chapter 3

# chapter 3 - 3.1 Energy Conversion Devices In the first...

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3.1 Energy Conversion Devices In the first lesson, we saw that energy can be transformed from one form to another, and during this conversion, all the energy that we put into a device comes out. However, all the energy that we put in may not come out in the desired form. For example, we put electrical energy into a bulb and the bulb produces light (which is the desired form of output from a bulb), but we also get heat from the bulb (undesired form of energy from an electric bulb). Therefore, energy flow into and out of any energy conversion device can be summarized in the diagram below: When all forms of energy coming out of an energy conversion device are added up, it will be equal to the energy that is put into a device. Energy output must be equal to the input. This means that energy can not be destroyed or created. It can only change its form. In the case of an electric bulb the electrical energy is converted to light and heat. Efficiency is the useful output of energy. To calculate efficiency the following formula can be used:

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Example 1 An electric motor consumes 100 watts (a joule per second (J/s)) of power to obtain 90 watts of mechanical power. Determine its efficiency. Solution: Input to the electric motor is in the form of electrical energy and the output is mechanical energy. Using the efficiency equation, Or efficiency is 90%. Practice An electric motor consumes 100 watts (a joule per second (J/s)) of power to obtain 90 watts of mechanical power. Determine its efficiency. Step 1 Input to the electric motor is in the form of electrical energy and the output is mechanical energy. Using the given formula for efficiency The previous example about an electrical motor is very simple because both mechanical and electrical power is given in Watts. Units of both the input and the output have to match; if they do not, you must convert them to similar units. Example 2 The United States Power plants consumed 39.5 quadrillion Btus of energy and
produced 3.675 trillion kWh of electricity. What is the average efficiency of the power plants in the U.S.? Solution: Total Energy input = 39.5 x 10 15 Btus and the Useful energy output is 3.675 x 10 12 kWh. Recall that both units have to be the same. So we need to convert kWh into Btus. Given that 1 kWh = 3412 Btus, 3.2 Efficiency of Energy Conversion Devices Practice The United States power plants consumed 39.5 quadrillion Btus of energy and produced 3.675 trillion kWh of electricity. What is the average efficiency of the power plants in the U.S.? Step 1

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chapter 3 - 3.1 Energy Conversion Devices In the first...

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