B-2 Physics Thermodynamics 1.pptx PDF

Module: B-2 Physics Topic 2.3 Thermodynamics  INTRODUCTION On completion of this topic you should be able to: 2.3.1.1 Describe temperature and the operation of thermometers. 2.3.1.2 Describe the following tempera...

Module: B-2 Physics Topic 2.3 Thermodynamics  INTRODUCTION On completion of this topic you should be able to: 2.3.1.1 Describe temperature and the operation of thermometers. 2.3.1.2 Describe the following temperature scales: Celsius Fahrenheit Kelvin. 2.3.1.3 Define heat. 30-03-2024 Slide No. 2  INTRODUCTION On completion of this topic you should be able to: 2.3.2.1 Define specific heat and describe heat capacity 2.3.2.2 Describe the following methods of heat transfer: convection conduction radiation 2.3.2.3 Describe volumetric expansion 2.3.2.4 State the first and second laws of thermodynamics 2.3.2.5 Describe the following regarding gases: ideal gas laws specific heat at constant volume and constant pressure work done by expanding gas 30-03-2024 Slide No. 3  INTRODUCTION On completion of this topic you should be able to: 2.3.2.6 Describe the following: Isothermal and Adiabatic expansion and compression Engine cycles Constant volume and constant pressure Refrigerators and heat pumps Latent heats of fusion and evaporation Thermal energy Heat of combustion 2.3.2.7 Describe the following: Latent Heats of fusion and evaporation Thermal energy Heat of combustion 30-03-2024 Slide No. 4  HEAT The smallest particles of most substances, atoms and molecules, are constantly in random motion. Heat is defined as the kinetic energy associated with this motion. The more heat energy there is in a material, the faster its molecules move. Heat is one form of energy, and in many cases the production of heat and its subsequent release can do useful work. 30-03-2024 Slide No. 5  HEAT Energy concerning the application, loss or transfer of heat is termed thermal energy. According to the law of conservation of energy, thermal energy cannot be created or destroyed. It can be converted from, and to, other forms of energy. The work done by an expanding gas is one of the basic principles behind propulsion. 30-03-2024 Slide No. 6  CONDUCTION Water is a poor conductor of heat. Conduction requires physical contact between a body having a high level of heat energy and a body having a lower level of heat energy. 30-03-2024 Slide No. 7  CONVECTION Convection is the process by which heat is transferred by bulk movement of a fluid. 30-03-2024 Slide No. 8  RADIATION Conduction and convection involve the transfer of energy between particles of matter. A third way of transferring energy from one body to another is by radiation which does not require a medium. Radiation refers to the emission of energy from the surface of most objects. 30-03-2024 Slide No. 9  RADIATION The acceleration creates waves of energy to be propagated at high speed and without a medium called Electromagnetic Radiation. At a certain frequency of this wave motion, approx 1013 Hz, the energy is propagated as heat. (Actually called Infra-Red as it is just lower than red light). Conduction and convection take place relatively slowly while radiation takes place at the speed of light. 30-03-2024 Slide No. 10  KINETIC THEORY OF MATTER It was discovered that the smallest particles of most substances, molecules, are constantly in random motion. (For elements read atoms.) Heat is described as the kinetic energy associated with this motion. The more heat energy there is in a material, the faster its molecules move, and changes will occur to the substance. More energy allow the molecules to move further apart and change state. 30-03-2024 Slide No. 11  VOLUMETRIC EXPANSION With few exceptions, solids expand when they are heated and contract when they are cooled. When solids increase in size, they not only increase in length but also breadth and thickness. This is called volumetric expansion. Expansion is calculated using the formula E = kL(T2 –T1) 30-03-2024 Slide No. 12  BI-METALLIC STRIP If two dissimilar substances are joined and heated, they will expand at different rates and create stress in the structure. The bending operates the electrical contacts for thermostats, and keeps the clock wheel in balance for constant speed at differing temperatures. The two metals in the bi-metallic strip expand different amounts with heating. Hence the strip bends with temperature changes. 