Thermal Physics
Thermodynamics – study of heat and work and those properties of a substance that bear relation
to heat and work.
Terms:
1. Surrounding – all matter and space outside of a system.
2. System - a group of molecules connected to each other particles
• Open system – a system that has internal interaction
• Closed system – a physical system that does not allow the transfer of matter.
• Isolated system – a physical system that does not interact or exchange energy with
the surrounding.
3. Control Volume – focused volume in space from which the substance flows.
4. Control Surface – the surface that surrounds the control volume.
5. Phase – a quantity of matter having the same chemical composition.
6. Property – a quantity that describes a substance.
Thermodynamic Property
1. Intensive Property – a property that does not depend on the mass of a substance.
2. Extensive Property – a property that does depend on the mass of a substance.
Working Substance – a substance to which heat can be stored or heat can be extracted.
Pure substance – a substance that is made up of only one kind of particle and has a fixed
and constant structure.
Properties of a working substance
1.
2.
3.
4.
5.
6.
Mass and Weight
Volume
Pressure
Temperature
Specific Volume, Density, Specific Weight
Internal Energy, “u”, “J”
The sum of kinetic, potential, chemical, electrical, nuclear, and other energy
associated with the atoms and molecules of a system. An increase in internal energy
results in a rise in temperature or a change in phase.
Internal energy is zero if T is constant.
7. Flow Work – work due to the change in volume.
8. Enthalpy, “H”, J/kg
The total heat and heat content of a substance which is equal to the sum of the
internal energy of a body and the product of pressure and specific volume.
When a process occurs at constant pressure, the heat evolved (either released or absorbed)
is equal to the change in enthalpy. Enthalpy (H) is the sum of the internal energy (U) and the
product of pressure and volume (PV) given by the equation:
H=U+PV
Enthalpy is a state function that depends entirely on the state functions T, P, and U.
Enthalpy is usually expressed as the change in enthalpy (ΔH) for a process between initial
and final states:
ΔH=ΔU+ΔPV
If temperature and pressure remain constant through the process and the work is
limited to pressure-volume work, then the enthalpy change is given by the equation:
ΔH=ΔU+PΔV
Also at constant pressure, the heat flow (q) for the process is equal to the change in
enthalpy defined by the equation:
ΔH=q
9.
Entropy, kJ/K – a measure of randomness of molecules of a substance or measures the
fraction of the total energy of a system that is not available for doing work. The increase in
entropy is called entropy production.
WORK, HEAT, AND POWER
a. Work - the quantity of energy transferred from one system to another
b. Heat – a form of energy due to temperature difference. BTU, calories, joule
-Transfer of energy from high temperature to low temperature
Where: m = mass in kg
C = specific heat
c. Specific Heat, C – the heat required to change the temperature of 1 kg of a
substance by 1oC.
Conversions Units of Heat
1 BTU = 778 ft-lb
= 252 cal
1 kcal = 4.187 kJ
1 N-m = 1 J
= 1.055 KJ
d. Power – time rate of doing work
Conversion Units of Power
1 hp =
550 ft-lb/sec
33, 000 ft-lb/min
2545 BTU/hr
42.4 BTU/min
746 watts
TEMPERATURE
✓ The degree of hotness or coldness of a body or environment.
✓ A measure of the warmth or coldness of an object or substance with reference to some
standard value.
✓ A measure of the average kinetic energy of the particles in a sample of matter expressed
in terms of units or degrees designated on a standard scale.
✓ A measure of the ability of a substance, or more generally of any physical system, to
transfer heat energy to another physical system.
✓ Any of various standardized numerical measures of this ability, such as the Kelvin,
Fahrenheit, and Celsius scale.
Problems:
1.
2.
3.
4.
750 deg R to deg C
90K to deg R
Pure iron melts at 1,535 C. What is the temperature in Fahrenheit?
A general rule of thumb used by pilots is for every 1,000 feet of altitude, the temperature falls
3.5 F. If the temperature at sea level is 78 F, what would you expect the temperature to be at
10,000 feet in Celsius
THERMAL EXPANSION
✓ tendency of matter to change in shape, area, and volume with response to change in
temperature
Temperature is a monotonic function of the average molecular kinetic energy of a
substance. When a substance is heated, the kinetic energy of its molecules increases. Thus, the
molecules begin vibrating/moving more and usually maintain a greater average separation.
