lec 05 25_042013.pptx

LECTURE NO: 05(25th April, 2013)

Engr.Abdur Rafaiabdurrafai29@gmail.com

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ENERGY, ENERGY TRANSFER, ANDGENERAL ENERGY ANALYSISObjectives of Ch. 2Introduce the concept of energy and define its

variousforms. Discuss the nature of internal energy. Define the concept of heat and the

terminology associated with energy transfer by heat.

Discuss the three mechanisms of heat transfer:

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Conduction ,convection , and radiation.Define the concept of work, including electrical

work and several forms of mechanical work. Introduce the first law of thermodynamics,

energy balances, and mechanisms of energy transfer to or from a system.

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FORMS OF ENERGYEnergy can exist in numerous forms such as

thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear, and their sum constitutes the total energy E of a system. The total energy of a system on a unit mass basis is denoted by e and is expressed as

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Thus the total energy of a system can be assigned a value of zero (E=0) at some convenient reference point. The change in total energy of a system is independent of the reference point selected.

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Types of EnergyThe macroscopic forms of energy are those

a system possesses as a whole with respect to some outside reference frame, such as kinetic and potential energies.

The microscopic forms of energy are those related to the molecular structure of a system and the degree of the molecular activity, and they are independent of outside reference frames.

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Internal EnergyThe sum of all the microscopic forms of

energy is called the internal energy of a system and is denoted by U.

The macroscopic energy of a system is related to motion and the influence of some external effects such as gravity, magnetism, electricity, and surface tension.

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Kinetic and Potential Energy

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The energy that a system possesses as a result of its motion is called kinetic energy (KE). When all parts of a system move with the same velocity, the kinetic energy is expressed as

The energy that a system possesses as a result of its elevation in a gravitational field is called potential energy (PE)

Control volumes typically involve fluid flow for long periods of time, and it is convenient to express the energy flow associated with a fluid stream in the rate form. This is done by incorporating the mass flow rate m., which is the amount of mass flowing through a cross section per unit time. It is related to the volume flow rate V,

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Internal EnergyInternal energy is defined earlier as the sum

of all the microscopic forms of energy of a system. It is related to the molecular structure and the degree of molecular activity and can be viewed as the sum of the kinetic and potential energies of the molecules.

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The molecules of a gas move through space with some velocity, and thus possess some kinetic energy. This is known as the translational energy. The atoms of polyatomic molecules rotate about an axis, and the energy associated with this rotation is the rotational kinetic energy. The atoms of a polyatomic molecule may also vibrate about their common center of mass, and the energy associated with this back-and-forth motion is the vibrational kinetic energy.

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The portion of the internal energy of a system associated with the kinetic energies of the molecules is called the sensible energy.

The internal energy is also associated with various binding forces between the molecules of a substance, between the atoms within a molecule, and between the particles within an atom and its nucleus.

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If sufficient energy is added to the molecules of a solid or liquid, the molecules overcome these molecular forces and break away, turning the substance into a gas. This is a phase-change process.

The internal energy associated with the phase of a system is called the latent energy.

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The internal energy associated with the atomic bonds in a molecule is called chemical energy.

The tremendous amount of energy associated with the strong bonds within the nucleus of the atom itself is called nuclear energy.

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Example

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