2. THE FIRST LAW OF THERMODYNAMICS
Energy is conserved. This is the first law of thermodynamics (also called the
conservation of energy principle)
•
1st law of thermodynamics: Energy can neither be created nor
destroyed during a process; it can only change forms.
Energy cannot be created or
destroyed; it can only
change forms.
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3. Energy Balance
The net change (increase or decrease) in the total energy of the system
during a process is equal to the difference between the total energy
entering and the total energy leaving the system during that process.
Per unit mass:
Rate form:
Differential forms:
(kJ)
(kJ/kg)
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4. Energy Balance
The work (boundary) done on
an adiabatic system is equal to
the increase in the energy of the
system.
The energy change of a system
during a process is equal to the
net work and heat transfer
between the system and its
surroundings.
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5. Energy Balance
The work
(electrical) done
on an adiabatic
system is equal
to the increase
in the energy of
the system.
In the absence of any
work interactions, the
energy change of a
system is equal to the
net heat transfer.
The work (shaft)
done on an
adiabatic system
is equal to the
increase in the
energy of the
system.
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6. Mechanisms of Energy Transfer, Ein and Eout
(Left hand side of the energy balance equation)
•
Heat transfer
•
Work transfer
•
Mass flow
(kJ)
• For closed systems, energy is
transferred in and out across the
system boundary by two means
only: by work and by heat.
• The energy content of a control
volume can be changed by mass
flow as well as heat and work
interactions.
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7. Energy Change of a System, Esystem
(Right hand side of the energy balance equation)
Energy is an extensive property that includes the
kinetic, gravitational potential energy, and Internal energy.
Internal, kinetic, and
potential energy changes
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8. Closed System Energy Balance
E2 – E1 = (Qin – Qout)+(Win – Wout)
Qin – Qout =Qnet
Wout – Win =Wnet
or
Win – Wout =-Wnet
E2 – E1 = Qnet– Wnet
For a closed system in a cycle:
For a cycle ∆E = 0, thus Qnet= Wnet
or in its rate form:
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11. ENERGY CONVERSION EFFICIENCIES
Efficiency is one of the most frequently used terms in thermodynamics, and it
indicates how well an energy conversion or transfer process is accomplished.
Efficiency of a water heater: The ratio of
the energy delivered to the house by hot
water to the energy supplied to the water
heater.
•
•
Generator: A device that converts
mechanical energy to electrical energy.
Generator efficiency: The ratio of the
electrical power output to the mechanical
power input.
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12. •
•
•
Using energy-efficient appliances conserve
energy.
It helps the environment by reducing the
amount of pollutants emitted to the
atmosphere during the combustion of fuel.
The combustion of fuel produces
• carbon dioxide, causes global warming
• nitrogen oxides and
hydrocarbons, cause smog
• carbon monoxide, toxic
• sulfur dioxide, causes acid rain.
The efficiency of a cooking
appliance represents the
fraction of the energy
supplied to the appliance that
is transferred to the food.
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13. Efficiencies of Mechanical and Electrical Devices
Mechanical efficiency
For pumps and turbines, the effectiveness of the
conversion process between the mechanical work
supplied or extracted and the mechanical energy of
the fluid is expressed by the pump efficiency and
turbine efficiency,
The mechanical
efficiency of a fan is the
ratio of the kinetic
energy of air at the fan
exit to the mechanical
power input.
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16. Summary
• Forms of energy
Macroscopic = kinetic + potential
Microscopic = Internal energy (sensible + latent + chemical + nuclear)
•
•
•
•
Energy transfer by heat
Energy transfer by work
Mechanical forms of work
The first law of thermodynamics
Energy balance
Energy change of a system
Mechanisms of energy transfer (heat, work, mass flow)
• Energy conversion efficiencies
Efficiencies of mechanical and electrical devices (turbines, pumps)
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