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UNIT HEAT AND TEMPERATURE The discovery of fire and the controlled use of fire are among the most valuable human discoveries Nearly a million years ago, humans already knew how to use fire to create light and heat, to cook plants and animals, and to keep predators away On basis, whenever standing close to a heat source, we feel hot However, if we touch a good thermal flask on its surface, we not feel hot at all Have you ever asked yourself why there is a difference; in other words, “what is heat” and “how is heat transferred from an object to the other”? Back To answer these questions, we should recall the basic principle that heat can only be transferred via a specific environment of matter According to the Kinetic Theory of Matter, matter is composed of a large number of atoms or molecules These atoms or molecules carry (contain) both the kinetic energies resulted from their motion and potential energy resulted from interaction or the changing of position of the forces among them Back 3.1: A reconstruction of Homo erectus mak Back A body of matter can be viewed as a system of atoms, and each system has a typical amount of energy known the “internal energy” of that system, which is sum of the total kinetic and potential energies of all the atoms or molecules in the system at rest as a whole When that internal energy is transferred between two bodies as a result of their different temperatures, this energy is called heat [2] Heat is thus the amount of internal energy flowing from a body at a higher temperature to a body at a lower one, raising the temperature of the latter and lowering that of the former substance, provided that the volumes of the bodies remain constant as shown in Fig 3.3 Fig.3.3: Transfering of energy from one part of a substance to another [2] Heat is energy in transform: when the body cools down, its internal energy decreases, when it is heated, its internal energy increases Whenever two bodies with different temperature are brought into contacts, thermal energy always flows from the hotter body to the cooler one until they are both at the same temperature When this occurs, we say two bodies are in thermal equilibrium Therefore, in principle, heat does not flow from a lower to a higher temperature environment unless another form of energy has transferred in the opposite direction, or work is also presented Back [3] Until the beginning of the 19th century, the effect of heat on the temperature of a body was explained by postulating the existence of an invisible substance or form of matter termed caloric According to the caloric theory of heat, a body at a higher temperature contains more caloric than one at a lower temperature So, when two bodies at different temperatures are brought into contacts, hotter body loses some caloric to the cooler one until they are both at the same temperature Back [3] Although the caloric theory successfully explained some phenomena of heat transfer, experimental evidence was presented by the American – born British physicist Benjamin Thompson (later known as Count von Rumford) in 1798 and by the British chemist Sir Humphry Davy in 1799 suggesting that heat, like work, is a form of energy in transform Between 1840 and 1849 the British physicist James Prescott Joule, in series of highly accurate experiments, conclusively confirmed that heat is a form of energy in transform and that it can cause the same changes in a body as work [4] The sensation of warmth or coldness of a substance on contact is determined by the property known as temperature Although it is relatively easy to compare the relative temperature of two substances by the sense of touch, it is impossible to evaluate the absolute magnitude of the temperatures by subjective reactions Temperature depends on the average kinetic energy of the molecules of the substance Back [5] Thermal systems change as the thermodynamic variables change (P,V,T) It’s possible to have processes in which only two parameters change, another parameter keeps being unchanged, these processes are called isoprocess The process of state transformation in which the temperature is kept unchanged is called isothermal process It is Boyle- Mariotte’s law “a certain amount of gas, the pressure inverses proportionally to the volume or PV = const while T=const.” In addition, the process of state transformation when the volume is unchanged is the isochoric process Back [5] This can call as Charles’s law “a certain amount of gas, the pressure is directly proportional to the absolute temperature, while V=const.” Moreover, the process transforming the state without changing the air pressure is called isobaric process It is Guy – Lussac’s law “a certain amount of gas, the volume is directly proportional to the absolute temperature l ; P=const.” Back [6] The international thermodynamic temperature scale: In 1954, the triple point of water – that is, the point at which the three phases of water (vapor, liquid, and ice) are in equilibrium – was adopted by international agreement as 273,16 K(see Fig 3.5) In