1. Thermal Physics

2. Thermal Energy
Physics I
Chapter 21

3. Kinetic Theory What is the kinetic theory of matter?
Kinetic theory explains the behavior of solids, liquids, and gases.
ALL particles of matter are in constant random motion.
We don’t observe the movement directly, but we can measure it indirectly
Pressure and temperature (macroscopic) are indicators of particle movement (microscopic)

4. Kinetic Theory: “big indicates little”
Macroscopic properties: big, easy to observe
(like temperature and pressure)
Microscopic properties: small, difficult to observe
(like motion, velocity, and momentum of atoms)

5. Temperature The measure of an object’s average kinetic energy.
how hot or cold something is, with respect to a standard (temperature scale – ⁰F, ⁰C, or K))
Measured with a thermometer

6. Temperature and Molecular Kinetic Energy
All the particles in a given object have different KE’s.
Temperature measures the average KE of all the particles in an object.
So…
Higher KE = Higher Temperature
Lower KE = Lower Temperature

7. Change of State (Phase) of Matter
There are 3 states (phases) of matter:
Solids, liquids and gases
When the temperature of a solid is increased enough, it will become a liquid
When temperature is increased even more, it becomes a gas.

8. Phase Change
To change phase, we must add or remove ENERGY in the form of heat
When adding heat energy…
If phase DOES NOT change, temperature and KE increase
If phase DOES change, temperature remains constant and PE increases

9. Phase Change and Temperature
Heat transferred during a change of state doesn't change the temperature.

10. Temperature Scales and Water
Celsius Scale -
Freeze at 0⁰
Boil at 100⁰
Fahrenheit Scale –
Freeze at 32⁰
Boil at at 212o
Kelvin Scale –
Freeze at 273 K
Boil at 373 K
Freezing is freezing is freezing…
0 ⁰C = 32 ⁰F = 273 K
100 ⁰C = 212 ⁰F = 373 K

11. Pressure
A measure of the force of particle collisions over the surface of a container
Pressure = Force/Area
P = F/A (units: N/m2)

12. Pressure and Molecular Kinetic Energy
Pressure increases when the average kinetic energy (temperature) of the particles increase
More KE means…
More velocity, and therefore more momentum
Particles hit with more force and are moving faster

13. Observations… As the volume changes: Describe how pressure changes
Describe how the motion of the molecules change

14. Discuss & write…
Describe Use the kinetic theory of matter to describe the effect of cold winter weather on the air pressure inside a car tire.
Describe Cake batter and bread dough can “rise” dramatically during baking. Use the kinetic theory of matter to describe one way in which the heat of an oven can help to produce this increase in size.

15. Heat
Heat is the amount of energy transferred between two objects as a result of differences in temperature
represented by “Q”
Measured in Joules (J) or calories (c)
Direction of energy flow is always from hot to cold
+Q – heat has been absorbed (gained)
- Q – heat has been lost

16. Thermal Equilibrium
Thermal energy transfers between two objects until the reach the same temperature
Thermal equilibrium occurs when the average kinetic energy of the atoms and molecules is the same

17. Specific Heat
When heat flows into an object, thermal energy increases, increasing the temperature
Increase depends on the material and its mass
Different materials require different amounts of heat to change temperature

18. Specific Heat
The amount of energy that must be added to a material to raise the temperature of a unit mass of the material by one unit temperature
Measured in J/g●K or J/g●ºC
Kelvin or Celsius, doesn’t matter… the magnitude of each degree is the same.

19. Calculating Heat Changes
Q = mc∆T
Q = heat gained(+) or lost(-)
m = mass (g)
c = specific heat of material (J/g●K or J/g●ºC)
∆T = temperature change (K or ºC)
IMPORTANT: Notice that mass is measured in GRAMS!!!!!

20. Example 1
How much heat is absorbed by 60 g of copper (c = J/gºC) when its temperature is raised from 20ºC to 80ºC ?
ANS: J

21. Example 2
A child is given a 175 g silver spoon which she promptly puts in her mouth. The spoon was initially at room temperature (20°C) and the child’s mouth is 37°C. If 684 Joules of energy is gained by the spoon, what is the specific heat of silver?

22. Law of Heat Exchange Remember the Law of Conservation of Energy
The sum of heat loss and heat gain in a closed system is zero.
When 2 bodies of unequal temp. are mixed, the cold body absorbs heat from the warm body (loses heat) until an equilibrium temperature is reached.
Qloss + Qgain = O

23. Example 3
If 30 grams of water (c = 1 cal/g°C) at 12⁰C is mixed with 80 grams of water at 88 ⁰C , what will the final temperature be?

24. Discuss and Write
Describe Use the kinetic theory of matter to describe why the high specific heat of water makes it an ideal substance for thawing frozen food or for cooling down overheating machinery.

25. Happens in most solids, liquids & gases
Thermal Expansion
Increase in the size due to an increase in temperature
If temp then size
Happens in most solids, liquids & gases
Water is an exception – it expands
as it becomes a solid!

26. Thermal Expansion Materials expand when heated
Materials contract when cooled
Expansion joints
Ring sizes

27. Heat Transfer & Thermodynamics
Chapters 22 & 24

28. Heat Transfer
Conduction – transfer of thermal energy due to particle collision
Convection - transfer of thermal energy due to motion of fluid
Radiation – transfer of thermal energy by electromagnetic waves

29. Heat Transfer

30. Heat Transfer

31. Absolute Zero
A theoretical temperature at which no further thermal energy can be removed from an object
Usually shown as -273ºC
Kelvin Scale is based on Absolute zero

32. First Law of Thermodynamics
Whenever heat flows into a system, the gain of thermal energy equals the amount of heat transferred.
Whenever heat flows out of a system, the loss of thermal energy equals the amount of heat transferred.
So, the net heat put into a system is equal to the change in internal energy of the system plus the work done BY the system.
Simply, the Law of Conservation of Energy restated.

33. First Law of Thermodynamics
Heat added = increase in internal energy + work done by the system

34. Second Law of Thermodynamics
Natural processes go in a direction that maintains or increases the total entropy of the universe.
Recall: Entropy –
Measure of disorder; the more entropy, the higher the temperature.
Simply, heat flows from high to low temperature