Today’s title brought to you by my refrigerator, which beeps if you leave the door open for too long. I imagine it’s the light bulb letting you know that it’s tired of being on.
This modern marvel automates the scolding our parents always gave us when we spent too long contemplating the apple-juice-vs.-orange-juice decision: “Close the fridge, you’re letting the cold out.” Which is an environmentally friendly, if not altogether scientifically accurate, chastisement. It would be more accurate to say that you’re letting the heat in.
While cold air can flow out of a fridge when you open the door, a chilled milk carton sitting on the shelf doesn’t emanate “cold.” If you propped the door open, the carton would warm up. But it wouldn’t warm up because it released all the “cold” it was holding onto, it would warm up because it absorbed energy from the warm air around it.
Temperature is a measure of the average kinetic (motion-related) energy of the particles in a substance. The faster the particles in a substance are moving, the higher the temperature of the substance. So if the temperature of a substance rises, it means that the substance’s particles sped up. In order to move faster, the particles had to absorb energy from somewhere else. When you leave the refrigerator door open, the warm air particles will transfer energy to the cold particles in the milk carton. This energy transfer is what we call heat. Heat always flows from warmer substances to cooler substances. In the case of the milk carton, heat will flow from the warm air to the cold milk carton, cooling down the air and warming up the carton, until the two have reached the same temperature. Yay thermal equilibrium!
It’s heat flow that causes objects to feel hot or cold. Your body isn’t a thermometer; you don’t feel absolute temperature. Something feels hot if heat flows into your hand when you touch it, and something feels cold if heat flows out of your hand when you touch it.
Take three cups of water: one cold, one hot (but not too hot! hot water straight from the tap is fine!), and one room temperature. Put your left hand in the cold water and your right hand in the hot water. Leave them there for 20-30 seconds, then place both of your hands in the room temperature water. If your hands felt absolute temperature, they should feel the same when you put them in the room temperature water. They won’t. Is heat flowing into or out of your right hand? What about your left hand? How does the direction of heat flow explain what’s going on?