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Thermal energy

Thermal energy

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Thermal energy

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  1. Thermal energy Heat and heat effects

  2. KINETIC MOLECULAR THEORY: (KMT) • 1) All matter is composed of very small particles. • 2) The particles of matter are in constant random motion and possess kinetic energy. • 3) Particles collide with each other and the container walls with perfectly elastic collisions. • 4) Empty space exists between particles which is large compared to the size of the particles. • 5) The particles move faster with an increase of temperature absorbing the heat energy transforming it into kinetic energy of the particles.

  3. KINETIC MOLECULAR THEORY: (KMT) • At a specific state change, heat is no longer used to increase kinetic energy of atoms or molecules but rather used to break bonds. • There is no increase in temperature during a change of state. • Thermal energy is definedas the total kinetic and potential energy of the atoms or molecules of a substance. It depends on mass, temperature, nature and state.

  4. Definitions • Heat: A measurement of energy transfer from a hot body to a cold one. • Temperature: A measure of the average kinetic energy of the atoms or molecules of a substance. • T increases as the motion of the particles increases. • Thermometers are used to measure temperature. They contain mercury or alcohol. • Mercury freezes at -39oC and boils at 357oC. • Alcohol freezes at -117oC and boils at 79oC

  5. Thermometer scales • Anders Celsius (1701-1744) based his temperature scale on water 0oC to 100oC and used 100 divisions on a scale; 1 division/degree. • It is possible to use the known boiling and freezing temperatures of any liquid to make your own thermometer!! • William Thompson Kelvin (1824-1907) invented a scale with a zero point of -273.14oC, the coldest temperature possible; absolute zero when matter collapses. • One Kelvin degree = one Celsius degree.

  6. Methods of Heat Transfer • Temperature decides the direction of spontaneous energy transfer by; • Conduction • Convection • Radiation

  7. Conduction • Takes place within materials and from one to another while in contact; metals make the best conductors. • An insulator does not conduct heat well (wood, wool, paper, cork). • When heated, electrons move rapidly in conductors (as electrons are held loosely) which collide with other electrons transferring kinetic energy and therefore temperature.

  8. Conduction • Ex: Touch wood and metal; the metal feels cool as it transfers your heat. Tiles and carpet are similar. • Blankets slow the transfer of heat. • Snow slows down the escape of heat from Earth’s surface. • Liquids and gases are generally insulators. • Cold is the absence of heat

  9. Convection • Air in contact with a heat source rises to warm the region above occurs in all fluids via currents. • A fluid heated from below, expands and becomes less dense, and then rises.

  10. Convection • Ex: You can place your hand around a candle flame but not above it: • Air is an insulator but convection occurs above the candle flame. • Wind is the result of convection currents stirring the atmosphere (hot air rises and cool air rushes in underneath).

  11. Convection • Earth heats unevenly as it absorbs heat differently. (causes convection currents) • At the beach: the sand warms faster than the water so the air rises up causing a greater air pressure above the shore. • At a ~2 km height, air blows out to the water, on the beach, a wind blows onshore (cyclic). • At night, the process reverses as sand gives up heat faster than water.

  12. radiation • Radiant energy is transmitted by radiation in the form of waves • Ex: Radio, micro, infrared, visible, ultraviolet, x-rays, gamma rays( in order from long to short wavelengths.) • All objects continually give off radiant energy of different wavelengths.

  13. radiation • Low temperature objects emit longer wavelengths. • When an object becomes hot enough, the wavelength decreases to visible light (like light bulbs). (Red is ~500oC and white at 1200oC ..... white hot!!) • Objects absorb andreflect Infra Red (IR) and the visible light of the sun. • Absorption increases the internal energy of the object.

  14. HEAT CAPACITY • The heat capacity of a material determines the amount of heat that can be added to a sample of matter. • It is the amount of heat, Q, required to raise an object’s temperature 1oC measured in J/oC

  15. Specific Heat Capacity • The amount of heat required to raise the temperature of a mass of 1 kg by 1oC measured in J/kgoC. • Different materials can absorb heat differently. • Water has a very high specific heat capacity making it useful for radiators: • cw= 4184 J/kgoC

  16. Specific Heat Capacity • Q = mc∆T • where Q is in Joules • m is in kg • c is in J/kgoC • ∆T is inoC • 1 cal= 4.184 J

  17. Factors affecting Heat Capacity • 1) Mass: an increase in mass will increase heat capacity • 2) Temperature change: a large ∆T requires more heat • 3) Material: different materials require different Q to raise the T by oC, so a different c value. (specific heat capacity). • *You should be able to explain these factors using KMT.

  18. example • Find the amount of heat transferred to 200 g of water heated from 20oC to 60oC. • Q = mc∆T • = (.200 kg) (4184 J/kgoC) (60-20)oC • = 3.3 x 104J

  19. Heat of mixtures • The cold component gains heat (heat always flows from hot to cold). • We will assume no energy is lost to the surroundings to simplify these problems. • Qlost= Qgained • - (mc∆T)hot = (mc∆T)cold

  20. Example 1 • You mix 100 g of water at 80oC with 100 mL of water at 20oC .What is the final temperature of this mixture? • Qlost = Qgained • -(mc∆T)hot= (mc∆T)cold • -(.100 kg)(4184J/kgoC)(Tf- 80oC) = (0.100kg)(4184J/kgoC)(Tf - 20oC) • -(Tf-80oC) = (Tf - 20oC) • -2Tf = -20oC - 80oC • Tf = 50oC

  21. Example 2 • Jello™ Party!! Jello™ has a specific heat capacity of 480 J/kgoC and the human body has a specific heat capacity of 3470 J/kgoC. You dive into 100 kg of Jello™ and your mass is 55 kg. Your body temperature is 37oC and the Jello™ is 10oC, Find the final temperature of this mixture. • Qlost= Qgained • -(mc∆T)body = (mc∆T)Jello • = - (55kg)(3470J/kgoC)(Tf - 37oC) = (100kg)(480J/kgoC)(Tf - 10oC) • Tf= 32oC

  22. Heat of fusion • The quantity of heat required to melt a unit of mass of a solid at constant temperature. • The heat released by a unit mass of a liquid crystallizing at constant temperature. • Standard units are: (J/kg) • Q = mLf

  23. Heat of vaporization • The quantity of heat required to vaporize a unit of mass of a liquid at constant temperature • The heat released by a unit of mass of a gas condensing at a constant temperature • Standard units are: (J/kg)oC • Q = mLv

  24. Example • How much heat is required to melt 100 g of ice and then raise its temperature to 70.0oC? • As we have to change state and raise temperature; • Q = mLf + mc∆T • = (100g)(80 cal/g)(4.184 J/cal) + (0.100kg)(4184J/kgoC)(70.0oC) • = 6.28 x 104 J • Unit analysis in this unit will be very helpful!