Materials which contract with increasing temperature are unusual; this effect is limited in size and
only occurs within a limited range of temperature. The degree of expansion divided by the change
in temperature is called the material’s Coefficient of Thermal Expansion and generally varies with
temperature.
Linear Expansion:
Where:
Y = elongation due to temperature change
𝛼 = coefficient of linear expansion (m/m-℃)
Superficial/Area Expansion:
∆𝑨 = 𝜷𝑨(𝑻𝟐 − 𝑻𝟏 )
Where: 𝛽 = coefficient of thermal expansion (1/℃)
The coefficient of thermal expansion used in area expansion is equal to 2𝛼
Cubic/Volume Expansion:
Where: 𝛽 = coefficient of thermal expansion (1/℃)
The coefficient of thermal expansion used in area expansion is equal to 3𝛼
Linear Expansion
Aluminum: 0.000023 (m/moC)
steel: 0.000012 (m/m oC)
copper: 0.000017 (m/m oC)
Example:
1. A steel train rail is 400 m long. In March it is at -30 oC and 40oC in July. Find the change
in length in mm. (𝛼= 11.7 m/m C)
Area Expansion:
The coefficient of thermal expansion used in area expansion is equal to 2𝛼
Example
1. At 20 oC, the length of a sheet of steel is 50 cm and the width is 30 cm. If the coefficient
of linear expansion for steel is 10-5 oC-1, determine the change in area and the final area at
60 oC.
2. At 30 oC, the area of a sheet of aluminum is 40 cm2 and the coefficient of linear expansion
is 24 x 10-6 / oC. Determine the final temperature if the final area is 40.2 cm2.
Volume Expansion:
The coefficient of thermal expansion used in area expansion is equal to 3𝛼
Example:
1. At 30 oC the volume of an aluminum sphere is 30 cm3. The coefficient of linear expansion
is 24 x 10-6 /oC. If the final volume is 30.5 cm3, what is the final temperature of the
aluminum sphere?
2. The density of mercury at exactly 0℃ is 13600 kg/𝑚3 , and its volume expansion
coefficient is 1.82𝑥10−4 /℃. Calculate the density of mercury at 50℃.
HEAT
✓
✓
✓
✓
Total kinetic energy of all molecules
Depends on the mass and energy of the particles
Heat flows from hot to cold until the heat is balanced and it is called equilibrium.
Absolute zero is the temperature where there is the absence of heat.
Specific Heat, C
✓ the heat required to change the temperature of 1 kg of a substance 1oC.
✓ the ability of the substance to absorb heat
Example:
1. How much heat does 25 g of aluminum give off from 100℃ to 20℃? For aluminum,
C=880 J/kg ℃.
2. A certain amount of heat is added to a mass of aluminum (c=0.21 cal/g,℃), and its
temperature is raised 57℃. Suppose that the amount of heat is added to the same mass of
copper (c=0.093 cal/g,℃). How much does the temperature of the copper rise?
3. Determine the temperature that results when 150 g of ice at 0℃. Is mixed with 300 g of
water at 50℃.
Heat Capacity
✓ Defined as the amount of heat to be supplied to a given mass to raise the
temperature of the body by one degree.
Example:
1. The specific heat of water is 4180 J/kg C. How much is the heat capacity of 2 kg water?
2. The specific heat of aluminum is 900 J/kg C. How much is the heat capacity of 2 gram
aluminum? Ans. C=1.8 J/deg C
PHASE CHANGES
Enthalpy Change Accompanying a Change in State
When a liquid vaporizes the liquid must absorb heat from its surroundings to replace
the energy taken by the vaporizing molecules in order for the temperature to remain constant.
This heat required to vaporize the liquid is called enthalpy of vaporization (or heat of
vaporization). For example, the vaporization of one mole of water the enthalpy is given as:
ΔH = 44.0 kJ at 298 K
When a solid melts, the required energy is similarly called enthalpy of fusion (or
heat of fusion). For example, one mole of ice the enthalpy is given as:
ΔH = 6.01 kJ at 273.15 K
ΔH=ΔU+pΔV