Summary of Lecture
Energy, Energy Transfer, and General Energy Analysis
1 Unit and Symbol of Energy:
Energy, E = unit of force × unit of distance = N × m = Joul or J
Energy E J N m
Specific form: e= = ; or
mass m kg kg
Energy E J
Rate form: E = = ; or watt or W
time t s
Forms of Energy: (a) Macroscopic: The macroscopic forms of energy are those a system possesses as a
whole with respect to some outside reference frame. For example,
system’s kinetic energy and potential energy.
(b) Microscopic: The microscopic forms of energy are those related to the molecular
structure of a system and the degree of the molecular activity, and
they are independent of outside reference frames. For example,
molecular translational, rotational, vibrational kinetic energies.
Internal Energy: The sum of all the microscopic forms of energy is called the internal energy (U) of a
Total Energy: Sum of all Macroscopic and Microscopic forms of energy. Total energy is denoted by E.
Total Energy, E = U + KE + PE + other formsof energy (magnetic,chemical,etc.)
Total Energy, E = U + mV 2+ mgz + other formsof energy (magnetic,chemical,etc.); J or kJ ]
E 1 J kJ
Specific Energy,e= = u+ V 2+gz + other formsof specificenergy; or
m 2 kg kg
Energy Transfer Processes: Energy transfer processes from a system to its surrounding or from
surrounding to a system or between two systems, Examples,
1. Energy transfer by Heat (Closed or Open system)
2. Energy transfer by Work (Closed or Open system)
3. Energy transfer by Mass (Open system only)
Heat: Heat is defined as the form of energy that is transferred between two systems (or a system and its
surroundings) by virtue of a temperature difference. Heat is easy to recognize: Its driving force is a
temperature difference between the system and its surroundings or between two systems.
Symbol used for Heat = Q (J or kJ) and Specific Heat = q = Q/m (J/kg or kJ/kg)
Symbol used for Heat per unit time or Heat transfer rate = Q=Q/t (J/sec or watt)
Heat transfer from the system to Heat transfer from the Thermal energy vs. heat
the surrounding surrounding to the system
2 Work: Energy can cross the boundary of system in the form of heat or work. Therefore, if the energy
crossing the boundary of a system is not heat, it must be work. Then we can simply say that an
energy interaction that is not caused by a temperature difference between a system and its
surroundings is work. Example, a rising piston, a rotating shaft, and an electric wire crossing the
system boundaries are all associated with work interactions.
Electric Work Shaft Work Piston or moving
Symbol used for Work = W (J or kJ) and Specific Work = w = W/m (J/kg or kJ/kg)
Symbol used for Work per unit time or Power = W =W/t (J/sec or watt)
Old Sign Convention (positive or negative) for Heat and Work:
W=30 kJ means 30 kJ work done by the
(Positive) W=-20 kJ means 20 kJ work done on
(Negative) the system
Q=10 kJ means 10 kJ heat transferred to
Q=-15 kJ means 15 kJ rejected by the
Modern Sing Convention:
Energy goes into the system (Positive or Negative) and Energy leaving the system (Negative or Positive)
Important Notes on Heat and Work:
Heat and work are not system’s properties like internal energy, specific volume, enthalpy, etc. Heat and
work are energy transfer mechanisms between a system and its surroundings, and there are many
similarities between them:
1. Both are recognized at the boundaries of a system as they cross the boundaries. That is, both heat and
3 work are boundary phenomena.
2. Systems possess energy, but not heat or work.
3. Both are associated with a process, not a state. Unlike properties, heat or work has no meaning at a
4. Both are path functions (i.e., their magnitudes depend on the path followed during a process as well as
the end states).
State 1 Power
∀ ∀ supply
∀ 2 ∀
Process A Process B
Classical example of Heat and Work transfer:
Example of arrangement Example of
work transfer System System boundary System heat transfer to
to a system a system
Different Types of Works: (1) Electrical Work, (2) Displacement Work, (3) Shaft Work
(4) Spring Work, (5) Moving boundary Work, and (6) Flow Work, (7) Magnetic Work, etc.
Electrical Work: Electrons crossing the system boundary do electrical work on the system. When N
coulombs of electrical charge move through a potential difference V volt with a current I amp,
The electrical Power: W electricalI ; (watt)
The electrical work:W = V× N or in general W = V I dt
Displacement Work: Work done by a constant or variable force F on a body displaced a distance s in
the direction of the force.
For constant force: Wdisplacements; (J)
For variable force: displacemen∫ds ; (J)
4 Electrical work and power Displacement work
Shaft Work: Work transfer due to a rotating shaft is Shaft Work. Energy transmission with a rotating
shaft is very common in engineering practice. Imagine a force F acting through a moment
arm r generates a torque T. The work done during n revolutions is determined as follows:
Work transfer or Work done: W shtaftForce Distance = × (2 π r n )2 π n T ; (J or kJ)
Power transfer or Shaft Power: W shtaft = ×(2 π r n =2 π n T ; (watt or kW)
(Note: n= revolution per unit time)
Spring Work: Spring force is a variable force which depends on the spring compression or expansion
of spring and spring constant k (N/m). Therefore, spring force is equal to kx.