Prime Movers

1
Prime Movers

• Thermal System

2
Thermal Energy, Heat, and Temperature

• WHAT IS THE DIFFERENCE???
• Thermal energy is the total energy of motion
  (vibration) of molecules in an object.
• Heat is the energy that flows from hot object to
  a cooler object (thermal energy in motion).
• After heat is transferred from one body to
  another it is no longer heat but rather thermal
  energy.
• Temperature is a measure of the average energy of
  motion (or speed of vibrations) of molecules in
  an object, measured by thermometers in F, C or
  Kelvin.

3
Thermal Energy vs. Temperature

• Which has more thermal energy?
• Hot iron
• Swimming pool with cool water
• The temperature of the iron is higher (faster
  molecules) but the swimming pool has more total
  energy (more molecules in motion)

4
Temperature

• Temperature difference (?T) - Heat energy flows
  from regions of high temperature to regions of
  low temperature. Temperature difference is
  considered a force-like quantity because energy
  is transferred (forced) from one area to another.

5
Zeroth Law of Thermodynamics

• Thermal Equilibrium
• When two objects are in contact, heat energy will
  be transferred from the hotter to the lower
  temperature object until both objects are at the
  same temperature (molecules move at the same
  approximate speed).

6
Thermal Expansion

• The higher the temperature, the faster the motion
  of molecules
• The faster they move, the more they want to
  spread out
• Objects expand when heated

7
Thermometers

• A thermometer (thermo-meter) consists of a glass
  bulb and capillary tube that contains a liquid.
  This liquid is usually mercury or colored
  alcohol. When the temperature of the liquid
  rises, the liquid expands and rises in the tube.
  A calibrated scale is provided to read the
  temperature in F , or C .

8
How the scales developed Fahrenheit

• Fahrenheit described how he calibrated the scale
  of his mercury thermometer
• Placing the thermometer in a mixture of sea
  salt, ice, and water a point on the scale will be
  found which is denoted as zero. A second point is
  obtained if the same mixture is used without
  salt. Denote this position as 30. A third point,
  designated as 96, is obtained if the thermometer
  is placed in the mouth so as to acquire the heat
  of a healthy man." (D. G. Fahrenheit,Phil. Trans.
  (London) 33, 78, 1724)
• On this scale, Fahrenheit measured the boiling
  point of water to be 212. Later he adjusted the
  freezing point of water to 32 so that the
  interval between the boiling and freezing points
  of water could be represented by the more
  rational number 180. Temperatures measured on
  this scale are designated as degrees Fahrenheit
  ( F).

9
How the scales developed Celsius

• On the Celsius scale, the freezing point of water
  is taken to be zero and the boiling point of
  water at sea level is taken to be 100 degrees.
• It is also called the Centigrade scale because
  there are 100 degrees between freezing and
  boiling.

10
How the scales developed Kelvin

• There is another scale the Kelvin scale, which
  has the same degree scale as Celsius, but the
  zero is called absolute zero because it is the
  point at which all molecules would theoretically
  stop moving completely.
• On this scale, the freezing point of water is 273
  Kelvins, so absolute zero would be -273 degrees
  Celsius.
• Absolute zero has never actually been achieved,
  but scientists have gotten to about 2 or 3
  Kelvins.

11
Temperature Conversions

• TC 5/9 (TF - 32)
• TF (9/5 TC) 32

12
Example 1

• Convert 50F to C.
• TC 5/9 (TF - 32)
• 5/9 (50 - 32)
• 10 C

13
Example 2

• Convert 20C to F.
• TF (9/5 TC) 32
• 9/5(20) 32
• 68 F

14
Transfer of Heat

• Conduction (Contact)
• Some materials conduct heat better than others
• Some materials store thermal energy better (dont
  let it transfer)
• Insulators delay the transfer of heat
• In other words, it takes more energy to raise the
  temperature of some things vs. others (higher
  specific heat).
• For example, it takes more energy to change the
  temperature of water than it does to change the
  temperature of iron.

15
Transfer of Heat

• This explains why during the day sand is hot
  while the ocean is cool. More energy is required
  to raise the temperature of water.
• At night the water is warm and the sand is cold
  because the water stores the energy longer.

16
Phases

• Technically there are 4 phases, but we will only
  worry about 3 Solid, Liquid, and Gas.

17
Change of Phase
18
Temperature During Phase Change

• Latent Heat
• Because all of the energy being absorbed is used
  to change the phase of a substance, the
  temperature throughout the phase change stays the
  same.
• That means that while youre boiling water, even
  though youre constantly adding heat, the
  temperature of the water cant increase!
• That also means that as long as there is ice in
  your cup, the temperature of the water wont
  increase.