Chap2a jan13

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CHAPTER 2aCHAPTER 2a

ENERGY, ENERGY TRANSFER & GENERAL ENERGY ANALYSIS

ENERGY, ENERGY TRANSFER & GENERAL ENERGY ANALYSIS

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CONTENTSCONTENTS

Forms of Energy

Energy Transfer by Heat

Energy Transfer by Work

Mechanical Forms of Work

The First Law of Thermodynamics

Energy Conversion Efficiencies

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LESSON OBJECTIVESLESSON OBJECTIVES

At the end of this lesson, you should be able to:

State the various forms of energy

Describe the nature of internal energy

Describe the energy transfer by heat and work

Explain mechanical work

FORMS OF ENERGYFORMS OF ENERGY

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FORMS OF ENERGYFORMS OF ENERGY

Energy exists in numerous forms (thermal, mechanical, chemical, kinetic, potential, electric, magnetic & nuclear)

The sum of the energies is the total energy, E (kJ) Or for a unit mass,

Grouping of Energy forms

macroscopic

microscopic

energy of a system as a whole with respect to some outside reference frames, e.g. KE, PE

• related to molecular structure of a system and the degree of molecular activity

• independent of outside reference frames• The sum is the Internal Energy, U

m

Ee (kJ/kg)

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FORMS OF ENERGY (cont’d)FORMS OF ENERGY (cont’d)

Kinetic energy (KE)

- result of motion relative to some

reference frame (unit J)

where

= velocity of the system relative to

some fixed reference frame (m/s)

m = mass of an object (kg)

Potential energy (PE)- due to elevation in a gravitational

field (unit J)

where

g = gravitational acceleration, 9.81 m/s2

h = elevation of center of gravity of a system relative to some arbitrarily plane (m)

Macroscopic forms of energy

212212

2

2

1

2

1

mKEKEKE

mKE

mgzPE

1212 zzmgPEPEPE

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Sensible Energy

- Kinetic energy of moleculesLatent Energy- Associated with

phase of a system

Microscopic forms of energy

Chemical energy: The internal energy associated with the atomic bonds in amolecule.Nuclear energy: The tremendous amount of energy associated with the strong bonds within the nucleus of the atom itself.

The internal energy of a system is the sum of all forms of the microscopic energies.

Internal = Sensible + Latent + Chemical + Nuclear

Thermal = Sensible + Latent


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Total Energy of a system

mgzmUPEKEUE 2

2

1

gzupekeue 2

2

1

Total Energy of a system per unit mass

Most of the closed system remains stationary, so for that system;


Energy in Closed SystemEnergy in Closed System

Most of the closed systemremains stationary, so for that system;

0 PEKE

PEKEUE

Energy Interaction

Forms of energy not stored in the system

It is also called dynamic forms of energy

Recognized as it crosses boundary, represent energy lost or gain

The only two forms of energy interactions associated with a closed system are heat transfer and work.

The difference between heat transfer and work: An energy interaction is heat transfer if its driving force is a temperature difference. Otherwise it is work.

Energy can cross the boundaries of a closed system in the form of heat and work.


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ENERGY TRANSFER BY HEATENERGY TRANSFER BY HEAT

Heat Transfer Means of energy transfer caused by temperaturedifference between the system and the surroundings

Direction of heat transfer: Higher T to Lower T

Energy is recognized as heat transfer only as it crosses the system boundary

Temperature difference is the driving force for heat transfer. The larger the temperature difference, the higher is the rate of heat transfer.

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ENERGY TRANSFER BY HEAT (cont’d)


Adiabatic process is a process during which there is no heat transfer, Q=0

How a process can be adiabatic?

Well insulated system

No temperature difference

Adiabatic is not necessarily means isothermal process. Temperature of the system can still be changed by other means

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Amount of heat transferred is denoted by Q (kJ)

For a unit mass:

m

Qq (kJ/kg)

Q positive indicates heat inputQ negative indicates heat lost

Sign Convention for Heat:

Specifying the directions of heat using in and out



ENERGY TRANSFER BY HEATENERGY TRANSFER BY HEAT

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MECHANISMS OF HEAT TRANSFER

Conduction Convection Radiation

The transfer of energy due to the emission of electromagnetic waves (or photons).

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.

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.



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ENERGY TRANSFER BY WORKENERGY TRANSFER BY WORK

Work is an energy interaction between a system and its surroundings that is not caused by temperature difference

WORK Energy transfer associated with a force acting through a distance, e.g. rotating shaft, rising piston

The work done by, or on, a system is defined as (unit kJ):

2

1

.s

sdsFW or

m

Ww Work done per unit mass

The work done per unit time is Power (unit kJ/s or kW):

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Sign convention for work:

The work done by, or on, a system is defined as (unit kJ):

W positive indicates work done by system (work output)W negative indicates work done on the system (work input)

Specifying the directions of work using in and out

ENERGY TRANSFER BY WORK (cont’d)ENERGY TRANSFER BY WORK (cont’d)

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Heat vs. Work

Both are recognized at the boundaries of a system as they cross the boundaries. That is, both heat and work are boundary phenomena.

Systems possess energy, but not heat or work.

Both are associated with a process, not a state.

Unlike properties, heat or work has no meaning at a state.

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

Properties are point functions; but heat and work are path functions (their magnitudes depend on the path followed).


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Electrical power in terms of resistance R, current I, and potential difference V.

Electrical work

Electrical power

When potential difference and current change with time

When potential difference and current remain constant


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MECHANICAL FORMS OF WORKMECHANICAL FORMS OF WORK

There are two requirements for a work interaction between a system and its surroundings to exist:

there must be a force acting on the boundary.

the boundary must move.

Work = Force Distance When force is not constant

If there is no movement, no work is done.

In thermo, most of the work is mechanical work, which associated with moving boundary work

Other common forms of workShaft workSpring work

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A force F acting through a moment arm r generates a torque T

Shaft work is proportional to the torque applied and the number of revolutions of the shaft.

This force acts through a distance s

The power transmitted through the shaft is the shaft work done per unit time

Shaft work:

SHAFT WORKSHAFT WORK

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When the length of the spring changes by a differential amount dx under the influence of a force F, the work done is:

For linear elastic springs, the displacement x is proportional to the force applied

k: spring constant (kN/m)

Substituting and integrating yield

x1 and x2: the initial and the final displacements

The displacement of a linear spring doubles when the force is doubled.

SPRING WORKSPRING WORK

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Different forms of work transfer could occur in a system simultaneously during a process.

The total or net work done by the system = algebraic sum of all work

NET WORK DONE BY A SYSTEMNET WORK DONE BY A SYSTEM

...WWWWWsshebtotal

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1. How can a closed system and its surroundings interact?

2. What are the sign conventions used for energy transfer?

3. Express total energy and for each of the terms, indicate the unit.

Class TakeawayClass Takeaway

Data & Analytics

Chap2a jan13