30-03-2024 Slide No. 13  UNITS OF HEAT Calorie (cal): one calorie is the quantity of heat required to raise the temperature of one gram of water by one degree Celsius. British thermal unit (Btu): one Btu is the quantity of heat required to raise the temperature of one pound of water by one degree Fahrenheit. Joule (J): the SI unit for all forms of energy. Energy provides the capacity for work to be done. One joule of energy can do one joule of work. The heat produced by burning one litre of gasoline is approximately 8 x 106 cal, 3 x 104 Btu, or 3 x 107 J (30 MJ). 30-03-2024 Slide No. 14  HEAT AND TEMPERATURE Temperature represents the degree of heat possessed by one mass over another. When heat flows from one body to another, the hotter is said to be at a higher temperature. However, a cup of water at 90°C contains less heat than a swimming pool at 20°C. For this reason we define two properties, one called the Specific Heat of a substance, and the other, the Heat Capacity. 30-03-2024 Slide No. 15  SPECIFIC HEAT The specific heat of a substance is the amount of heat required to change the temperature of one unit of mass of a substance by one degree eg: The number of joules required to raise the temperature of 1 gram of material by 1°C. The specific heat of water is: 1.00 calorie / gram°C To raise temp of one gram of water by 1°C, you must provide 1.00 calorie of heat. Because gasses are compressible, the amount of heat require to change temperature depends on pressure and volume. Specific heat for a gas is defined either at Constant Pressure or Constant Volume. 30-03-2024 Slide No. 16  HEAT CAPACITY Heat capacity is the amount of heat needed to raise the temperature of an entire body by one degree Celsius. Heat capacity of an object may be calculated if the mass of the object & the specific heat of the substance is known. 30-03-2024 Slide No. 17  TEMPERATURE Temperature represents the average kinetic energy of molecules such that, as heat is added to a substance, its temperature rises, and vice versa. The increased molecular motion causes matter to expand, and we use this to measure temperature. Adding or subtracting heat from matter will cause it to change state from solid to liquid to gas. Temperature scales have been devised using the change of state of water. An amount of mercury in a glass tube will be at a certain height if placed in melting ice, and rise to a higher position in boiling water. 30-03-2024 Slide No. 18  THERMOMETERS Bulb thermometers rely on the simple principle that a liquid changes its volume relative to its temperature. All bulb thermometers use a fairly large bulb and a narrow tube to accentuate the change in volume. Mercury is used in most bulb thermometers because mercury will expand more than glass when heated and contracts more than glass when cooled. Thus, the length of mercury column in the glass tube provides a measure of the surrounding temperature. 30-03-2024 Slide No. 19  TEMPERATURE SCALES The difference in height of the column of mercury can be calibrated in any way, however four temperature scales have been devised: degrees Celsius (°C); degrees Fahrenheit (°F); Kelvin (K). Rankin (Fahrenheit equivalent of Kelvin) With the Kelvin scale, the unit ‘degrees’ and its symbol (°) is not used. It is said that water boils at 373 K. 30-03-2024 Slide No. 20  TEMPERATURE SCALES The centigrade or Celsius scale is divided into 100 increments between the freezing point of pure water (0°) and the boiling point of pure water (100°). The Kelvin scale also has 100 increments between the freezing and boiling point of water, but zero on the Kelvin scale represents the temperature at which all molecular activity ceases (absolute zero) at – 273°C. To convert temperatures between Kelvin and Celsius scales is relatively easy: K = °C + 273 °C = K – 273 30-03-2024 Slide No. 21  TEMPERATURE SCALES The Fahrenheit scale has 180 increments between the freezing point and boiling point of water. The freezing point is at 32°F and the boiling point is 212°F. 30-03-2024 Slide No. 22  TEMPERATURE SCALES To convert between Kelvin/Celsius to Fahrenheit takes a little more calculation There are 5 Celsius degrees to every 9 Fahrenheit degrees. To convert between these scales, the same fractional factors apply, but because 0°C = 32°F, the formulas are: ° F = 9/5 °C + 32 ° C = 5/9 (°F – 32) 30-03-2024 Slide No. 23  LATENT HEAT When ice is heated, the heat that initially enters the substance is used to melt the ice. As the ice melts the temperature remains constant at 0°C. The amount of heat required to melt the ice is called the latent heat of fusion. Once the ice has melted and further heat is added, the temperature of the water slowly increases from 0°C to 100°C. This is called sensible heat, that is, the increase in temperature with the addition of heat can be sensed. 