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Workshop 7. Heat, Work, and Efficiency

A machine's energy output cannot be greater than its input. In this session, look at the energy that goes into useful work, examine how some always ends up as heat, and see why systems are never 100% efficient.



Program Questions

Answers

1. How can we get work from heat?

We get work from heat by exploiting a temperature difference between two parts of a system, usually, by expanding and contracting a gas.

2. Why do hot things cool down and cold things warm up?

Heat is the disordered motion of molecules. Hot places — ones where the molecules are moving more vigorously — will scatter their kinetic energy by the randomly bumping and jostling slower moving, or cooler, molecules. This makes the hot places cool down. When the cooler molecules start moving faster, they become warmer.

3. What factors limit a system's efficiency in getting work from heat?

The key factor is the temperature difference between the system's heat source and the heat sink. The greater the temperature difference, the higher the possible efficiency.

4. How are heat, work and friction related?

Friction converts the ordered energy of mechanical motion into the disordered — and less useful — energy of heat.

 

WWWFurther Explanations

Host

1. Molecules in the Classroom

Elevator

2. How Does Heat Travel?

Molecules

3. Efficiency

Toy Car

4. Entropy

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