Energy Efficiency

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Energy efficiencies are not 100% and sometimes they are pretty low. The table belows shows typical efficiencies of some of the devices that are used in day to day life.

Table 3‑1 Typical Efficiencies of Some of the Commonly Used Devices



Electric Motor


Home Gas furnace


Home Oil Furnace


Home Coal Stove


Steam Boiler in a Power Plant


Overall Power Plant


Automobile Engine


Electric Bulb





From our discussion on national and global energy usage patterns in Lesson 2, we have seen that:

Yet the energy efficiency of a power plant is about 35%, and the efficiency of automobiles is about 25%. Thus, over 62% of the total primary energy in the U.S. is used in relatively inefficient conversion processes.

Why power plant and automobile design engineers allowing this? Can they do better?

There are some natural limitations when converting energy from heat to work.

3.3 Measuring Thermal Energy

Thermal energy is energy associated with random motion of molecules. It is indicated by temperature which is the measure of the relative warmth or coolness of an object.

A temperature scale is determined by choosing two reference temperatures and dividing the temperature difference between these two points into a certain number of degrees.

The two reference temperatures used for most common scales are the melting point of ice and the boiling point of water.

  • On the Celsius temperature scale, or centigrade scale, the melting point is taken as 0°C and the boiling point as 100°C, and the difference between them is divided into 100 degrees.

  • On the Fahrenheit temperature scale, the melting point is taken as 32°F and the boiling point as 212°F, with the difference between them equal to 180 degrees.

It is important to realize, however, that the temperature of a substance is not a measure of its heat content, but rather, the average kinetic energy of its molecules resulting from their motions.


Below is a 6-ounce cup with hot water and 12 ounce cup hot water at the same temperature.

  1. Do they have the same heat content?

  2. Do they have the same amount of energy?

Click the play button to obtain a magnified view of what is happening. Draw your conclusions and then check your answer below.


Flash: Show 6 and a 12 ounce cups (clear would be good) with thermometers in them, show close up of thermometer and temperature (same temperature). Then show close up of each, with molecules. The 12 ounce cup should show a lot more molecules than the 6 ounce, though both should be moving around a bit.

Answer: They do not have the same heat content. Because they are at the same temperature the average kinetic energy of the molecules is the same; however, the water in 12 ounce cup has more molecules than the 6 ounce cup and thus has greater motions or heat energy.


3.4 Kelvin Scale

When water molecules freeze at 0°C, the molecules still have some energy compared to ice at -50°C. In both cases, the molecules are not moving, so there is no heat energy.

So what is the temperature at which all the molecules absolutely have zero energy? A temperature scale can be defined theoretically for which zero degree corresponds to zero average kinetic energy. Such a point is called absolute zero, and such a scale is known as an absolute temperature scale. At absolute zero, the molecules do not have any energy.

The Kelvin temperature scale is an absolute scale having degrees the same size as those of the Celsius temperature scale. Therefore, all the temperature measurements related to energy measurements must be made on Kelvin scale.

Combine thermometers with an animation. Press play and observe what happens:

Animation goes here:

Show ice – thermometers show temperature for water freezing.
Place ice into a pan on the stove and it melts – thermometers show temperature for melting water.
Water in ban starts to boil – thermometers show temperature for boiling water.

Based on your observations, answer the following questions:

At what temperature does water freeze?

___ Kelvin ____ Celsius ____ Fahrenheit

At what temperature does ice melt?

___ Kelvin ____ Celsius ____ Fahrenheit

At what temperature does water boil?

___ Kelvin ____ Celsius ____ Fahrenheit


Add pop-up “More Information” text box with the above screen:

You can convert a temperature in Celsius (c) to Kelvin (k) with this formula:

k = c + 273.15

You can also change a temperature in Kelvin to Celsius:

c = k - 273.15

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