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Chapter 16: Thermal Energy and Heat

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Chapter 16: Thermal Energy and Heat Section 1: Thermal Energy and Matter Temperature We think of temperature in terms of hot and cold, but what exactly is temperature? – PowerPoint PPT presentation

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Title: Chapter 16: Thermal Energy and Heat


1
Chapter 16 Thermal Energy and Heat
  • Section 1 Thermal Energy and Matter

2
Temperature
  • We think of temperature in terms of hot and cold,
    but what exactly is temperature?
  • Temperature is a term that describes the average
    kinetic energy of the atoms or molecules that
    make up a substance.
  • As the atoms move faster, we perceive an object
    to be hotter.
  • If atoms are moving more slowly, we perceive an
    object to be cooler.

3
Motion of the Molecules
  • Molecules that are moving very fast (we think of
    these things as warm or hot) bump into the
    molecules of objects that are moving more slowly.
  • When a fast atom bumps into a slow atom, some of
    the energy is transferred to the slower moving
    atom (remember the Law of Conservation of
    Momentum?).

4
Thermal Energy Temperature
  • The sum of all of the kinetic and potential
    energy of all the molecules in an object is known
    as thermal energy.
  • As the temperature of an object increases, the
    kinetic energy of the molecules in the object
    increase. As a result of this, the total thermal
    energy of the object increased due to the
    increase in temperature.

5
Thermal Energy and Mass
  • If two objects with different masses are at the
    same temperature, the object with the larger mass
    will have more thermal energy.
  • This is the case because the larger object will
    be composed of more molecules and the sum of the
    thermal energy will be greater.

6
Heat
  • Heat is thermal energy that flows from something
    at a higher temperature to something at a lower
    temperature.
  • Heat always goes from HOT ? COLD
  • So if your mom says Close the door, youre
    letting the cold in!, is this a correct
    statement?
  • NO, in fact you are letting the heat out!

7
Specific Heat
  • Specific heat is the amount of heat required to
    heat a 1 kg object one degree Celsius.
  • Specific heat is measured in J / kg.C.
  • Water is a common substance that has a very high
    specific heat. It takes 4,184 joules of energy to
    raise one kg of water one degree C. Water can
    absorb a great deal of heat without changing
    temperature.

8
Specific Heat values
  • The lower the specific heat value of a substance,
    the less energy it takes to raise its
    temperature.
  • Metals in general have a low specific heat as
    they are good conductors of heat.
  • Water has a relatively high specific heat as it
    takes a lot of energy to change its temperature.

Some specific heat values (J/kg.C) Mercury
140 Steel 450 Water (at 15C) 4184
9
Calculating Changes in Thermal Energy
  • A formula?.........but, of course
  • Change in thermal energy mass x specific heat
    x change in temperature
  • OR
  • Q m C Dt
  • if Q is positive, the temperature of the object
    increases
  • If Q is negative, the temperature of the object
    decreases
  • Q change in thermal energy (J), m mass (kg),
  • C Specific Heat (J/g . C), ?T (Tf - Ti)

10
Measuring Specific Heat
  • Specific Heat can be measured using a device
    called a calorimeter.
  • There is a picture and a description of a
    calorimeter in your book on page 478.
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