Chapter 1

1
Chapter 1 Section 4

• Temperature in Thermal Systems

2
Objectives

• Define thermal energy.
• Name the property that determines the temperature
  of an object.
• Convert Fahrenheit temperatures to Celsius vice
  versa.
• Explain the difference between heat thermal
  energy.
• Explain the relationship between heat transfer
  and temperature change.
• Solve heat transfer problems.

3
Energy

• Energy is the ability to do work.
• Potential energy is energy of position. When you
  lift an object, it has potential energy because
  you lifted it higher.
• Kinetic energy is energy of motion. A moving
  object has energy because of its speed.

4
Thermal Energy

• The atoms of a substance are in constant motion,
  even if the substance is frozen.
• Because the atoms are moving, they have kinetic
  energy.
• The total energy of all the atoms in a substance
  is its thermal energy.

5
Temperature

• The temperature of an object is a measure of the
  average kinetic energy of all the particles in
  the object.
• As energy is added to the object, the particles
  speed up thus increasing the kinetic energy of
  the particles.
• As a result, the temperature rises. The object
  gets hotter.

6
Measuring Temperature

• A thermometer uses expansion contraction of a
  liquid to measure temperature (usually colored
  alcohol or mercury).
• When placed in contact with a hot object or
  substance, heat flows from the object to the
  thermometer. The liquid in the thermometer
  expands.

7
Measuring temperature - cont

• The heat flows until the thermometer and the
  object are in thermal equilibrium, i.e. they are
  the same temperature.
• Likewise, for a cold object, heat flows from the
  thermometer to the object until thermal
  equilibrium is reached. The liquid contracts.

8
Temperature Scales

• Celsius (formerly centigrade) is based on the
  freezing point (FP) and boiling point (BP) of
  water.
• FP 0o C BP 100o C
• Fahrenheit is still used in the US.
• FP 32o F BP 212o F

9
Converting temperatures

• To convert Celsius to Fahrenheit use the formula
  TF 9/5 TC 32
• To convert Fahrenheit to Celsius use the formula
  TC 5/9 (TF - 32)

10
Kelvin temperature scale

• Also known as the absolute temperature scale.
• To convert Celsius to Kelvin Tk TC 273
• Thus 0 K -273o C
• 0 K is known as absolute zero.

11
Heat

• Heat is a form of energy.
• Heat flows spontaneously from a high temperature
  source to a low temperature source.
• The rate at which heat flows increases as the
  temperature difference between the objects or
  areas increases.

12
Prime Movers

• In mechanical systems, force is the prime mover.
• In fluid systems, pressure difference is the
  prime mover.
• In electrical systems, potential difference or
  voltage is the prime mover.
• In thermal systems, temperature difference is the
  prime mover.

13
Heat transfer methods

• Conduction objects are in direct contact. Heat
  (kinetic energy) is transferred as atoms collide
  with other atoms. Examples iron in a fire
  metal spoon in a hot liquid.
• Convection heat is transferred by moving large
  quantities of fluid (liquid or gas). Examples
  weather systems, forced air heaters, convection
  ovens, hair dryers.
• Radiation the only heat transfer method that
  does not require a medium. Example sunshine.

14
Units

• The SI unit of heat is the joule (J)
• Other commonly used units are the calorie (cal)
  and the British thermal unit (BTU).
• 1 cal is the amount of heat that must be added to
  1 gram of water to raise its temperature 1o C. 1
  cal 4.184 J
• 1 BTU is the amount of heat that must be added to
  1 lb of water to raise its temperature 1o F.

15
Specific heat

• The specific heat of a substance is the amount of
  heat needed to raise a unit of mass a unit of
  temperature.
• By the definition of the calorie, the specific
  heat of water is 1 cal/g.oC.
• By the definition of the BTU, the specific heat
  of water is 1BTU/lb.oF.

16
Heat formula

• Q mCDT where
• Q heat (energy)
• m mass
• C specific heat (see chart page 71)
• DT temperature difference

17
Heat formula cont.

• This formula assumes that there is no phase
  change.
• A phase change is going from solid to liquid
  liquid to solid or liquid to gas gas to liquid.
• If a phase change occurs, a different formula is
  used.

18
Phase Change Diagram
19
Phase change formulas

• Hv heat of vaporization the amount of heat
  needed to change state from liquid to gas.
• Hf heat of fusion the amount of heat needed
  to change state from solid to liquid.
• Q mHv or Q mHf
• Chart pg 75 lists Hf Hv for some materials.

20
Summary

• The thermal energy of a body is the total kinetic
  energy of all the particles in it.
• The temperature of a body is the average kinetic
  energy of all the particles in it.
• Three temperature scales are Celsius, Fahrenheit
  and Kelvin.
• Heat flows due to a temperature difference.

21
More Summary

• Heat flows from high temperature to low
  temperature.
• The heat needed to raise the temperature of a
  substance is given by Q mCDT, assuming no phase
  change.
• For a phase changes, Q mHf for solid liquid
  change or Q mHv for liquid gas change.
  During phase change, temp does not change.