1 / 29

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.

daniel-kelley
Télécharger la présentation

Heat & Thermodynamics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related