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Heat & Thermodynamics

Heat & Thermodynamics. Heat. Heat is the transfer of energy between a system and its environment because of a temperature difference between them The symbol Q is used to represent the amount of energy transferred by heat between a system and its environment. Heat.

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Heat & Thermodynamics

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  1. Heat & Thermodynamics

  2. Heat • Heat is the transfer of energy between a system and its environment because of a temperature difference between them • The symbol Q is used to represent the amount of energy transferred by heat between a system and its environment

  3. Heat • The process by which energy is exchanged between objects because of temperature differences is called heat • Objects are in thermal contact if energy can be exchanged between them • Thermal equilibrium exists when two objects in thermal contact with each other cease to exchange energy

  4. Units of Heat • Calorie • A calorie is the amount of energy necessary to raise the temperature of 1 g of water from 1° C . • A Calorie (food calorie) is 1000 cal • US Customary Unit – BTU • BTU stands for British Thermal Unit • A BTU is the amount of energy necessary to raise the temperature of 1 lb of water from 63° F to 64° F • 1 cal = 4.186 J • This is called the Mechanical Equivalent of Heat

  5. Internal Energy • Internal Energy, U, is the energy associated with the microscopic components of the system • Includes kinetic and potential energy associated with the random translational, rotational and vibrational motion of the atoms or molecules • Also includes any potential energy bonding the particles together

  6. Zeroth Law of Thermodynamics • If objects A and B are separately in thermal equilibrium with a third object, C, then A and B are in thermal equilibrium with each other. • Allows a definition of temperature

  7. First Law of Thermodynamics • We cannot get a greater amount of energy out of a cyclic process than we put in

  8. Second Law of Thermodynamics • We can’t break even • Means that Qc cannot equal 0 • Some Qc must be expelled to the environment • Means that e must be less than 100%

  9. Perpetual Motion Machines • A perpetual motion machine would operate continuously without input of energy and without any net increase in entropy • Perpetual motion machines of the first type would violate the First Law, giving out more energy than was put into the machine • Perpetual motion machines of the second type would violate the Second Law, possibly by no exhaust • Perpetual motion machines will never be invented

  10. Celsius Scale • Temperature of an ice-water mixture is defined as 0º C • This is the freezing point of water • Temperature of a water-steam mixture is defined as 100º C • This is the boiling point of water • Distance between these points is divided into 100 segments or degrees

  11. Kelvin Scale • When the pressure of a gas goes to zero, its temperature is –273.15º C • This temperature is called absolute zero • This is the zero point of the Kelvin scale • –273.15º C = 0 K • To convert: TC = TK – 273.15 • The size of the degree in the Kelvin scale is the same as the size of a Celsius degree

  12. Fahrenheit Scales • Most common scale used in the US • Temperature of the freezing point is 32º • Temperature of the boiling point is 212º • 180 divisions between the points

  13. Comparing Temperature Scales

  14. Converting Among Temperature Scales

  15. Ideal Gas • A gas does not have a fixed volume or pressure • In a container, the gas expands to fill the container • Most gases at room temperature and pressure behave approximately as an ideal gas

  16. Moles • It’s convenient to express the amount of gas in a given volume in terms of the number of moles, n • One mole is the amount of the substance that contains as many particles as there are atoms in 12 g of carbon-12

  17. Avogadro’s Number • The number of particles in a mole is called Avogadro’s Number • NA=6.02 x 1023 particles / mole • Defined so that 12 g of carbon contains NA atoms • The mass of an individual atom can be calculated:

  18. Ideal Gas Law, Chemistry Version • PV = n R T • R is the Universal Gas Constant • R = 8.31 J / mole.K • R = 0.0821 L. atm / mole.K • Is the equation of state for an ideal gas

  19. Ideal Gas Law, Physics Version • P V = N kB T • kB is Boltzmann’s Constant • kB = R / NA = 1.38 x 10-23 J/ K • N is the total number of molecules • n = N / NA • n is the number of moles • N is the number of molecules

  20. Specific Heat • Every substance requires a unique amount of energy per unit mass to change the temperature of that substance by 1° C • The specific heat, c, of a substance is a measure of this amount

  21. Heat and Specific Heat • Q = m c ΔT • ΔT is always the final temperature minus the initial temperature • When the temperature increases, ΔT and ΔQ are considered to be positive and energy flows into the system • When the temperature decreases, ΔT and ΔQ are considered to be negative and energy flows out of the system

  22. A Consequence of Different Specific Heats • Water has a high specific heat compared to land • On a hot day, the air above the land warms faster • The warmer air flows upward and cooler air moves toward the beach

  23. Phase Changes • A phase change occurs when the physical characteristics of the substance change from one form to another • Common phases changes are • Solid to liquid – melting • Liquid to gas – boiling • Phases changes involve a change in the internal energy, but no change in temperature

  24. Sublimation • Some substances will go directly from solid to gaseous phase • Without passing through the liquid phase • This process is called sublimation • There will be a latent heat of sublimation associated with this phase change

  25. Methods of Heat Transfer • Methods include • Conduction • Convection • Radiation

  26. Conduction example • Energy transferred by the movement of molecules that are in direct contact with each other.

  27. Convection • Energy transferred by the movement of a substance • When the movement results from differences in density, it is called natural conduction • When the movement is forced by a fan or a pump, it is called forced convection

  28. Convection Current Example • The radiator warms the air in the lower region of the room • The warm air is less dense, so it rises to the ceiling • The denser, cooler air sinks • A continuous air current pattern is set up as shown

  29. Radiation • Radiation does not require physical contact • All objects radiate energy continuously in the form of electromagnetic waves due to thermal vibrations of the molecules

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