A heat engine is any device that converts heat into work. One consequence of the second law of thermodynamics is that the efficiency of a heat engine is always less than 100 percent. The best an engine can do is to convert most of the input energy into useful work. Thermal energy that is not converted into work is called waste heat. Waste heat is lost to the surrounding environment. In fact, a heat engine can do work only if some waste heat flows to a colder environment outside the engine.

Spontaneous changes will always make a system less orderly, unless work is done on the system. For example, if you walk long enough, your shoelaces will become untied. But the opposite won't happen; shoelaces don't tie themselves. Disorder in the universe as a whole is always increasing. You can only increase order on a local level. For instance, you can stop and tie your shoelaces. But this requires work. Because work always produces waste heat, you contribute to the disorder of the universe when you stop to tie a shoelace!

Third Law of Thermodynamics

The efficiency of a heat engine increases with a greater difference between the high temperature inside and the cold temperature outside the engine. In theory, a heat engine could be 100 percent efficient if the cold outside environment were at absolute zero (0 kelvins). But this would violate the third law of thermodynamics. The third law of thermodynamics states that absolute zero cannot be reached. Scientists have been able to cool matter almost all of the way to absolute zero. Figure 10 shows the equipment used to produce the record lowest temperature, just 3 billionths of a kelvin above absolute zero!

Figure 10 The third law of thermodynamics states that absolute zero cannot be reached. This physicist is adjusting a laser used to cool rubidium atoms to 3 billionths of a kelvin above absolute zero. This record low temperature was produced by a team of scientists at the National Institute of Standards and Technology.

Section 16.2 Assessment

Reviewing Concepts

1. Why is conduction in gases slower than conduction in liquids or solids?

2. Give three examples of convection currents that occur in natural cycles.

3. What happens to radiation from an object as its temperature increases?

4. State the first law of thermodynamics.

5. In your own words, what is the second law of thermodynamics?

6. State the third law of thermodynamics.

7. Why does a metal spoon feel colder than a wooden spoon at room temperature?

8. Why is solar energy transferred to Earth by radiation?

Critical Thinking

9. Applying Concepts If your bedroom is cold, you might feel warmer with several thin blankets than with one thick one. Explain why.

10. Relating Cause and Effect If every object is radiating constantly, why aren't all objects getting colder?

Connecting Concepts

Conservation of Energy Review energy conservation in Section 15.2. Describe how the first and the second laws of thermodynamics are consistent with the law of conservation of energy.

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