Energy Concepts

Prepared By
Engr. Raymond S. Abesamis, ME, CLSSYB, MIE

College
ENERGY
It is basically defined as the capacity to do work. It is the capacity of the substance or system to do an
effect.

LAW OF CONSERVATION OF ENERGY

It states that “energy can neither be created nor destroyed, it just transform into other form.”

Types of Energy
1. Potential Energy – it is the energy of a body or substance because of its position or elevation
measured with respect to a certain datum line. This energy is also called gravitational potential energy.

Energy Concepts
• For Total Potential Energy
~ kJ, kW, Btu, Btu/min

• For change of Potential Energy, total value

~ kJ, kW, Btu, Btu/min

• For change of Potential Energy, unit mass

~,
where: m = mass of body or substance, kg, lb,
= change in elevation of body, m, ft
= initial position of the body, m, ft
= final position of the body, m, ft
g = acceleration due to gravity,

Energy Concepts
2. Kinetic Energy – it is basically defined as the energy of a body because of its
velocity.

• For Total Kinetic Energy of a Body

~ kJ, kW, Btu, Btu/min
• For change of Kinetic Energy, total value
() ~ kJ, kW, Btu, Btu/min
• For change of Kinetic Energy, unit mass
~,
where: m = mass of body or substance, kg, lb,
= initial velocity of the body, m/s, ft/min
= final position of the body, m/s, ft/min

Energy Concepts
3. Internal Energy – it is the sum of the energies of all the molecules of a body or substance. It is also the sum
of the various forms of energy that a molecule has.

where: U = total internal energy , kJ, kW, Btu, Btu/min

= change of total internal energy, kJ, kW, Btu, Btu/min
= initial specific internal energy, kJ/kg, Btu/lb
= final specific internal energy, kJ/kg, Btu/lb
= specific heat at constant volume, ,
= initial temperature, K, °R
= final temperature, K, °R

4. Work Energy – it is basically defined as energy in transition. It exists only when a force is moving through
a distance.

where: W = work energy, N·m = Joules (J), kN-m = kJ, Btu, ft-lb
F = force applied to a body, N,
s = distance travelled by the body, m, ft, in
Note:
• Work is positive (+) when work is done BY the system
• Work is negative (-) when work is done TO the system.

Energy Concepts
Classification of Work Energy
a. Non-flow Work
- it is a work done to or by a non-flow closed system during a reversible process without the flowing of a
working substance or through a moving boundary
- a typical example of non-flow work is a piston-cylinder arrangement wherein the substance expands (or
compresses) against resistance and does work.

For Total Work:

For unit mass:

Energy Concepts
b. Flow Work or Flow Energy – it is the work done in pushing a fluid across a boundary
usually into or out of the system.

Flow work for any given boundary

Change of flow energy between boundaries

Change of flow energy, unit mass analysis

Energy Concepts
Example No. 1
Determine the work done by a 1-kg fluid system as it expands slowly within the piston-cylinder
arrangement from an initial pressure and volume of 60-kPaa and 0.028-m³, respectively, to a final
volume of 0.123-m³ in accordance with the following relations: a.) , b.) , c.) , and d.) .

Energy Concepts
Example No. 1
Determine the work done by a 1-kg fluid system as it expands slowly within the piston-cylinder
arrangement from an initial pressure and volume of 60-kPaa and 0.028-m³, respectively, to a final
volume of 0.123-m³ in accordance with the following relations: a.) , b.) , c.) , and d.) .

Energy Concepts
Example No. 1
Determine the work done by a 1-kg fluid system as it expands slowly within the piston-cylinder
arrangement from an initial pressure and volume of 60-kPaa and 0.028-m³, respectively, to a final
volume of 0.123-m³ in accordance with the following relations: a.) , b.) , c.) , and d.) .

Energy Concepts
Example No. 2
In a steady-flow open system, a fluid moves between two boundaries. In boundary 1: . In boundary 2: .
Determine a.) the mass flow rate, b.) the velocity in boundary 2, c.) the change in kinetic energy
between the boundaries, and d.) the change in flow energy between boundaries.

Energy Concepts
Example No. 2
In a steady-flow open system, a fluid moves between two boundaries. In boundary 1: . In boundary 2: .
Determine a.) the mass flow rate, b.) the velocity in boundary 2, c.) the change in kinetic energy
between the boundaries, and d.) the change in flow energy between boundaries.

Energy Concepts
Example No. 2
In a steady-flow open system, a fluid moves between two boundaries. In boundary 1: . In boundary 2: .
Determine a.) the mass flow rate, b.) the velocity in boundary 2, c.) the change in kinetic energy
between the boundaries, and d.) the change in flow energy between boundaries.

Energy Concepts
Example No. 3
A piston-cylinder device containing air undergoes a series of two processes. The first process is
constant volume with the pressure decreased from 300-kPaa to 60-kPaa. The second process is
compression in accordance with and the pressure increases to 300-kPaa with a final volume of 0.10-
m³. Determine the work done.

Energy Concepts
Example No. 3
A piston-cylinder device containing air undergoes a series of two processes. The first process is
constant volume with the pressure decreased from 300-kPaa to 60-kPaa. The second process is
compression in accordance with and the pressure increases to 300-kPaa with a final volume of 0.10-
m³. Determine the work done.

Energy Concepts
5. Heat Energy – it is basically defined as the energy in motion. It moves from a body of higher
temperature body to another body of lower temperature.
Note:
• Work can be converted entirely into heat, or ideally entirely into other forms of energy.
• Heat cannot be converted entirely into work, only part of it will be converted into work energy.

SPECIFIC HEAT - it is the amount of heat energy required to raise the temperature of a unit
mass substance by one degree Kelvin.

a. Constant Volume Specific Heat,

It is the change of the molecular internal energy for a unit mass or one mole of a substance
per degree change of temperature with the volume that remains constant from the initial state to
the final state.
~ kJ, Btu
For unit mass analysis:
) ~ kJ/kg, Btu/lb

Energy Concepts
b. Constant Pressure Specific Heat,
It is the change of enthalpy for a unit mass or one mole substance
per degree change of temperature between two states without
changing the pressure.
~ kJ, Btu
For unit mass analysis:
~ kJ/kg, Btu/lb
c. Specific Heat Ratio
It is the ratio of constant pressure specific heat to that of constant
volume specific heat.

d. Specific Heat Relation (For Ideal Gas)

Energy Concepts
Example No. 4
Assume that 4-kg of a substance receives 253-kJ of heat at constant volume undergoes a
temperature change of 66-K. Determine the average specific heat of the substance during the
process.

Energy Concepts
Example No. 5
For a constant pressure system where mass is 36-kg, 45-kJ of heat is required to raise its
temperature by 3-K. Find the constant pressure specific heat.

Energy Concepts
Example No. 6
A 1.5-kg gaseous non-flow closed system receives heat while its pressure remains constant at
360-kPaa. The internal energy and temperature are increased by 215-kJ and 65-K, respectively. If
the work done is 112-kJ, find:
a.The constant pressure specific heat
b.The change in volume, and
c.The , R, and k

Energy Concepts
Energy Concepts