30-03-2024 Slide No. 24  LATENT HEAT Once the water starts to boil, the heat that enters the substance is used to convert the liquid into a gas and the temperature of the water remains constant until all the liquid has evaporated. The amount of heat required to boil, or vaporise, the liquid is called the latent heat of vaporisation (or evaporation). 30-03-2024 Slide No. 25  LATENT HEAT To restate, when a liquid is at its boiling temperature, or a solid is at its melting temperature, a change of state will only occur if further heat is added. No temperature change occurs during the change of state, even though heat is added. This heat that causes a substance to change its state with no change in temperature is known as ‘latent heat’. Sensible heat is heat, when applied, causes a temperature change that can be sensed. Whereas latent heat is used to break down intermolecular bonds, sensible heat is stored in intermolecular forces, increasing kinetic energy of the molecules. 30-03-2024 Slide No. 26  LATENT HEAT 540 calories of latent heat will cause one gram of water at 100°C to change to steam at 100°C. One gram of steam at 100°C condenses to water at 100°C if it loses 540 calories of heat. For sensible heat, 100 calories of heat changes one gram of water at 0°C to one gram of water at 100°C. 30-03-2024 Slide No. 27  THE REFRIGERATION CYCLE The compressed gas heats up as it is pressurized. The coils on the back of the refrigerator let the hot refrigerant gas dissipate its heat. The refrigerant gas INSIDE condenses into refrigerant liquid (dark blue) at high THE pressure. FRIDGE The high-pressure refrigerant liquid flows through the expansion valve. C The liquid refrigerant immediately boils and vaporizes using its own latent heat (light blue), its temperature dropping to -27 F. This makes the inside of the refrigerator cold. The cold refrigerant gas is BACK sucked up by the OF THE compressor, and the cycle FRIDGE repeats. 30-03-2024 Slide No. 28  AIR CONDITIONING (Cold Cycle) OUTSIDE Evaporates to cold gas INSIDE using its own Latent Heat 20°C 35°C 40°C -20°C Extracts Condenses to a heat from liquid giving up the room heat to the air Hot gas 60°C 10°C A is the EVAPORATOR B is the COMPRESSOR or HEAT PUMP 30-03-2024 Slide No. 29  AIR CONDITIONING (Reverse Cycle) HEAT PUMP Evaporates to cold gas INSIDE using its own Latent Heat OUTSIDE 25°C 5°C 25°C -10°C Extracts Condenses to a heat from liquid giving up the air heat to the room Hot gas 40°C 5°C A is the EVAPORATOR B is the COMPRESSOR or HEAT PUMP 30-03-2024 Slide No. 30  BOYLES LAW A gas can be easily compressed. As it is compressed, its pressure increases and its volume decreases, assuming temperature remains constant. This relationship acts in accordance with Boyle’s Law which states that the volume of a confined body of an ideal gas varies inversely as pressure varies, assuming temperature remains constant. This can be expressed by the formula: V1 = P2 V2 P1 (temperature constant) Halving V will double P. 30-03-2024 Slide No. 31  BOYLES LAW Isothermal processes are those processes taking place at constant temperature. This can really only occur when a material being compressed is in contact with a heat sink. This can be expressed by the formula: V1 = P2 V2 P1 (temperature constant) Halving V will double P. 30-03-2024 Slide No. 32  CHARLES’ LAW Just as changes in gas volume are related to pressure changes, they are also related to temperature changes. Charles’ Law states: ‘The volume of a gas varies in direct proportion to its temperature, assuming pressure remains constant’ Note the flexible container to ensure constant pressure. 30-03-2024 Slide No. 33  CHARLES’ LAW If a gas is confined in a solid container, so that the volume remains constant, Charles’ Law becomes: P1 = T1 (volume constant) P2 T2 Welding gas bottles out in the sun could over pressurise, hence the need for relief valves. However, an adiabatic temperature change is one that occurs without the addition or removal of heat. For example: Temperature drop that occurs when a gas under pressure is released to a lower pressure; Increase in cylinder pressure when filled at too high a rate. 30-03-2024 Slide No. 34  GENERAL GAS LAW The general gas law is derived by combining Boyle’s and Charles’ laws. For an ideal gas, it is expressed by the equation: For a snow making machine, the pressure drop is more significant than the volume increase, and the temp reduces below freezing. 