Physics 102
Download
1 / 23

Heat - PowerPoint PPT Presentation


  • 202 Views
  • Uploaded on

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

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' Heat' - mandel


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.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

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
PAL #2 Galileo Thermometer

  • How does it work?

  • Limitations


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


Measuring heat
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)


Heat and temperature
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


Specific heat
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


Today s pal
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.


Calorimetry
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


Example quenching a dagger
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)


How does heat move
How Does Heat Move?

  • Heat (like information) is transferred in different ways

  • Conduction

  • Radiation

  • Convection


Conduction
Conduction

  • Why?

    • They interact and collide with other atoms and electrons and pass the energy on


Conduction rate factors
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


Radiation
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


Radiation rate factors
Radiation Rate Factors

  • Surface area

  • Emissivity

    • Radiation is strongly dependant on T



Convection
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


  • Convection rate factors
    Convection Rate Factors

    • Fluidity

    • Energy exchange with environment

      • ?

      • How rapidly will the material lose heat?

      • Small temperature difference, not enough density difference to move


    Today s pal1
    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?


    Conduction diagram
    Conduction Diagram

    A

    T2

    T1

    Q

    L


    Conductive heat transfer
    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


    Radiative heat transfer
    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


    Radiation exchange
    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


    Next time
    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


    ad