Peoples Physics Concepts

(Marvins-Underground-K-12) #1

http://www.ck12.org Chapter 18. Thermodynamics and Heat Engines


18.1 Temperature



  • Explain the concept of temperature, where it comes from and how it relates to our everyday world.


Students will learn the concept of temperature, where it comes from and how it relates to our everyday world.

Key Equations


KEavg=^12 mv^2 avg=^32 kT
The average kinetic energy of atoms (each of massmand average speedv) in a gas is related to the temperatureT
of the gas, measured in Kelvin. The Boltzmann constantkis a constant of nature, equal to 1. 38 × 10 −^23 J/K
TF=^95 TC+ 32 ◦F ; conversion from Celsius to Fahrenheit
TC=^59 (TF− 32 ◦F); conversion from Fahrenheit to Celsius

Guidance



  • When an object feels cold to the touch, it is because heat is flowing from you to the object.

  • When an object feels hot to the touch, it is because heat is flowing from the object to you.

  • Some objects (like metals) conduct heat better than others (like wood). Thus if you stick a metal rod in the
    fireplace and hold the other end, the heat is conducted well and you get burned. On the other hand, if you
    place a wood stick in the fire and hold the other end you’ll be OK.

  • The temperature of a gas is a measure of the amount of average kinetic energy that the atoms in the gas possess.

  • If you heat something, you increase its internal energy, so you increase the movement of molecules that make
    up this thing, thus it expands. This is called heat expansion; most everything expands as heated and contracts
    as cooled.

  • Most materials expand as they are heated. This can cause bridges to collapse if they are not designed to have
    a place to expand in the summer months (like the placing of metal ’teeth’ at intervals on the Golden Gate
    Bridge).

  • Watercontractsfrom 0◦Cto 4◦Cand then expands from 4◦Cto 100◦C. Remembering that density is mass
    divided by volume explains why water at 4◦Cis more dense than water below and above 4◦C. This also
    explains why lakes freeze on the top first and not throughout. As the water on the top of the lake drops below
    4 ◦C, it is now more dense than the water below it, thus it sinks to the bottom, allowing the warmer water to
    rise up to the top and cool down in the winter weather. Only when the entirety of the lake is at 4◦C, then the
    lake can start to freeze. It freezes from the top down, because water below 4◦Cis less dense than water at 4◦C.

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