L2-Calorimetry Physics Department Helwan University PDF

Physics Department Helwan University Contents Chapter I: Heat Phenomena and Thermal Physics Chapter II: Heat and Matter Chapter III: Calorimetry Chapter IV: Thermometry Chapter V: Heat Transfer Chapter VI: Thermal Analysis P...

Physics Department Helwan University Contents Chapter I: Heat Phenomena and Thermal Physics Chapter II: Heat and Matter Chapter III: Calorimetry Chapter IV: Thermometry Chapter V: Heat Transfer Chapter VI: Thermal Analysis Physics Department Helwan University Calorimetery Physics Department Helwan University Introduction Calorimetry is derived from Latin "calor" meaning “heat” and Greek “metron” meaning “measure”. Calorimetry is the science of measuring the quantity of heat exchanged, involved in either change of temperature or state: 1) generated (exothermic process), 2) consumed (endothermic process) or 3) dissipated by the materials. Physics Department Helwan University Sources of Heat Sun is largest source of renewable energy & this energy is abundantly available in all parts of earth. Sun is in one of the best alternatives to non-renewable sources of energy. Biggest share of solar energy reaching earth is absorbed at surface. Amount that actually reaches surface varies according to weather conditions, amount of particulate matter & water vapour in air, time of day, season of year, earth’s distance from sun. Solar Energy Physics Department Helwan University Solar Sources of Heat Way to trap solar energy: Large-scale energy collectors Bond with integral heat storage for supplying thermal energy. Principle: as water or air is heated → become lighter → rise upward. Ordinary pond: sun heat water & heated water within pond rises → top → loses heat into atmosphere → pond water remains at the atmospheric temperature. Water is very poor conductor of heat → if this circulation can be stopped → heat can be trapped in bottom of lake. Solar pond: Natural tendency of hot water to rise to surface is restricted by dissolving salt in bottom layer of pond making it too heavy to rise. 1) Top zone is surface zone or Upper Convective Zone at atmospheric temperature & has little salt content. 2) important gradient zone or Non-Convective Zone 3) Bottom storage zone or Lower Convective Zone zone is very hot, 80 -100° C, & is very salty. Physics Department Helwan University Solar Sources of Heat Bond has major advantages: 1) The heat storage is massive, so energy can be extracted day and night → source of 'base load' solar power → no batteries or other storage needed. 2) Solar ponds can have very large heat collection area at low cost. 3) Major production potential is during peak electrical power demand in mid summer. Heat is extracted by heat exchanger at bottom of pond. Heat energy can power engine, provide space heating or produce electricity via low-pressure steam turbine. Heated saltwater can be pumped to location where heat is needed. After heat is used, water can be returned to solar pond & heated again. Physics Department Helwan University Sources of Heat Microwave Heating A) Mechanism: Electromagnetic field → Materials containing polar molecules having electric dipole moment → molecules rotate to continuously align with field → dipole rotation. As field alternates → molecules reverse direction → molecules push, pull & collide with other molecules → distributing energy to adjacent molecules & atoms in material. Agitating molecules increases temperature. “Energy in form of electromagnetic radiation is converted to heat energy in matter". Mechanism operates efficiently in microwave oven on water but much less on ice (molecules are not free to rotate) and on fats and sugars (have less molecular dipole moment). The effect is used to heat solids, liquids or gases contain electric dipoles. Physics Department Helwan University Sources of Heat Microwave Heating B) Uses: i- Heating: # 1930s: high-frequency electric fields → heating dielectric materials. # “Diathermy" means "electrically induced heat“: - Ultrasonic diathermy: ultrasound for purpose of therapeutic deep heating. - Electric diathermy uses high frequency alternating electric or magnetic fields, sometimes with no electrode or device contact to the skin, to induce gentle deep tissue heating Physics Department Helwan University Sources of Heat Microwave Heating B) Uses: ii- Microwave Oven: # Kithen application that cooks or heats food by dielectric heating. # Heat polarized molecules within food. # Use of microwave frequency electric fields for highly efficient dielectric heating. Magnetron tube creates → microwaves → # Excitation is fairly uniform, waveguide directs them to stirrer fan → leading to food being more evenly heated throughout. oven cavity. Physics Department Helwan University Quantity of Heat Change in Quantity of heat Mass temperature gained or lost Q  m. T by a substance Then: Q = S m T S = constant depending on nature of material  Specific heat. Unit for quantity of heat: Putting m = 1 & T = 1 and assuming standard substance (water) to have S = 1 is ”the amount of heat required to raise