Energy Efficiency

Overall Efficiency of a Light Bulb

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Overall Efficiency of a Light Bulb


Step Efficiency

Cumulative Efficiency or Overall Efficiency

Extraction of coal





94% (0.96 X 0.98)*100

Electricity Generation


33% (0.96 X 0.98 X 0.38)

Transmission of Electricity




Incandescent bulb






AUDIO: It can be seen that to generate 6.2 units of light from a relatively efficient fluorescent bulb, we used up 100 units of energy from coal from the ground. This also means that during various conversion steps 93.8 units of energy is dissipated into the environment.


A similar analysis on Automobile shown in Figure 3-7 and Table 3-3 shows that only about 10% of the energy in the crude oil in the ground is in fact turned into mechanical energy moving people.

Figure – missing from word document but in printed document – need to get from Sarma:

Titled “Overall Automobile Efficiency”- shows sequence of steps in converting chemical energy in crude oil in the ground to movement of a car:

Production of crude Transportation to refinery Refining Transportation of gasoline Engine Transmission Movement


Overall Efficiency of an Automobile


Step Efficiency

Overall or Cumulative Efficiency

Production of Crude















    1. Perhaps audio to explain the above more fully??


BUT There is a crossword puzzle at the very end…
    1. Selected References:

Hinrichs, R. A., “Energy,” Saunders College Publishers, Philadelphia, PA, 1992.

Aubrecht, G. L., “Energy,” Prentice Hall, Inc., Englewood Cliffs, NJ, 1995.

Fay, J.A. and Golomb, D. S., “Energy and the Environment,” Oxford University Press, New York, NY, 2002.

Christensen, J. W., “Global Science: Energy Resources Environment”, 4th edition, Kendall/Hunt Publishing Company, Dubuque, IA, 1996.

    1. Questions for Review and Discussion

  1. A heat engine has Carnot efficiency of 30%. Useful output from the engine is 1000J. How much heat is wasted?

  2. How can we improve the Carnot efficiency of a heat engine by changing the hot and cold reservoir temperatures?

  3. Most of the energy conversion devices that we use in our day-to-day life can be classified as Heat Engines. Give two examples

  4. The following diagram shows the energy flow to and from a furnace.

Coal furnace

Heat energy

Rejected though the tail pipe = 29,045,000 J

Energy input is 30,000,000 cal

Calculate is the efficiency of the furnace. You need show your work very clearly to get complete credit

    1. Multiple Choice Questions

  1. Approximately what percentage of electricity does an incandescent light bulb convert into visible light?

        1. 5

        2. 20

        3. 40

        4. 90

  1. The following step efficiencies apply to the use of gasoline in a car: Crude production: 96%, Refining 87%, Transportation 97%, Engine efficiency 25%. What is the total efficiency of the process?

      1. 40%

      2. 30%

      3. 20%

      4. 50%

  1. The flame temperature in an automobile is 1,000 °C, and the exhaust is emitted at 70 °C. What is the Carnot efficiency?

  1. 25%

  2. 65%

  3. 73%

  4. 33%

  1. If the energy input of a system is 50 calories and the output is 25 calories, what is the system efficiency?

  1. 100%

  2. 50%

  3. 200%

  4. 25%

  1. Heat engines are inefficient because the energy conversion is

  1. From low entropy to high entropy

  2. From high entropy to low entropy

  3. From low temperature to high temperature

  4. From high temperature to low temperature

  1. Automobile engine efficiency increases in

    1. Summer

    2. Winter

  2. The useful output from a heat engine is 238 cal. The energy that is wasted is 5667 J? What is the Carnot efficiency of the engine?

    1. 4%

    2. 15%

    3. 17.6%

    4. none of the above

  1. Three energy conversion processes take place in succession. The first has an efficiency of 50%, the second is 40% efficient, and the third 5%. What is the overall efficiency (%) of the entire process? (2 points)

  1. 10

  2. 40

  3. 50

  4. 5

  1. The turbine is the _______ efficient component in a power plant

  1. Least

  2. Highest

  1. The flame temperature in an automobile is 1652 °F, and the exhaust is emitted at 212 °F. What is the Carnot efficiency?

  1. 88.8%

  2. 87.2%

  3. 68.2%

  4. 25%

  1. The function of a generator in a power plant is to convert

  1. Chemical energy to mechanical energy

  2. Mechanical energy to electrical energy

  3. Thermal energy to mechanical energy

  4. None of the above

  1. Which of these is False (2 points)

  1. In a power plant_________energy is converted into_______energy.

    1. Chemical, Mechanical

    2. Chemical, Electrical

    3. Mechanical, Electrical

    4. Mechanical, Thermal

  1. Which of the following devices is least energy efficient?

    1. Power plant

    2. Electric motor

    3. Light bulb

  1. Most energy conversion process produce----------- as by product

  1. Light

  2. Heat

  3. Sound

  4. Motion

Energy Basics Review Puzzle


1. Conversion of chemical energy to this form is notoriously inefficient.

3. This law states that we cannot create or destroy energy

7. Temperature is a measure of this form of energy

9. Rate at which energy is spent

10. Ability to do work

11. Useful energy divided by the total energy is called

12. This source of energy supplies most of the US energy needs


2. This primary source is used for most of the electricity generated in this country

4. Second largest primary source for electricity generation

5. Measure of disorder

6. Most households use this fuel for home heating

8. The form of energy in gasoline

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