30-03-2024 Slide No. 35  THERMAL ENERGY Energy concerning the application, loss or transfer of heat is termed thermal energy. According to the law of conservation of energy, thermal energy cannot be created or destroyed, but it is converted from, and to, other forms of energy. For example, thermal energy may be created from electrical, chemical, mechanical or nuclear energy. It can be converted to mechanical or kinetic energy. The heat in a thermal process can also add energy to chemical reactions. 30-03-2024 Slide No. 36  LAWS OF THERMODYNAMICS The first law of thermodynamics is similar to the law of conservation of energy: HOT IN ‘Heat energy cannot be destroyed, it can only be changed from one form of energy to another.’ HEAT SINK OUT The second law of thermodynamics states that: ‘heat cannot flow from a body of a given temperature to a body of a higher temperature.’ That is, heat will only flow from a warmer body to a cooler body. This is a logical process and the theory behind it is used in car radiators, heat exchangers, oil coolers etc. 30-03-2024 Slide No. 37  HEAT OF COMBUSTION Any time fuel is burnt (combustion), heat is produced. Sometimes the heat is useful and sometimes the heat is unwanted. We say that heat is a by-product of the combustion process. Combustion can range from lighting a match through to the furnace of a coal-fired power station. Combustion can use liquid, solid or gaseous fuel. The domestic fireplace is desirable combustion! 30-03-2024 Slide No. 38  HEAT OF COMBUSTION When fuel is burnt in a combustion engine to produce power, the presence of heat is inevitable. Often heat is wasted and needs to be dissipated for the engine to work optimally. For example, most car engines have water circulating around the engine. The water is cooled by a radiator, allowing the engine temperature to remain within a specified range. If too much heat is allowed to build up, the engine can be damaged. 30-03-2024 Slide No. 39  HEAT OF COMBUSTION In a gas turbine (jet) engine, the heat of combustion is necessary to expand gases and do work while flowing through the engine. It is the higher volume of gas which drives a turbine, making the engine self- sustaining and it is the gas which contributes to the reactive force of thrust. Nevertheless, it is also important for gas turbine engines for maximum operating temperatures to be observed. Materials used for construction of engine components cannot withstand temperatures above a certain range. 30-03-2024 Slide No. 40  WORK DONE BY EXPANDING GASES Sometimes heat is not an unwanted by- product of combustion. Sometimes the expansion of gas created by the heat is the prime purpose for the combustion. For example, when a gun is discharged, the heat produced by the ignition of a small gunpowder charge increases the volume of gas available to push the bullet out of the barrel. Heat is necessary for the process to occur. Likewise, a gas turbine engine relies on heat to produce a gas from the fuel air mixture. The expanded volume of gas drives the engine turbines and contributes to the reactive force of thrust. In this sense, expanding gases do work similar to other mechanical processes. 30-03-2024 ☻Slide No. 41  WORK DONE BY EXPANDING GASES Work is calculated by multiplying the force applied by distance: W = Fs The greater the force applied to an object or the greater the distance an object moves, the more work has been done. Power is the time rate of doing work. A man may expend 5,000 joules pushing a wheelbarrow for 1 hour. The rifle has expended the 5,000 joules in a split second. It has generated a great deal more power than wheelbarrow man. 30-03-2024 Slide No. 42  ENGINE CYCLES In both piston and gas turbine engines, the compression of air is necessary before fuel is introduced and ignited. In both these types of engines there is a significant increase in temperature of the air medium due to this compression process. Collisions between the molecules themselves and between molecules and the walls of the container are greatly increased. It is this increased kinetic activity which causes a temperature rise in a compressed gas. 30-03-2024 Slide