temperature of one gram of water through one degree” = “calorie”. This definition presumes that same amount of heat is required to raise temperature of one gram of water through 1 oC in any temperature range not true. Unit recommended by International Union of Pure and Applied Physics: "The amount of heat necessary to raise the temperature of 1 gram of water from 14.5 oC to 15.5 oC“ = 15 oC calorie. Physics Department Specific Heat Helwan University Q = S m T S = Q / m T Definition: Putting m = 1 & T = 1 S=Q “quantity of heat required to raise temperature of unit mass of substance through one degree. Unit: C.G.S. system cal/gm.K. Mean: From T1 to T2: S' = Q / m (T2 – T1) True: Small range T: S(T) = (1 / m). dQ/d T T2 Q =  dQ = m  S(T)dT T1 Physics Department Helwan University Specific Heat Quantity of heat Q1 mass m of water (S=1) raise temperature by T: Q1 = m x l x T Quantity of heat Q2 same mass m of substance raise temperature by same amount T: Q2 = m x S x T Then, the ratio: S = Q2 / Q1 “Ratio” of quantity of heat required to raise temperature of given mass of substance through given temperature range to that required to raise equal mass of water through same temperature range. Physics Department Helwan University Heat Capacity Different substances have different specific heat. Bodies of same specific heat (S) and different masses (m) are distinguished by their “heat capacity” (C) = mS = amount of heat required to raise temperature of the body through one degree. C = mS Putting m = 1 C=S Specific heat capacity is heat capacity of unit mass of substance. Physics Department Helwan University Water Equivalent Q = mw (water). Sw (water) T = m (body). S (body) T mw (water) = m (body). S (body) Water equivalent = C (heat capacity of body) Water equivalent (grams) = Heat capacity (calories) product of mass & specific heat of body: 1) expressed in calories gives heat capacity, 2) expressed in grams gives its water equivalent. Physics Department Helwan University Atomic Heat Capacity Atomic heat = (atomic weight) x (specific heat) In 1819, Duling and Petit’ Law: “Atomic heat is constant for all elements in the solid state”. Atoms of different elements (different atomic weights) have same atomic heat capacity independent on mass but on number of atoms. Physics Department Helwan University Phase Changes & Heat Content Phase change (vaporization of liquid, fusion or sublimation of solid and transition from one crystalline modification to another) change of heat content: = latent heat of vaporization, fusion, etc…. Latent heats vary with the various types of transformation. Tendency is toward omission of term latent, why? This quantity represents difference in the heat content of 1 gram (or 1 mole) of two phases under consideration at temperature and pressure at which phase change takes place. Physics Department Helwan University Phase Changes & Heat Content General term ”heat of transformation” (L) is applied both to heat of fusion and heat of vaporization. It represents heat absorbed or liberated in phase change of unit mass. Effect of adding or absorbing heat to or from system is not to ascend or descend its temperature but to change its phase Heat Q absorbed or liberated in the change of phase of mass m is Q=mL Phase change is reversible: Amount of heat is considered (in any case) +ve or -ve depending on heat is added or removed. Physics Department Helwan University Conservation of Energy Measuring specific heat: 1) Heating sample of unknown specific heat (Sx) and mass (mx) to some known temperature Tx. 2) Placing it in vessel containing specific liquid of known specific heat (Sl) and mass (ml) at temperature Tl < Tx. 3) Measuring final temperature Tf of liquid after equilibrium has been reached. 4) Consider negligible amount of mechanical work is done during this process: Law of conservation of energy amount of energy that leaves (-ve) sample (Qhot) equal amount of energy that enters (+ve) liquid (Qcold): Qcold = - Qhot ml Sl (Tf – Tl) = - mx Sx (Tf – Tx) This technique is called Calorimetry and device in which process occurs is called calorimeter. Physics Department Helwan University Specific Heat of Gases Value of specific heat depends not only on nature of substance, but also on amount of external work done due to expansion of substance caused by the rise in temperature. In case of solids and liquids, change of volume (external work done = PV) during change of temperature is negligibly small. In case of gases, big changes of pressure and volume with change in temperature conditions under which heating takes place must be stated. It is customary to speak of two specific heats of a gas: 1) Specific heat at constant volume (Cv): whole heat supplied goes to increase the internal energy. 2) Specific heat at constant pressure (Cp): thermal equivalent of external work done by gas forms part of total heat supplied Cp > Cv. Physics Department Helwan University Contents Chapter I: Heat Phenomena and Thermal Physics Chapter II: Heat and Matter Chapter III: Calorimetry Chapter IV: Thermometry Chapter V: Heat Transfer Chapter VI: Thermal Analysis