# Want to see another example? (Give one more example like above in pop-up textbox.) I will get this info

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Want to see another example? (Give one more example like above in pop-up textbox.) I will get this info.
Instead of example, Sarma wants cautionary note – wants it to have the yellow caution tape in it.
Caution! This is a simple example because both variable are measured in Watts. If the two variables were measured differently, you would need to convert them to equivalent forms before performing the calculation.

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Vinit’s problems – 3-1

Example for equation 3.1

Illustration:

An electric motor consumes 100 watts (a joule per second (J/s)) of power to obtain 90 watts of mechanical power. Determine its efficiency?

Now lets see the solution............

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
 Efficiency = Useful Energy Output Total Energy Input = 90 W 100 W = 0.9

 = 90 %

Why dont you give it a shot now....................

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An electric motor consumes 92 watts (a joule per second (J/s)) of power to obtain 83 watts of mechanical power. Determine its efficiency?

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Illustration 3-1 is a very simple case because both mechanical and electrical power is given in Watts. Units of both the input and the output have to match.

I need to find an example
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Vinit’s Problem 3-2

Another Example for equation 3.1

Illustration:

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.?

Now lets see the solution............

Step 1: To find the efficiency, both the units of input energy and the output energy have to be same. So we need to convert kWh into Btus.
 1 kWh = 3412 Btus Therefore 3.675 x10^12 kWh = 3.675 x 10^12 kWh x 3412 Btus 1 kWh = 12539.1 x 10^12 Btus

Step 2: Use the formula for efficiency.
 Efficiency = Useful Energy Output Total Energy Input = 12539 x 10^12 Btus 39.5 x 10^15 Btus = 0.3174

 = 31.74 %

Why dont you give it a shot now....................

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The United States power plants consumed 37 quadrillion Btus of energy and produced 2 trillion kWh of electricity. What is the average efficiency of the power plants in the U.S.?

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