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حرارة وديناميكا حرارية المحاضرة الثانية د/عبدالرحمن لاشين قسم الفيزياء - كلية العلوم التطبيقية – جامعة أم القرى - المملكة العربية السعودية قسم الفيزياء - كلية العلوم – جامعة المنصورة – جمهورة مصر العربية

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W HAT IS H EAT (Q)? Form of energy because it can move things - E.g: Makes a hot air balloon rise. - Steam engines Measured in JOULES (J)

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S PECIFIC H EAT The amount of heat ( Q ) added into a body of mass m to change its temperature an amount T is given by Q=m c T c is called the specific heat and depends on the material and the units used. Note: since we are looking at changes in temperature, either Kelvin or Celsius will do.

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U NITS OF H EAT Heat is a form of energy so we can always use Joules. More common in thermodynamics is the calorie: By definition 1 calorie is the amount of heat required to change the temperature of 1 gram of water 1 C The Btu (British Thermal Unit.) It is the amount of heat required to change the temperature of 1 lb of water 1 F. Conversions: 1 cal =4.186 J 1Btu = 252 cal

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U NITS OF S PECIFIC H EAT Note that by definition, the specific heat of water is 1 cal/g C.

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Material J/kg C cal/g C Water41861 Ice Steam Silver Aluminum Copper Gold Iron Lead Brass Glass Wood Ethyl Alcohol Beryllium

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E XAMPLE C ALCULATION Compare the amount of heat energy required to raise the temperature of 1 kg of water and 1 kg of iron 20 C?

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H EAT T RANSFER Heat always moves from a warmer place to a cooler place. Hot objects in a cooler room will cool to room temperature. e.g: tea, coffee Cold objects in a warmer room will heat up to room temperature.e.g: butter, ice

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H EAT T RANSFER M ECHANISMS 1. Conduction: (solids--mostly) Heat transfer without mass transfer. 2. Convection: (liquids/gas) Heat transfer with mass transfer. 3. Radiation: Takes place even in a vacuum.

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C ONDUCTION Takes place in solid, liquid and gases but it is common in solid Needs physical contact Particles at the warm end vibrates faster and this then causes the next particles to vibrates faster and so on. e.g: spoon in tea In this way heat in an object travels from: the HOT end the cold end

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E XAMPLE

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C ONDUCTION When you heat a metal strip at one end, the heat travels to the other end. As you heat the metal, the particles vibrate, these vibrations make the adjacent particles vibrate, and so on, the vibrations are passed along the metal and so is the heat. We call this? Conduction

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C ONDUCTION

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C ONVECTION What happens to the particles in a liquid or a gas when you heat them? The particles spread out and become less dense. A liquid or gas.

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C ONVECTION It is the way in which particles in a GAS or LIQUID move upwards, carrying heat with them Think about when you boil water, the bubbles move upwards Or think of a gas heater in the room, the heat rises around the room

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Convection : the transfer of thermal energy (heat) through the bulk movement of matter. Convection occurs in FLUIDS (liquids and gases). Convection produces CURRENTS in both gases and liquids. Thermal Energy heat is carried by the particles as they move from one location to another.

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C ONVECTION Example: Heating water : a. When the water at the bottom of the pot (nearest the burner) is heated, the particles absorb energy by conduction as they touch the hot pot. b. The water particles vibrate more rapidly. c. The particles also move farther apart and the hot water becomes less dense than the surrounding cool water. d. This causes the heated (hot) water to rise.

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C ONVECTION e. The surrounding denser cooler water is forced downward near the burner by the rising hot water. f. This process continues to repeat. g. This FLOW creates a circular motion known as a convection current

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R ADIATION How does heat energy get from the Sun to the Earth? There are no particles between the Sun and the Earth so it MUST travel by radiation ? RADIATION

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The transfer of heat in rays, from a hot object, without needing a medium to pass through It travels in all directions from a hot object The hotter an object is, the more heat it will radiate out Does the surface affect the way heat is radiated?

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T HE BLACK BODY 22 A black body is an ideal body which absorbs all the incident radiation within itself. The black body is an ideal absorber of incident radaition. The black body is an ideal radiator

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R ADIATION Everything that has a temperature radiates energy. The radiation transferred by radiation from a black body is governed by the fourth power low σ is Stefan’s constant =5.6 ×10 -8 wm -2 k -4, e is the emissivity (between 0,1) and A is the surface area of the of the black body

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If a black body whose temperature T is in an enclosure at a temperature T o then, the net rate of loss of energy is given by:

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I DEAL G AS E QUATION /M OLAR & M OLECULAR M ASS

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Highly compressible. Occupy the full volume of their containers. When gas is subjected to pressure, its volume decreases. Gases always form homogeneous mixtures with other gases. Gases only occupy about 0.1 % of the volume of their containers. General Characteristics of Gases

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F OUR P HYSICAL Q UANTITIES FOR G ASES Phys. Qty.SymbolSI unitOther common units pressureP Pascal (Pa) atm, mm Hg, torr, psi volumeVm3m3 dm 3, L, mL, cm 3 temp.TK°C, °F molesnmol

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G AS : E QUATION OF S TATE It is useful to know how the volume, pressure, and temperature of the gas of mass m are related. The equation that interrelates these quantities is called the equation of state.

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I DEAL G AS M ODEL The ideal gas model can be used to make predictions about the behavior of gases. If the gases are at low pressures, this model adequately describes the behavior of real gases.

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T HE M OLE The amount of gas in a given volume is conveniently expressed in terms of the number of moles, n. One mole of any substance is that amount of the substance that contains Avogadro’s number of constituent particles. Avogadro’s number is N A = x The constituent particles can be atoms or molecules.

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T HE NUMBER OF MOLES The number of moles can be determined from the mass of the substance: M is the molecular weight, or the atomic mass expressed in grams/mole, for example: for He, M = 4.00 g/mol m is the mass of the sample. n is the number of moles.

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C ALCULATING THE N UMBER OF M OLES The number of moles, n, of a gas can be can be calculated using:- Where N is the total number of molecules and N A is Avogadro’s constant (=6.02 × )

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(1) A VOGADRO ' S L AW or V n ( T, p at constant)

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G AS L AWS Boyle’s law. Charles and Gay-Lussac’s law. Guy-Lussac’s law.

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B OYLE ' S L AW When a gas is kept at a constant temperature, its pressure is inversely proportional to its volume, PV=constant Robert Boyle ( )

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(2) C HARLES ' L AW V T ( n, p constant) Jacques Charles ( ), French When a gas is kept at a constant pressure, its volume is directly proportional to its temperature (Charles and Gay-Lussac’s law).

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When the volume of the gas is kept constant, the pressure is directly proportional to the temperature. (see gas thermometer) Guy-Lussac’s law

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We can combine these into a general gas law: The Ideal Gas Equation Boyle’s Law: Charles’s Law: Avogadro’s Law:

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R = gas constant, then The ideal gas equation is: R = L·atm/mol·K = J/mol·K Real Gases behave ideally at low P and high T. The Ideal Gas Equation

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E QUATION OF S TATE Recall that each phase can exist in a variety of states e.g. the temperature and pressure Thus the Ideal Gas Equation of State pV = nRT summarises the physically possible combinations of p, V and T for n moles of the ideal gas. Examples 9,11

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