No. 43  PISTON ENGINE (OTTO CYCLE) A typical piston engine turns reciprocating motion into rotary motion to drive a propeller in a four cycle operation as shown below. Intake Air and fuel are sucked into the cylinder through the intake valve. Compression This mixture (15:1) is compressed into a smaller volume. Power The compressed mixture is ignited by a spark and the piston is forced down Crankshaft by the rapid expansion of hot gas. Exhaust. The exhaust gases are forced out of the exhaust valve by the ascending piston which then descends in the next intake stroke. Multiple cylinders connected to the same crankshaft increase the number of power strokes per revolution. 30-03-2024 Slide No. 44  PISTON ENGINE (OTTO CYCLE) Plotting how pressure varies with volume shows how the cycle develops its power. The work done by the engine is equal to the enclosed area of the graph, Power is work/time so more rpm = more power. Note that the combustion process occurs at approximately constant volume, (3 - 4 on the graph), so the Piston engine is called a Constant Volume engine. 30-03-2024 Slide No. 45  GAS TURBINE ENGINE (BRAYTON CYCLE) A gas turbine is similar to a piston engine in that it compresses a mixture of air and fuel which is burnt to release its energy. However, the cycle differs from there. Intake - Air only is drawn in directly, at the front. Compression - This air is gradually squeezed into a smaller volume through a series of compressor stages (1 stage = a rotating element and a stationary element). Power - Fuel is added into a combustion chamber and ignited during the start procedure, it is continuously burnt with the compressed air. The hot gas expands through a turbine forcing it to rotate. The power part of the cycle. The turbine is connected to the compressor causing it to rotate and continually supply air for combustion. About 50% of the power produced is used by the turbine. Exhaust - The hot gasses leave the engine through a suitably shaped duct providing thrust because of the acceleration given to the mass of air originally taken in. 30-03-2024 Slide No. 46  GAS TURBINE ENGINE (BRAYTON CYCLE) Once again, plot P against V: This time the combustion takes place at approximately constant pressure (B – C) Hence the name Constant Pressure cycle 30-03-2024 Slide No. 47  TURBOPROP Extra turbines transfer the power to a propeller. 30-03-2024 Slide No. 48  TURBOFAN The extra turbines transfer the power to a multi-bladed and shrouded fan which accelerates the air mass. FAN 30-03-2024 Slide No. 49  TURBOSHAFT These are similar to turboprops and fan engine in that they utilise extra turbines to deliver the power to a variety of applications, such as electrical generators, ship’s propellers, and helicopter rotors. SHAFT 30-03-2024 Slide No. 50  DEVELOPMENT OF THRUST F = ma (Newton’s 2nd Law) and it can be shown that all of these engines provide thrust by accelerating a mass of air. F = m (V – U) which can be written F = m (V – U) t t m is air mass flow rate in kg/s, U is the air initial velocity entering the engine t V is the velocity at which the exhaust leaves. A prop gives a large mass of air with a small acceleration. A pure jet provides a small mass of air with a large acceleration. 30-03-2024 Slide No. 51  CONCLUSION On completion of this topic you should be able to: 2.3.1.1 Describe temperature and the operation of thermometers. 2.3.1.2 Describe the following temperature scales: Celsius Fahrenheit Kelvin. 2.3.1.3 Define heat. 30-03-2024 Slide No. 52  CONCLUSION On completion of this topic you should be able to: 2.3.2.1 Define specific heat and describe heat capacity 2.3.2.2 Describe the following methods of heat transfer: convection conduction radiation 2.3.2.3 Describe volumetric expansion 2.3.2.4 State the first and second laws of thermodynamics 2.3.2.5 Describe the following regarding gases: ideal gas laws specific heat at constant volume and constant pressure work done by expanding gas 30-03-2024 Slide No. 53  CONCLUSION On completion of this topic you should be able to: 2.3.2.6 Describe the following: Isothermal and Adiabatic expansion and compression Engine cycles Constant volume and constant pressure Refrigerators and heat pumps Latent heats of fusion and evaporation Thermal energy Heat of combustion 2.3.2.7 Describe the following: Latent Heats of fusion and evaporation Thermal energy Heat of combustion 30-03-2024 Slide No. 54  This concludes: Module: B-2 Physics Topic 2.3 Thermodynamics