Chapter 15 Heat Capacity Thermal Expansion Chapter 16 Heat Transfer

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1 Lecture 17 Chapter 15 Heat Capacity Thermal Expansion Chapter 16 Heat Transfer Internal Energy Internal energy of an object depends on: Temperature Mass Material Iron 1 kg 1000 grams Temperature Internal Energy 300 K 120,000 J 200 K 80,000 J 100 K 40,000 J 0 K 0 Joules Water Temperature Internal Energy 300 K 1,200,000 J Iron Temperature Internal Energy 300 K 120 J 1 kg 200 K 800,000 J 1 gram 200 K 80 J 100 K 400,000 J 100 K 40 J 0 K 0 Joules 0 K 0 Joules

2 Specific Heat Capacity Specific heat capacity is the amount of heat energy required to raise the temperature of one unit mass of a material by one degree. SI Unit: J/(kg K) or J/(kg C) Other Units: cal/(g C) Heat energy needed to raise temperature of material by T is: (Specific heat cap. of material) (mass) T Some specific heat capacity values. Specific heats of gases are complicated.

3 Example How much heat energy is needed to raise the temperature of 2 kg of copper (s.h.c. = 387 J/kg-K) from 10 to 30 C? Q = (s.h.c.) m T = (387 J/kg-K)(2kg)(20K) = 1.55 x 10 4 J How long would it take for a 1000 W heater to do this? Power = (Energy provided)/ t t = (Energy needed)/(power) = 15500J/(1000 J/s) = 15.5s Check Yourself Why does a piece of watermelon stay cool for a longer time than sandwiches do when both are removed from a cooler on a hot day? Why is it that the climate in the desert is so hot during the day yet so cold at night?

4 Thermal Expansion Due to increased molecular motion, most materials expand as temperature increases. Space allows for expansion Sidewalk buckles and cracks due to expansion on a hot summer day Demo: Expansion of a Ring Metal ball barely fits past the metal ring. Not surprising that heated ball won t pass through cold ring. Will cold ball pass through heated (expanded) ring?

5 Coefficients of Linear Expansion Thermal Expansion Differences A bimetallic strip has two metals of different coefficients of thermal expansion, A and B in the figure. It will bend when heated or cooled.

6 Demo: Heat, Cool, Break Glass expands when heated. If hot glass is cooled quickly, exterior cools before the interior. Exterior contracts faster than the interior, cracking the glass. Pyrex glass expands much less than regular glass. GLASS Cracks form COOL (quickly) HEAT Thermal Expansion The amount of thermal expansion of length L is: L = (expansion coefficient) L T An area gets linear expansion in both directions. Holes expand as well:

7 You have a (glass) jar and you can t get the metal lid off. What should you do: a) ask your friend b) run the jar & lid under cold water c) run the jar & lid under hot water You have a (glass) jar and you can t get the metal lid off. What should you do: a) ask your friend b) run the jar & lid under cold water c) run the jar & lid under hot water Because the metal has a substantially higher coefficient of thermal expansion than the glass, heating them will make both of them bigger, but the metal will be more bigger.

8 Water Density vs. Temperature This explains why lakes freeze from the top. Heat Transfer (Flow of Heat Energy) Three Methods Conduction - Thermal kinetic energy passed from particle-to-particle along a length of material. Convection - Thermal energy carried by moving fluid. Radiation - Thermal energy carried by electromagnetic waves.

9 Heat Transfer: Conduction Heat conduction can be visualized as occurring through molecular collisions. Thermal kinetic energy is passed along as hotter particles collide with colder ones. Crosssectional area Q L Conduction is heat flow by direct contact. Some materials are good thermal conductors, others are insulators. Conduction Tile floor feels colder than wood floor 98º 75º Wood is an insulator 98º 75º Tile is a conductor

10 Conduction Experimentally, it is found that the amount of heat Q that flows through a piece of material: Increases proportionally to the crosssectional area A Increases proportionally to the temperature difference T from one end to the other Increases steadily with time t Decreases with the length L of the piece Depends on the thermal conductivity of the material. More conductive more heat flows Thermal Conductivity Some typical thermal conductivities: Substances with high thermal conductivities are good conductors of heat; those with low thermal conductivities are good insulators. Vacuum has a thermal conductivity = 0.

11 Convection Convection is flow of fluid due to difference in temperatures, such as warm air rising. Fluid carries heat with it as it moves. Natural convection: Warm fluid will rise because it is less dense then cold fluid. Heat Transfer: Convection Convection occurs when heat flows by the mass movement of molecules from one place to another. It may be natural or forced (fans); both these examples are natural convection.

12 Convection Heat transfer in a fluid often occurs mostly by convection. Buoyancy causes warm air to rise, which carries thermal energy directly by its motion. Convection Oven Convection oven has a fan to enhance the circulation of the air, increasing the transfer of heat.

13 Fiberglass Insulation Air is a poor thermal conductor but easily transfers heat by convection. Fiberglass insulation is mostly air, with the fibers disrupting the convection flow. Radiation All objects give off energy in the form of radiation, as electromagnetic waves infrared, visible light, ultraviolet which, unlike conduction and convection, can transport heat through a vacuum. Objects that are hot enough will glow visibly first red, then yellow, white, and blue as temperature increases. Objects at body temperature radiate in the infrared, and can be seen with night vision binoculars.

14 Radiation has many different wavelengths, most of which are not visible to the eye. All radiation carries energy, and thus transfers heat. Radiation Heat Lamp Physics 1 (Garcia) SJSU Emission of Radiant Energy All objects radiate; higher the temperature, the higher the frequency. At room temperature, the radiated light is at frequencies too low for our eyes to see. Special cameras are sensitive to this infrared radiation º 75º Attics in this house were kept warm for growing marijuana.

15 Reflection of Radiant Energy White and silver objects reflect light, black objects and holes don t. White tubes look black inside. Hole in a box with white interior looks black because almost none of the light entering the hole reflects back out. Black objects are also the best emitters of radiation. White objects emit less radiation, and perfectly reflective objects don t emit at all. (Space blanket.) Controlling Heat Transfer Thermos bottle eliminates conduction and convection by having doublewalled sides with vacuum. Silvered interior walls minimize heat transfer by radiation.

16 Radiation If you are in sunlight, Sun s radiation will warm you. The intensity of solar radiation is 1000 W/m 2. In general, you will not be perfectly perpendicular to the Sun s rays, and will absorb energy at a rate that depends on your angle to the sun s rays. Seasons This angle effect is also responsible for the seasons.

17 Greenhouse Effect Glass is transparent to sunlight (short-wavelength). Glass is opaque to infrared radiation (long-wavelength) produced by objects inside greenhouse, trapping the heat. Physics 1 (Garcia) SJSU Earth s Greenhouse Effect Earth s atmosphere acts as a greenhouse, trapping solar energy. Most of the trapping is due to carbon dioxide and water vapor, which is why they re called greenhouse gasses.

18 Key Points of Lecture 17 Specific Heat Capacity Thermal Expansion Transfer of Heat by Conduction Transfer of Heat by Convection Transfer of Heat by Radiation Before Friday, read Hewitt Chap. 16. Homework Assignment #12 is due before 11:00 PM on Friday, Oct. 8.