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Energy Transport by Heat and Work and the Classical Sign Convention

Energy may cross the boundary of a closed system only by heat or work.

Energy transfer across a system boundary due solely to the temperature difference between a system and its surroundings is called heat.

Energy transferred across a system boundary that can be thought of as the energy expended to lift a weight is called work.

Heat and work are energy transport mechanisms between a system and its surroundings. The similarities between heat and work are as follows:

1.Both are recognized at the boundaries of a system as they cross the boundaries. They are both 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 state.

4.Both are path functions (i.e., their magnitudes depends on the path followed during a process as well as the end states.

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Since heat and work are path dependent functions, they have inexact differentials designated by the symbol . The differentials of heat and work are expressed as Q and W. The integral of the differentials of heat and work over the process path gives the amount of heat or work transfer that occurred at the system boundary during a process.

2

12

1,

2

12

1,

(not Q)

(not )

along path

along path

Q Q

W W W

That is, the total heat transfer or work is obtained by following the process path and adding the differential amounts of heat (Q) or work (W) along the way. The integrals of Q and W are not Q2 – Q1 and W2 – W1, respectively, which are meaningless since both heat and work are not properties and systems do not possess heat or work at a state.

The following figure illustrates that properties (P, T, v, u, etc.) are point functions, that is, they depend only on the states. However, heat and work are path functions, that is, their magnitudes depend on the path followed.

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• Internal energy: May be viewed as the sum of the kinetic and potential energies of the molecules.

• Sensible heat: The kinetic energy of the molecules.• Latent heat: The internal energy associated with the phase of a

system.• Chemical (bond) energy: The internal energy associated with

the atomic bonds in a molecule. • Nuclear energy: The internal energy associated with the bonds

within the nucleus of the atom itself.

What is thermal energy?

What is the difference between thermal energy and heat?

Property

• Any characteristic that can be ascribed to a system e.g. volume (V), temperature (T) and pressure (P).

• Extensive property: depends on the size of the system e.g. volume, mass

• Intensive property: independent of system size: pressure, temperature, density

• Non-property: work, heat

State

• Condition of the system as described by its properties.

• Usually only a subset of properties need to be specified to identify the state of a system.

Thermodynamic equilibrium (TE)

• Undergoes no changes when isolated from its surroundings.• At an equilibrium state, there is no unbalanced driving forces

between the system and the surroundings and between parts of the system.

• A system is in TE if following types of equilibrium are satisfied.– Mechanical equilibrium: balance of forces – Thermal equilibrium: system undergoes no changes in properties when

separated from the surroundings through walls that allow passage of heat. – Phase equilibrium: Mass of each phase within the system does not change– Chemical equilibrium: chemical composition of the system does not change

Process and cycle

• Change that a system undergoes from one equilibrium state to another equilibrium state.

• Cycle: A process that begins and ends at the same state e.g. the working fluids of power plants and refrigerators work in a cycle.

Quasiequilibrium process

• Quasiequilibrium or quasistatic process: A process conducted so slowly that the system is in thermodynamic equilibrium at every stage of this process.

Steady flow process• Steadynot changing with time• A process during which the fluid flows through an open

system (control volume) steadily.• Of engineering importance

Temperature and the thermometer

• Temperature is a property which characterizes “degree of hotness”.

• A thermometer contains a working substance which undergoes easily detectable changes in some property according to the “degree of hotness”.

• Therefore, temperature can be assigned numerical values based on the reading of a thermometer.

Two bodies in thermal equilibrium

• When a “hot” body is placed in contact with a “cold” body through a part of their boundary that allows passage of heat, the properties of the body change initially due to heat transfer between them. Eventually the heat transfer stops and the properties no longer change with time. The two bodies have reached thermal equilibrium.

Zeroth Law of Thermodynamics

If:• Body A is in thermal equilibrium with a body C • Body B is in thermal equilibrium with body CThen:• Bodies A and B are in thermal equilibrium with

each other.• If body C is a thermometer, then body A and body

B are in thermal equilibrium if they have the same “thermometer reading” i.e. temperature.

Two bodies are in thermal equilibrium if they have the same temperature (there is no need anymore to place them in contact).

Thermodynamic equilibrium (TE)

• Undergoes no changes when isolated from its surroundings.• At an equilibrium state, there is no unbalanced driving

forces between the system and the surroundings and between parts of the system.

• A system is in TE if following types of equilibrium are satisfied.– Mechanical equilibrium– Thermal equilibrium Temperature uniform – Phase equilibrium– Chemical equilibrium

Pressure and density

• Normal force per unit area exerted by fluid on a substrate

• Pressure is measured using manometers (rise of mercury column), Bourdon Tube, pressure transducers using pressure sensitive resistance, capacitance, piezoelectric e.m.f. etc.

• Density is the mass per unit volume of a macroscopic region surrounding a point within the material.

Chapter Summary

• The sum of all forms of energy of a system is called total energy, which is considered to consist of internal, kinetic, and potential energies. Internal energy represents the molecular energy of a system and may exist in sensible, latent, chemical, and nuclear forms.

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Chapter Summary

• The mass-dependent properties of a system are called extensive properties and the others, intensive properties. Density is mass per unit volume, and specific volume is volume per unit mass.

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Chapter Summary

• The sum of all forms of energy of a system is called total energy, which is considered to consist of internal, kinetic, and potential energies. Internal energy represents the molecular energy of a system and may exist in sensible, latent, chemical, and nuclear forms.

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Applications of Thermodynamics

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Power plants

The human bodyAir-conditioning

systemsAirplanes

Car radiators Refrigeration systems

Crossing Closed-System Boundries

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• (Fig. 1-13)

Energy, not mass, crosses closed-system boundries

Closed System with Moving Boundry

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Crossing Control Volume Boundaries

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Mass and Energy Cross Control Volume Boundaries

System’s Internal Energy

• (Fig. 1-19)

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System’s Internal Energy = Sum of Microscopic Energies