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Lecture# 3 The first law of thermodynamics

Lecture# 3 The first law of thermodynamics. FORMS OF ENERGY: 1-Total Energy:[E] But in unit mass [e].

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Lecture# 3 The first law of thermodynamics

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  1. Lecture# 3The first law of thermodynamics • FORMS OF ENERGY: • 1-Total Energy:[E] • But in unit mass [e] It’s the law of conservation of energy. Its statement is, “Although energy assumes many forms, the total quantity of energy is constant and when energy disappears in one form it appears in other forma”.

  2. 2-Internal energy of a system and is denoted by U. 3-The energy that a system possesses as a result of its motion relative to some reference frame is called kinetic energy (KE).

  3. 4-The energy that a system possesses as a result of its elevation in a gravitational field is called potential energy (PE) and is expressed as: the total energy of a system consists of the kinetic, potential, and internal energies and is expressed as:

  4. Heat is defined as the form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference.

  5. Then we can simply say that an energy interaction that is not caused by a temperature difference between a system and its surroundings is work. More specifically, work is the energy transfer associated with a force acting through a distance. A rising piston, a rotating shaft, and an electric wire crossing the system boundaries are all associated with work Interactions:

  6. Energy Transfer by Heat and Work 1-In what forms can energy cross the boundaries of a closed system? 2–C When is the energy crossing the boundaries of a closed system heat and when is it work? 2– What is an adiabatic process? What is an adiabatic system? 3– A gas in a piston–cylinder device is compressed, and as a result its temperature rises. Is this a heat or work interaction?

  7. Solve the problem: Water is being heated in a closed pan on top of a range while being stirred by a paddle wheel. During the process, 30 kJ of heat is transferred to the water, and 5 kJ of heat is lost to the surrounding air. The paddle-wheel work amounts to 500 N · m. Determine the final energy of the sys-tem if its initial energy is 10 kJ.

  8. Energy Balance of closed systems:

  9. Example 1:

  10. Example 2:

  11. PURE SUBSTANCE PROPERTIES OF PURE SUBSTANCES

  12. PHASES OF A PURE SUBSTANCE • Solid • Liquid • Gas. PHASE-CHANGE PROCESSES OF PURE SUBSTANCES

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