4. The First Law of Thermodynamics for Closed Systems
4.1 Internal energy in a system
The total energy of a system may consist of internal energy, kinetic energy, potential energy, and other forms of energy. For a system free of magnetic, electric, and surface tension effects, its total energy and corresponding specific energy can be expressed as
where
The first law of thermodynamics gives the relation between the total energy stored in a system and the energy transferred into or out of the system in the form of heat and work. In this chapter, we will firstly introduce the common methods of determining internal energy and work, and then the first law of thermodynamics and its applications to closed systems.

4.1.1 Using thermodynamic tables to determine specific internal energy u
For pure substances with available thermodynamic tables, the specific internal energy can be read from the thermodynamic tables, then the internal energy can be found from
where
Example 1
Complete the table, and label each state on the P-T, T-v and P-v diagrams.
Substance | T
oC |
P
kPa |
v
m3/kg |
u
kJ/kg |
x | Phase | |
1 | Water | 60 | 500 | ||||
2 | R134a | 40 | 0.1 |
Solution
1. Water at 60oC and 500 kPa
From Table A1: Psat = 0.01995 MPa = 19.95 kPa at 60oC. The given pressure P = 500 kPa > Psat ; therefore, water at the given state is a compressed liquid.
From Table A3: v = 0.001017 m3/kg and u = 251.08 kJ/kg for the given state.

2. R134a at 40oC and 0.1 m3/kg
From Table C1: vg = 0.019966 m3/kg at 40oC. The given specific volume v = 0.1 m3/kg > vg ; therefore, R134a at the given state is a superheated vapour.
From Table C2:
v = 0.080629 m3/kg and u = 410.00 kJ/kg at 40oC and 300 kPa
v = 0.123226 m3/kg and u = 411.22 kJ/kg at 40oC and 200 kPa
Use linear interpolation to find P and u at the given condition

In summary, the table below gives the final answers to the question.
Substance | T
oC |
P
kPa |
v
m3/kg |
u
kJ/kg |
x | Phase | |
1 | Water | 60 | 500 | 0.001017 | 251.08 | n.a. | Compressed liquid |
2 | R134a | 40 | 254.52 | 0.1 | 410.55 | n.a. | Superheated vapour |
4.1.2 Constant-volume specific heat
When a substance absorbs heat, its temperature tends to increase. Different substances require different amounts of heat for a given temperature rise. For example, it requires 4.18 kJ of heat to warm up 1 kg of water by 1oC. But it only requires 2.22 kJ of heat to warm up the same amount of gasoline by 1oC. In other words, water and gasoline have different energy storage capacities. Specific heat, also called heat capacity, is an important property used to quantify the energy storage capacity of a substance. Specific heat is defined as the energy required to raise the temperature of one unit mass (i.e., 1 kg) of a substance by one degree (i.e., 1oC, or 1 K),
where
The specific heat of a substance may be measured in an isochoric or isobaric process; they are therefore called constant-volume specific heat,
Constant-volume specific heat is defined as the energy required to raise the temperature of one unit mass (i.e., 1 kg) of a substance by one degree (i.e., 1oC, or 1 K) in an isochoric process. Mathematically, it is expressed as,
where
The constant-volume specific heat of selected ideal gases can be found in Appendix G, Table G1. For example, oxygen has
It is important to note that although
4.1.3 Using Cv to calculate Δu for ideal gases
A gas behaves like an ideal gas as its compressibility factor
The change in specific internal energy between two states in any process involving ideal gases can be found from
where
The above equation provides a convenient way for estimating
Example 2
Two kilograms of air is heated from 10oC to 40oC. Calculate the change in internal energy,
Solution
Air is treated as an ideal gas. From Table G1:
The change in internal energy in this process is 43.08 kJ. As
Practice Problems
Practice Problems
Specific heat, also called heat capacity, is a thermodynamic property to quantify the energy storage capacity of a substance. It is defined as the amount of heat required to raise the temperature of one unit mass of a substance by one degree.
Constant-volume specific heat is a property of a substance. It equals to the amount of energy required to raise the temperature of one unit mass of the substance by one degree in an isochoric process.