move freely, liquids do not have a distinct shape and will take the shape of the container. For gases molecules move freely and gases do not have a distinct shape, gases are compressible meaning that their density varies. Question.9 Thermodynamics and heat transfer: According to the various laws it is evident that the rate of heat transfer depends on temperature difference. Question.10 Internal energy (u): It is the kinetic energy possessed by molecules of a system Enthalpy (h): Enthalpy is the thermo potential of a closed system Question.11 Flow rate = 4kg/s, Heat of combustion = 46450kJ/kg Heat release rate = 46450kJ/kg X 4kg/s =185800 Kilo Joules per second Question.12 Fourier's Law: This theory depicts that the rate of heat transfer through a material is proportional to the temperature difference Thermal conductivity: Thermo conductivity is the ability of a material to conduct heat Metal, gas and insulating material Thermal conductivity of a metal is higher than insulating material and that for gases is low than metal but higher than insulating materials. Minus sign: This law has a minus sign because heat flows from high temperature to low temperature. Question.13 Stefen-Boltzman Law: This law states that the rate of radiation of heat by a black body is proportional to its temperature Emissivity: It is a ratio that compares the heat radiated by a body at a temperature T compared to that absorbed by a black body at temperature T. View factor: View factors are configuration factors that determine the proportion of heat exchange between two bodies, for this reason therefore the shape of a body will determine the radiation rate Blackbody: A back body is a body that absorbs all heat radiation Question.14 Newton's law of cooling: This law states that the rate of cooling is proportional to the difference in temperature of the system and the other cooler system. Heat transfer coefficient The heat transfer coefficient it a measure of the rate of transfer of heat energy in joules of a given surface area Nusselt number The Nusselt number is the ratio of convectional heat transfer to conductive heat transfer. Heat convections The two heat convections include natural or free convection whereby fluid motion causes transfer of heat due to changes in density and heat advent convection or forced convection whereby heat transfer is due to movement of the liquid resulting from other forces. Question.15 Heat of combustion: Heat of combustion is the heat produced when a one mole of compound example a hydrocarbon combusts in oxygen to produce water and carbon dioxide. An example CH4 + 2O2 CO2 + 2H2O The above reaction produces heat energy. Types of combustion: Slow - low amount of heat is released Rapid- large amount of heat is released Complete - all the reactants form new compound Incomplete- combustion is not complete Specific heat capacity: The specific heat capacity of compounds is the amount of energy required to change or increase the temperature from a given temperature to another. Latent heat: Latent heat is heat released or absorbed by a reaction, whereby some reactions are endothermic and others are exothermic Calorimetry: Is the measuring of heat released or absorbed during a reaction Combustion temperature: It is the temperature reached by a chemical reaction Chemical equilibrium: Chemical equilibrium is achieved when there are no reactions over time Question.16 The efficiency of an engine that produces 150J of work from 212J of energy: Efficiency = (output power / input power) X 100 Efficiency = (150 / 212) X 100 Efficiency = 70.75% Question.17 We determine the heat capacity of the two water masses, heat capacity of water is 4186 joules per KG 150 gram of water at 10 degrees Heat capacity = 0.15 X 10 X 4186 = 6279 100 grams of water at 85 degrees Heat capacity = 0.1 X 85 X 4186 = 35581 Total heat capacity = 6279 + 35581 = 41860 Temperature = heat capacity divided by mass X heat capacity Mass = 250 grams Final temperature = 40C Question.18 A closed Styrofoam cup, which is 6 mm thick and has a surface area of 390 cm2, contains 550 ml of hot coffee at 850C The air outside the cup remains at a constant temperature of 210C. Assuming that the coffee has a mass of 0.6 kg, and that the specific heat capacity is the same as that of water, determine The initial rate of heat flow through the Styrofoam (in SI units), And the time required for the coffee to cool from 850C to 700C 1. Heat transfer rate = h . A(T1 - T2) Where h is the heat transfer coefficient, A is the surface area T1 is the object temperature and T2 is the temperature of the air outside. Heat transfer coefficient of Styrofoam =0.17 Heat transfer rate = 0.17 x 390 x10-4m2 (85 - 21) Heat transfer =0.42432 joules per second. 2. Heat capcaicy of coffee: 85 degrees= 4186 x 0.6kg x 85 = 213486 70 degrees = 4186 x 0.6kg x 70 =175812 Energy transfer = 213486 - 175812 = 37674 Heat transfer rate = 0.42432 joules per second Time taken = 88786 seconds Question.19 Flames: It is a front of an exothermic reaction, there are two types of flames and they include premixed flames and diffusion flames, in premixed flames fuel and oxygen are mixed and therefore the flame is hotter while for the diffusion flames fuel diffuses with oxygen. Diffusion flames are laminar while premixed flames are turbulent. Question.20 A fluid is a substance that flows and that fluid viscosity varies, example water is less viscous than oil, and therefore viscosity of a fluid is the resistance of that fluid to flow. Question.21 Air is compressible while water is incompressible, this is because air molecules can be compressed and made more dense while this is not possible for water. Question.22 A concrete slab has a length of 24 m at -8 C on a winter's day. What is the change in length from winter to summer, when the temperature is 35 C The linear expansion coefficient of concrete is 1 X 10-5 C-1. Linear expansion: L/L = (a ) T Where L is length, a is the linear coefficient of the substance and T is the change in temperature: L/L = (1 X 10-5 C-1). 43 L/L = 4.3 X 10-4 L/24= 4.3 X 10-4 L = 1.032 X 10-2meters Question.23 Work = Force X Distance Work = 30 X 5 = 150 joules, energy used = 150 joules. Question.24 A piece of aluminium siding is 12.45 meters long on a cold winter's day, -18C. How much longer is it on a very hot summer's day, 37C Aluminium slab has a length of 12.45 m at -18 C . What is the change in length when the temperature is 37 C The linear expansion coefficient of concrete is 2.3 X 10-5 C-1. Linear expansion: L/L = (a ) T L/12.45 = (2.3 X 10-5 C-1). 55 L = 1.574925 X 10-2meters Question.25 Boundary layer of a fluid is the layer of the fluid on the boundary of a pipe; laminar flow is smooth while tubular flow is distorted and therefore not smooth. References: Charles Kittel (1999) Thermal physics, McMillan publishers, London Mark Zemansky (2006) Heat and thermodynamics, McGraw Hill press, New York Van Ness H (1999) Understanding Thermodynamics, Prentice hall publishers, New Jersey Read More