Heat & Thermodynamics

1. Heat & By iTutor.com T- 1-855-694-8886 Email- [email protected] Thermodynamics

2. Energy that flows from something warm to something cooler A hotter substance gives KE to a cooler one Heat is the transfer of energy between two objects due to temperature differences Heat is measured in Joules (J) because it is a form of energy Heat

3. Heat The name of the transfer process is heat . What gets transferred is energy. Heat is NOT a substance although it is very convenient to think of it that way. In fact, it used to be thought that heat was a substance. There is a circular nature to the definitions used: (a) energy does work or produces heat, but (b) heat is a transfer of energy.

4. Temperature Temperature is the property which determines the direction heat will flow when two objects are brought into contact Based on the motion of the particles in a substance Fast motion = high temperature Slower motion = lower temperature Relatively describes how the particles collide with the surface of the thermometer Collisions transfer energy

5. Temperature Scales Fahrenheit Common in the US Freezing point of water is 32 ºF Boiling point of water is 212ºF Celsius Common in the rest of the world (SI) Freezing point of water is 0 ºC Boiling point of water is 100ºC Kelvin Used for science Freezing point of water is 273K Boiling point of water is 373K

6. Temperature and the Phases of Matter In the Fahrenheit scale , water freezes at 32 degrees and boils at 212 degrees The Celsius scale divides the difference between the freezing and boiling points of water into 100 degrees (instead of 180).

7. Temperature and the Phases of Matter All thermometers are based on some physical property (such as color or volume) that changes with temperature. A thermometer is a device that changes its electrical resistance as the temperature changes. A thermocouple is another electrical sensor that measures temperature.

8. Heat and Thermal Energy Temperature is NOT the same as thermal energy. Thermal energy is energy stored in materials because of differences in temperature. The thermal energy of an object is the total amount of random kinetic energy for all the atoms in the object. Remember, temperature measures the random kinetic energy of each atom.

9. Heat and Thermal Energy Imagine heating a cup of coffee to a temperature of 100°C. Next think about heating up 1,000 cups of coffee to 100°C. The final temperature is the same in both cases but the amount of energy needed is very different.

10. First Law of Thermodynamics Energy loss is equal to energy gain.

11. Second Law of Thermodynamics

12. Energy and Heat flow There Energy , in the process we call heat or heat flow , is constantly flowing into and out of all objects, including living objects.  Heat flow moves energy from a higher temperature to a lower temperature . The bigger the difference in temperature between two objects, the faster heat flows between them.    When temperatures are the same there is no change in energy due to heat flow. Radiation and Conduction are the two methods of heat transfer. Convection is a special type of conduction. Heat has the units of energy; heat flow has the units of power .

13. Heat flow is energy moving. It has the same units as power - energy per unit time . It means that during the given amount of time, during which heat is flowing, a certain amount of energy is transferred or moved from one place to another place Energy and Heat flow

14. T hree easy things to know about the way heat flows: 1 )   There has to be a temperature difference. Energy only flows as heat if there is a temperature difference. 2 )    Energy as heat flows from a higher temperature to a lower temperature . 3)   The greater or larger the difference in temperature, the faster the energy flows . Heat flow

15. The science of how heat flows is called heat transfer . There are three ways heat transfer works: conduction , convection , and radiation . Heat flow depends on the temperature difference . Heat flow

16. Heat flow

17. Heat Conduction Conduction occurs when two object at different temperatures are in contact with each other. Heat flows from the warmer to the cooler object until they are both at the same temperature. Conduction is the movement of heat through a substance by the collision of molecules. At the place where the two object touch, the faster-moving molecules of the warmer object collide with the slower moving molecules of the cooler object.

18. Heat Conduction Conduction is the transfer of heat through materials by the direct contact of matter. Dense metals like copper and aluminum are very good thermal conductors . A thermal insulator is a material that conducts heat poorly.

19. Thermal Conductivity The thermal conductivity of a material describes how well the material conducts heat.

20. Heat Conduction Equation P H = k A (T 2 -T 1 ) L Area of cross section (m2) Length (m) Thermal conductivity (watts/m o C) Heat flow (watts) Temperature difference ( o C )

21. Convection Convection is the transfer of heat by the motion of liquids and gases. Convection in a gas occurs because gas expands when heated. Convection occurs because currents flow when hot gas rises and cool gas sink. Convection in liquids also occurs because of differences in density.

22. Convection When the flow of gas or liquid comes from differences in density and temperature, it is called free convection . When the flow of gas or liquid is circulated by pumps or fans it is called forced convection .

23. Convection Both free and forced convection help to heat houses and cool car engines.

24. Convection Convection depends on speed and surface area . Motion increases heat transfer by convection in all fluids . If the surface contacting the fluid is increased, the rate of heat transfer also increases.

25. Convection and Sea Breezes On a smaller scale near coastlines, convection is responsible for sea breezes . During the daytime, land is much hotter than the ocean. A sea breeze is created when hot air over the land rises due to convection and is replaced by cooler air from the ocean. At night the temperature reverses so a land breeze occurs.

26. Heat Convection Equation P H = h A (T 2 -T 1 ) Area contacting fluids (m 2 ) Heat transfer coefficient (watts/m 2o C) Heat flow (watts) Temperature difference ( o C)

27. Radiation Radiation is heat transfer by electromagnetic waves. Thermal radiation is electromagnetic waves (including light) produced by objects because of their temperature. The higher the temperature of an object, the more thermal radiation it gives off.

28. Radiation Radiation Heat from the sun is the best example of radiation . The sun is rays reach the earth without having any material medium in between the earth and the sun. The electromagnetic rays carry energy from the sun is surface to the earth and heat up the earth and its atmosphere .

29. Radiant Heat We do not see the thermal radiation because it occurs at infrared wavelengths invisible to the human eye. Objects glow different colors at different temperatures.

30. The graph of power versus wavelength for a perfect blackbody is called the blackbody spectrum. Radiant Heat A perfect blackbody is a surface that reflects nothing and emits pure thermal radiation.

31. Radiant Heat The total power emitted as thermal radiation by a blackbody depends on temperature ( T ) and surface area ( A ). Real surfaces usually emit less than the blackbody power, typically between 10 and 90 percent. The Kelvin temperature scale is used in the Stefan-Boltzmann formula because thermal radiation depends on the temperature above absolute zero.

32. Stefan-Boltzmann formula P = s AT 4 Surface area (m 2 ) Stefan-Boltzmann constant 5.67 x 10 -8 watts/m 2 K 4 ) Power (watts) Absolute temperature (K)

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