1166516167 - Chapter 13 Energy and Power Prof. N. Ghaddar...

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12/19/2006 1 Chapter 13 Energy and Power Prof. N. Ghaddar
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2 12/19/2006 Introduction ± In this chapter we will introduce the concept of energy. ± Types of energy and work are also introduced. ± Conservation of Energy and first law of thermodynamics is introduced. ± Efficiency is also explained
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3 12/19/2006 Work, Mechanical Energy, Thermal Energy ± Energy is classified into different categories: ² Kinetic Energy ² Potential Energy ² Elastic Energy ² Thermal Energy
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4 12/19/2006 Kinetic Energy (1/2) ± An object having a mass m and moving with a speed V has a kinetic energy: 2 1 2 Kinetic Energy mV = ± The SI unit for kinetic energy is the joule: () ()() 2 2 2 1 2 mm Kinetic Energy mV kg kg m N m joule J ss ⎛⎞ == = = == ⎜⎟ ⎝⎠
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5 12/19/2006 Kinetic Energy (2/2) ± When work is done on or against and object, it changes the kinetic energy of the object according to: 2 2 2 1 2 1 2 1 i f mV mV Work = ( 1 ) 2 ) i f m mass of the object V initial velocity position V final velocity position = = =
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6 12/19/2006 Potential Energy ± The work required to lift an object with a mass m by a vertical distance h is called gravitational potential energy . ± The change in potential energy of an object when its elevation is changed is given by: Change in potential Energy PE mg h = ∆=∆ ( 1 ) 2 ) i f m mass of the object V initial velocity position V final velocity position = = = () ()() 2 m Potential Energy mgh kg m N m joule J s ⎛⎞ = == = = ⎜⎟ ⎝⎠
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7 12/19/2006 Example 1
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8 12/19/2006 Elastic Energy (1/2) ± It is the energy that is stored in a spring, when compressed or stretched, and allows it to return to its unstretched position. ± It is given by: 2 1 2 Elastic energy kx = ( ) () tan / k spring cons t N m x deflection of spring from its unstretced position m = = 22 1 2 N Elastic Energy kx m N m J m ⎛⎞ = == = ⎜⎟ ⎝⎠
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9 12/19/2006 Elastic Energy (2/2) ± The energy stored in a spring is given by: 22 21 11 change in elastic energy EE kx kx =∆ =
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10 12/19/2006 Example 2
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11 12/19/2006 Conservation of Mechanical Energy ± The conservation of mechanical energy states that the total mechanical energy of a system is constant. ± The total energy of a system is given by: 0 KE PE EE ∆+ ∆= KE Change in kinetic energy PE Change in potential energy EE Change in elastic energy
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12 12/19/2006 Example 3
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13 12/19/2006 Forms of Energy Forms of Energy ± Energy is usually symbolized by E, representing total energy ± e is energy per unit mass m E e = E: extensive e: intensive
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14 12/19/2006 Forms of Energy Forms of Energy ± Macroscopic forms --possessed with respect to some outside reference frame. ± Kinetic energy, ± Potential energy, 2 m 2 1 KE V = 2 2 1 ke or V = gz pe or mgz PE = =
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15 12/19/2006 Forms of energy Forms of energy ± Microscopic forms are called internal energy (internal to the molecule) and represent the energy a molecule can have as it translates, rotates, and vibrates.
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1166516167 - Chapter 13 Energy and Power Prof. N. Ghaddar...

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