Thermo Chp 2 Lec 2

7/29/2019 Thermo Chp 2 Lec 2

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Heat and Work

The forms of energy, which constitute the total energy of a system,

can be contained or stored in a system, and thus can be viewedas

the static forms of energy.

The forms of energy not stored in a system can be viewed as the

dynamic forms of energy or as energy interactions. The only two forms of energy interactions associated with a closed

system are heat transfer and 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. work is the energy

transfer associated with a force acting through a distance. Or any

energy interaction that is not due to temperature difference.

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Heat and Work

Heat and work are energy transport mechanisms between a system

and its surroundings. The characteristic properties of 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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Heat and Work

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.

3

2

12

1,

2

12

1,

(not Q)

(not )

along path

along path

Q Q

W W W


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Sign Convention

A sign convention is required for heat and work energy transfers,

and the classical thermodynamic sign convention is used.

According to the classical sign convention,

heat transfertoa system and work done bya system are positive;

heat transferf roma system and work done on system are negative.

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Energy Transfer by Heat

Energy transfer across a system boundary due solely to the

temperature difference between a system and its surroundings is called

heat. The net heat transferred to a system is defined as

Heat is a boundary phenomena.

Thus in thermodynamics, the term heat simply means heat transfer.5

Q Q Qnet in out


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6

qQ

m


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A process during which there is no heat transfer is called an

adiabatic process.

The system is well insulated so that only a negligible amount of heat can pass

through the boundary.

Both the system and the surroundings are at the same temperature.

An adiabatic process should not be

confused with an isothermal process

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Forms of Heat Transfer

Heat is transferred by three mechanisms

Conduction

Convection

Radiation

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Energy Transfer by Work

The energy crossing the boundary of a closed system other than

heat, is work.

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Work Heat


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Energy Transfer by Work

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Forms of Work

Mechanical work

Electrical work

The rate of electrical work done by electrons crossing a system boundary is

called electrical power and is given by the product of the voltage drop in voltsand the current in amps.

The amount of electrical work done in a time period is found by integrating the rate of

electrical work over the time period.

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(W)eW V I

2

1(kJ)eW V I dt


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Mechanical Forms of Work

Definition of work

There are several different ways of doing work, each in some way

related to a force acting through a distance.

In elementary mechanics, the work done by a constant force Fon a

body displaced a distance s in the direction of the force is given by

In many thermodynamic problems, mechanical work is the only form

of work involved. It is associated with the movement of the boundary

of a system or with the movement of the entire system as a whole. 12

2

1

2

112 FdsWW

For constant force

For variable force


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Mechanical Forms of Work

Some common forms of mechanical work are :

Shaft Work

Spring Work

Work done on Elastic Solid Bars

Work done to Raise a Body

Work done to Accelerate a Body

Work Associated with the Stretching of a Liquid Film

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Shaft Work

Energy transmission with a rotating shaft is very common in

engineering practice.

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Spring Work

Combination of springs are used in various common engineering

machines.

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Work done on Elastic Solid Bar

Most solids, contract or elongate under the action of a force.

They can be modeled as linear springs

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Work Done to Raise or Accelerate a

body

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Thermo Chp 2 Lec 2

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