This lecture discusses key concepts related to applying the first law of thermodynamics to closed systems, including the different types of energy (kinetic, potential, internal) that make up the total energy of a closed system. It explains how energy can be exchanged between a closed system and its surroundings through two means: work, which causes movement, and heat, which is transferred due to temperature differences. Various forms of work - including expansion/compression of gases, electrical, shaft, and spring work - are described. The sign convention for defining work done and heat transferred is also covered.

1. Lecture 3
(Energy, and the 1st law of
T.D. for closed systems. I)
Dr. Ahmed Darwish
Mechanical Power Engineering
Winter 2023
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Thermal and Hydraulic Machines

2. Outline
• Some key concepts.
• The kinetic energy (KE) and the gravitational potential energy
(PE), and their change.
• The internal energy (U).
• The total energy of a closed system.
• System-surrounding energy interactions | Work.
• System-surrounding energy interactions | Heat.
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3. The key concepts behind applying the 1st law of T.D. on
closed systems
• Recall that there are three types of systems:
• In this chapter, we will focus on closed systems
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4. The key concepts behind applying the 1st law of T.D. on
closed systems
Also, recall that we mentioned there is
an energy interaction between the
closed system and its surrounding,
through the system boundary.
Today, we will learn that:
• Two types of energy can be
exchanged with the surroundings
that are: heat and work.
• The system itself has an energy, E.
We call this energy as the total
energy of the closed system.
• This total energy is composed of:
the kinetic energy of the system (KE),
its gravitational potential energy
(PE), and its internal energy (U).
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5. The kinetic energy (KE) and the gravitational potential
energy (PE), and their change
• Kinetic energy, KE: The energy that a system
possesses as a result of its motion relative to
some reference frame.
• Potential energy, PE: The energy that a system
possesses as a result of its elevation in a
gravitational field.
Kinetic energy per unit mass
Kinetic energy
Potential energy per unit mass
Potential energy
z
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6. The kinetic energy (KE) and the gravitational potential
energy (PE), and their change
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Example:
consider a system having a mass of
1 kg whose velocity increases from
15 m/s to 30 m/s while its elevation
decreases by 10 m at a location
where g = 9.7 m/s2. Determine the
change of its kinetic energy and
potential energy?
Solution

7. The internal energy (U)
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• It is the sum of all the microscopic
forms of energy.
• These microscopic forms include
the kinetic energies of the
molecules, the atomic bonds in a
molecule, and the strong bonds
within the nucleus of the atom
itself.

8. The total energy of a closed system
(total energy of a closed system)
energy of a closed system per unit mass
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The total energy of a system is the sum of its kinetic,
potential and internal energies:

9. System-surrounding energy interactions | Work
| Definition
❑ The thermodynamic definition of work is that:
Work is done by a system on its surroundings if the sole effect on everything external to
the system could have been the raising of a weight.
❑ In simple words, can this energy cause a movement (e.g. a force acting through a
distance)
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Examples of work

10. System-surrounding energy interactions | Work
| Forms of work
• Expansion or compression of gases
• Electrical work
• Shaft work
• Spring work
• Work done on elastic solid bars
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11. System-surrounding energy interactions | Work
| Expansion or compression of gases
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12. System-surrounding energy interactions | Work
| Electrical work
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For an electrical system that has an
applied voltage, V, and I as the current
flowing, the electrical energy can goes
into/comes out of the system is
expressed as:
𝑊
𝑒 = 𝑉 𝐼 Δ𝑡

13. System-surrounding energy interactions | Work
| sign convention
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14. System-surrounding energy interactions | Work
| Example
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15. System-surrounding energy interactions | Work
| Example
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16. System-surrounding energy interactions | Work
| Example
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17. System-surrounding energy interactions | Heat
• Heat: The form of energy that is transferred between two systems (or
a system and its surroundings) due to a temperature difference.
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18. System-surrounding energy interactions | Heat
| modes of heat transfer
• Conduction: The transfer of energy from the more energetic particles of a substance to
the adjacent less energetic ones as a result of interaction between particles.
• Convection: The transfer of energy between a solid surface and the adjacent fluid that is
in motion, and it involves the combined effects of conduction and fluid motion.
• Radiation: The transfer of energy due to the emission of electromagnetic waves (or
photons).
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19. System-surrounding energy interactions | Heat
| sign convention
• Formal sign convention: Heat transfer to a
system and work done by a system are
positive; heat transfer from a system and
work done on a system are negative.
• Alternative to sign convention is to use the
subscripts in and out to indicate direction.
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(+)
(+)
(-)
(-)

20. System-surrounding energy interactions | Heat
| The adiabatic process
• An adiabatic system does not
exchange heat with its
surroundings.
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What about!
A potato initially at room
temperature (25°C) is being baked
in an oven that is maintained at
200°C.
Is there any heat transfer during
this baking process?

21. Tutorial and Assignment Problems
Tutorial Assignment
2-11 2-16
2-14 2-31
2-33 2-36
2-36 2-73
2-40 2-83
2-67 2-81
2-69
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22. • Some of the content of this lecture is taken, as it is or after some editing, from the
slides made by Prof. Abdelgawad and Dr. Mohamed Badrt and Dr. Mahmoud Nady
Abdelmoez.
• Permissions have been granted.
Acknowledgement
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23. Thank you
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