Thermo-Ch2 - 2-1 Chapter 2 ENERGY ENERGY TRANSFER AND...

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2-1 Chapter 2 ENERGY, ENERGY TRANSFER, AND GENERAL ENERGY ANALYSIS Forms of Energy 2-1C Initially, the rock possesses potential energy relative to the bottom of the sea. As the rock falls, this potential energy is converted into kinetic energy. Part of this kinetic energy is converted to thermal energy as a result of frictional heating due to air resistance, which is transferred to the air and the rock. Same thing happens in water. Assuming the impact velocity of the rock at the sea bottom is negligible, the entire potential energy of the rock is converted to thermal energy in water and air. 2-2C Hydrogen is also a fuel, since it can be burned, but it is not an energy source since there are no hydrogen reserves in the world. Hydrogen can be obtained from water by using another energy source, such as solar or nuclear energy, and then the hydrogen obtained can used as a fuel to power cars or generators. Therefore, it is more proper to view hydrogen is an energy carrier than an energy source. 2-3C The macroscopic forms of energy are those a system possesses as a whole with respect to some outside reference frame. The microscopic forms of energy, on the other hand, are those related to the molecular structure of a system and the degree of the molecular activity, and are independent of outside reference frames. 2-4C The sum of all forms of the energy a system possesses is called total energy . In the absence of magnetic, electrical and surface tension effects, the total energy of a system consists of the kinetic, potential, and internal energies.

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2-2 2-8 The specific kinetic energy of a mass whose velocity is given is to be determined. Analysis Substitution of the given data into the expression for the specific kinetic energy gives kJ/kg 0.45 = = = 2 2 2 2 /s m 1000 kJ/kg 1 2 ) m/s 30 ( 2 ke V 2-11 The total potential energy of an object is to be determined. Analysis Substituting the given data into the potential energy expression gives kJ 19.6 = = = 2 2 2 /s m 1000 kJ/kg 1 m) 20 )( m/s 8 . 9 ( kg) (100 PE mgz
2-3 2-12 A river is flowing at a specified velocity, flow rate, and elevation. The total mechanical energy of the river water per unit mass, and the power generation potential of the entire river are to be determined. Assumptions 1 The elevation given is the elevation of the free surface of the river. 2 The velocity given is the average velocity. 3 The mechanical energy of water at the turbine exit is negligible. Properties We take the density of water to be ρ = 1000 kg/m 3 . 90 m River 3m/s Analysis Noting that the sum of the flow energy and the potential energy is constant for a given fluid body, we can take the elevation of the entire river water to be the elevation of the free surface, and ignore the flow energy. Then the total mechanical energy of the river water per unit mass becomes kJ/kg 0.887 = + = + = + = 2 2 2 2 2 mech /s m 1000 kJ/kg 1 2 ) m/s 3 ( m) 90 )( m/s (9.81 2 V gh ke pe e

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Thermo-Ch2 - 2-1 Chapter 2 ENERGY ENERGY TRANSFER AND...

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