Energy Use Investigation 8.3:


Picture of a refrigerator

Josh and Bert met after school and were looking for a snack in the kitchen at Bert's house. Josh said, “This kitchen is hot! Can’t you open a window or something?”

“What good would that do?” Bert said. “It’s even hotter outside. Besides, we shouldn’t open the window because the air conditioning is on.”

Josh said, “Let’s open the refrigerator door. That should cool things down.” Bert pulled open the refrigerator and they both stood in front of it and enjoyed the feeling of the cool air.

Bert said, “I guess no food will spoil if we leave the refrigerator open for a few minutes. Listen to it hum!”

“You know,” said Josh, “our science book says that energy is never used up. It just changes from one kind of energy to another. So where does the energy go that runs the refrigerator?”

“The refrigerator takes electrical energy and uses it to make things cold. So I guess it turns electrical energy into cold energy,” Bert said.

“No, that can’t be right,” Josh said. “I know that heat is a kind of energy, but I've never heard of cold energy.”

In charged Bert’s mother. “Close that refrigerator!” she said. “It’s too hot in here already!”

Does Bart’s mother’s request make sense?

Josh knew that energy is never used up. Bert was aware that heat is a form of energy. They have an inkling of the first law of thermodynamics: (also called the law of conservation of energy) that energy can neither be created nor destroyed, but can change from one form to another. The second law of thermodynamics, that everything tends to go from a state of order to disorder, is harder to grasp. But one of its consequences is that heat flows from hotter places to cooler places. A refrigerator, however, seems to do just the opposite. It makes itself cold on the inside by moving heat from the cooler inside to the warmer outside.

The second law of thermodynamics does allow for heat to move from a cooler place to a hotter place, but it always requires work—the input of more energy. The energy that does the work to move the heat also adds to the total amount of heat in the system.

Bert’s mother understood that by forcing the refrigerator to keep running, the boys were producing even more heat in the kitchen.