1 / 23

Heat

Physics 102 Professor Lee Carkner Lecture 3. “If you can’t stand the heat, get out of the kitchen.” -Harry S. Truman. Heat. PAL #2 Galileo Thermometer. How does it work? Limitations. Heat. What is heat? Same temperature, no heat

mandel
Download Presentation

Heat

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. Physics 102 Professor Lee Carkner Lecture 3 “If you can’t stand the heat, get out of the kitchen.” -Harry S. Truman Heat

  2. PAL #2 Galileo Thermometer • How does it work? • Limitations

  3. Heat • What is heat? • Same temperature, no heat • Heat used to be thought of a fluid (caloric) that could flow to change temperature • Heat is represented by the letter Q

  4. Measuring Heat • Common unit of heat is the calorie: • Amount of heat necessary to increase the temperature of 1 gram of water by 1 C • In nutrition the Calorie is used • Case sensitive! • For rates of heat transfer (Q/t), unit is the Watt (W)

  5. Heat and Temperature • If you heat a metal spoon and a wooden spoon for the same time, which will have a higher T? • The specific heat

  6. Specific Heat • The specific heat is defined as: • c has units of J/kg C • Need to know the mass of the stuff (m) and the change in temperature (DT) Q =mcDT

  7. Today’s PAL • A certain amount of heat Q will warm 1 g of material A by 3 degrees C and 1 g of material B by 4 degrees C. Which material has the greater specific heat? Explain.

  8. Calorimetry • Insulated container that prevents heat transfer from outside • Since calorimeter is insulated, negative heat lost cancels out positive heat gained • Q1 + Q2 + Q3 … = 0 • Heat gained always positive, heat lost always negative • Make sure units for T and m match units for c

  9. Example: Quenching a Dagger • Suppose a silver dagger of mass ms at Ts is immersed in a mass mw of water at Tw. What is the final temperature of the water? Qsilver + Qwater = 0 csmsDT + cwmwDT = 0 csms(Tf - Ts) + cwmw(Tf- Tw) = 0 csmsTf -csms Ts + cwmwTf - cwmw Tw = 0 csmsTf + cwmwTf = csms Ts + cwmw Tw Tf = (csms Ts + cwmwTw)/(csms+ cwmw)

  10. How Does Heat Move? • Heat (like information) is transferred in different ways • Conduction • Radiation • Convection

  11. Conduction • Why? • They interact and collide with other atoms and electrons and pass the energy on

  12. Conduction Rate Factors • Free electrons • Density • Cross sectional area • Large window loses more heat than small • Temperature difference • Thickness • Heat takes less time to move through thinner material

  13. Radiation • How does the energy from the Sun get to Earth? • How can energy be transported with no physical contact? • Photons are emitted by the Sun and absorbed by you • All objects emit photons

  14. Radiation Rate Factors • Surface area • Emissivity • Radiation is strongly dependant on T

  15. The Surface of the Sun

  16. Convection • Hot air is less dense than the cooler air above it • After cooling the air may fall back down • Examples: baseboard heating, boiling water, Earth’s atmosphere

  17. Convection Rate Factors • Fluidity • Energy exchange with environment • ? • How rapidly will the material lose heat? • Small temperature difference, not enough density difference to move

  18. Today’s PAL • A hot piece of metal is at the bottom of a canister that can be completely filled with: • solid iron • liquid water • air • a vacuum • Consider the heat flow from the bottom to the top. • In which situation(s) would there be no conduction? • In which situation(s) would there be no convection? • In which situation(s) would there be no radiation?

  19. Conduction Diagram A T2 T1 Q L

  20. Conductive Heat Transfer • The rate of heat transfer via conduction is: • where: • T1 is the temperature of the hot side and T2 is the temperature of the cold side • A is the cross sectional area • L is the thickness • k is the thermal conductivity • High k = large heat transfer • Low k = small heat transfer

  21. Radiative Heat Transfer • The amount of heat radiated out from an object is called the power (P): • where • s = the Stefan-Boltzmann constant • 5.6696 X 10-8 W/m2 K4 • A is the surface area • e is the emissivity (number between 0 and 1) • 0 = • perfect reflector • 1 = • perfect absorber or black body

  22. Radiation Exchange • All objects emit and absorb radiation Pnet = sAe(T4-T42) • Where T2 is the temperature of the surroundings • Note that T must be in Kelvin

  23. Next Time • Read: 13.6-13.11 • Homework: CH 14, P: 13, 47, CH 13, P: 29, 48 • Help sessions start next week • Tuesday and Thursday 6-8pm Science 304

More Related