Tess and Harry on the Dock
“My cocoa got cold,” said Tess.
“That’s because of the second law of thermodynamics,” said Harry.
“Can’t it ever just be because I left it out on the dock on a chilly night?”
“That’s another way of saying the same thing.”
“Okay, fine, tell me about the second law of thermodynamics.”
“It says that if you leave a cup of hot cocoa on the dock on a chilly night, the cocoa will get cold.”
“They wrote a whole scientific law just for that?”
“More or less.”
“But that’s just common sense.”
“Is it?”
“Sure. If I leave something hot someplace cold, then it’s going to cool off.”
“But why shouldn’t it be the other way around?”
“What do you mean?”
“Even chilly air has some heat in it, right?”
“Does it?”
“If you left an ice cube on the dock tonight, what would happen?”
“It would melt.”
“Why?”
Tess thought for a moment. “Because the heat in the air would pass into the ice cube?”
“Exactly. Which means there is heat in the air.”
“Okay.”
“So why doesn’t that heat pass into your cocoa and warm it up? After all it passed into the ice cube.”
“But it can’t. You can’t warm something up with something that’s colder than the thing you want to warm up.”
“Exactly, and that’s the second law of thermodynamics—beautifully put, I might add. You can’t warm something up with something that’s colder than the thing you want to warm up.”
“That’s the way they write it in the physics books?”
“Not exactly, but it amounts to the same thing.”
“So, eventually wouldn’t the air and the cocoa be the same temperature?”
“That’s right. They will reach thermodynamic equilibrium.”
“What’s thermodynamic equilibrium?”
“It means they’re the same temperature.”
“That’s what I just said. Why does everything have to have big, long words to describe it when there are short words that mean the same thing?”
“Finally a question I can’t answer, my dear Tess. If you ever find out, I hope you’ll let me know.”
“If the cocoa and the air will eventually be the same temperature, doesn’t that mean that sooner or later everything will be the same temperature?”
“An astute leap for a ten-year-old. Yes, many people believe that ultimately the entire universe will reach thermodynamic equilibrium.”
“And will it be warm like the cocoa or cold like the air?”
“Put it this way,” said Harry. “I think you’ll need a jacket.”