cryogenics, or low- temperature research, temperatures as low as 0,003 K have been produced by the demagnetization of paramagnetic materials Momentary high temperatures estimated to be greater than 100,000,000 K have been achieved by nuclear explosions Back [6] Back As we have known that heat energy always travels from [7] the hotter object (higher temperature) to the colder object (lower temperature) Heat transfer between objects of different temperatures until an equilibrium point is reached as shown in Fig 3.6 However, one question is raising that how can the heat transfer? Heat travels from one object to another object in three ways (1) by conduction, (2) by convection, and (3) by radiation Back CONDUCTION: Heat can move through a substance by conduction especially in solids matter such as metals when we heat up a piece of metal (see Fig 3.7.) The electrons in a piece of metal can leave their atoms and move inside the metal body as free electrons The parts of the body’s atoms left behind are now charged metal ions The ions are packed closely together and they vibrate continually The hotter the metal, the more kinetic energy these vibrations have This kinetic energy is transferred from hot parts of the metal to cooler parts by the free electrons These move through the structure of the metal, colliding with ions as they go Back CONDUCTION: In this process, heat will continue to transfer until every part of the object has the same temperature While metals are good conductors for heat, non-metals and gases are usually poor heat conductors Poor heat conductors are called insulators Heat energy is conducted from the hot end of an object to its cold end Back CONVECTION Convection is another mean of heat transfer It occurs only in fluids (liquid and gas) When the liquid is heated, the particles of the liquid receive energy, the liquid also expands and becomes less dense The hot particles rises up against gravity then transfer some energy to the cooler particles around, and cool down After that, the hot fluid becomes more dense and falls (sinks) down to the bottom While this process occurs, the cooler particles at the top moves down to replace the hotter ones and starts a new cycle An example of convection is the Earth atmosphere During the day, the ground heats up more quickly than water in the sea, because water has a greater specific heat The air in contact with the warm ground is heated Back CONVECTION It expands and become lighter As a result, the warm air rises up, resulting in air currents and cool air moves down to fill the space This creates a air cycle or sea breeze In this way, thermal convection cycle is set up, which transfer heat away from land (see Fig 3.8) Back CONVECTION The next example is the convection current occurs in the aesthenophere The inner core of the Earth generates huge amount of heat energy and travels out through the outer core and to the aesthenophere (mantle) where it is much cooler It heats the viscous material at the bottom of the mantle and make up the mantle moves The hotter material at the bottom of matle moves towards the top and begin cool down as heat lost to the crust The cooler material at the top of the mantle starts sink to the bottom, it warms up and then moves towards to the top again This cyling of hot and cool material is called the covection current As a results, the trench and the ridge is created by the mantle convection (see firgure 3.8) Back CONVECTION Back RADIATION The transfer of heat by convection and conduction requires a material medium for the process to take place However, the Sun can transfer its heat to the Earth through the vacuum Because heat from the Sun reaches the Earth via electromagnetic waves The process of transferring heat energy through space by means of electromagnetic wave is known as radiation (see Fig 3.9) Electromagnetic waves carry energy and can travel through a vacuum The heat we get from the Sun is transmitted through the vacuum of space by radiation Back RADIATION Another example of heat transfer by radiation is heat from a campfire or wood stove We can readily feel warmth of the fire on exposed skin Some of the radiant energy that falls upon matter may be absorbed and converted to heat Dark-colored objects are good absorbers and poor reflectors of heat energy Black rough surfaces absorb heat and transfer heat well White or light-colored objects and highly polished, smooth surfaces such as aluminum reflect most of the heat rather than absorb it Back RADIATION Back ... British physicist Benjamin Thompson (later known as Count von Rumford) in 1798 and by the British chemist Sir Humphry Davy in 1799 suggesting that heat, like work, is a form of energy in transform... Between 1840 and 1849 the British physicist James Prescott Joule, in series of highly accurate experiments, conclusively confirmed that heat is a form of energy in transform and that it can cause the... say two bodies are in thermal equilibrium Therefore, in principle, heat does not flow from a lower to a higher temperature environment unless another form of energy has transferred in the opposite