Heat & temperature (intro)
The idea
Temperature tells you how hot or cold something is, and a thermometer measures it as a number — a higher number means hotter stuff. Heat is something different: heat is warmth on the move. Whenever a warm thing and a cool thing touch, warmth flows from the warmer one into the cooler one. That flow is what melts an ice pop in your hand, and what slowly steals the warmth from your cocoa on a chilly morning.
The direction never changes: heat always moves from warmer to cooler, never the other way. That is the big mix-up to fix — when you hold an ice cube, it feels like cold is creeping into your hand, but really warmth is leaking out of your hand into the ice. Your hand loses heat; the ice gains it and melts. Left alone together, warm things cool down and cool things warm up, until everything matches.
Worked example
Your room reads 20 °C on the thermometer. You set out a cup of warm water that reads 40 °C and a glass of icy water that reads 5 °C, then leave both alone for an hour. What does the thermometer show afterward, and which way did heat move?
- Compare each drink with the room at the start: the warm water (40 °C) is warmer than the room (20 °C), and the icy water (5 °C) is cooler than the room.
- Use the rule that heat flows from warmer to cooler. The warm cup should leak heat into the cooler room air, and the warmer room air should leak heat into the icy glass.
- Predict the changes: the warm water should cool down toward 20 °C, and the icy water should warm up toward 20 °C — both drifting toward the room from opposite sides.
- Check after an hour: the warm cup reads about 24 °C and the icy glass about 17 °C, with the ice melted away. Both numbers moved toward 20 °C, just as the rule predicted.
- Notice what did not happen: the warm water never got warmer, and the icy water never got icier on its own. Heat moved one way only — from warmer to cooler.
Answer. After an hour both drinks sit near room temperature — about 24 °C and 17 °C — because heat leaked out of the warm water into the room, and from the room into the icy water.
Check your understanding
- Why does an ice pop melt faster in your hand than on a plate, and where is the heat coming from?
- What is really happening when someone says to close the door so the cold does not get in?
- How are temperature and heat different from each other, and which one does a thermometer measure?
- What do you expect a glass of lemonade and a bowl of soup to read on a thermometer after sitting in the same room all afternoon, and why?