Two sealed beakers, one connecting rod — watch heat flow until they reach equilibrium.
| Time (s) | T — A (°C) | T — B (°C) | Difference (°C) |
|---|---|---|---|
| Click Play to start recording temperatures every 20 seconds. | |||
Heat moves from hot things to cold things. It never moves the other way by itself. The bigger the temperature gap between the beakers, the faster heat moves. As the beakers get closer to the same temperature, heat moves more slowly. That's why each curve is steep at first and flattens out later.
Both beakers are sealed and insulated, so no heat can escape into the room. Whatever heat Beaker A loses, Beaker B gains. That means if you add the two temperatures together, you always get 100°C. Eventually the beakers reach the same temperature, 50°C. Scientists call this thermal equilibrium.
A thermal conductor lets heat move through it quickly. A thermal insulator slows heat down. Scientists measure this with a number called thermal conductivity, or k, measured in watts per meter-kelvin (W/m·K). A bigger k means heat moves faster through that material.
| Material | k (W/m·K) |
|---|---|
| Copper | 385 |
| Aluminum | 205 |
| Steel | 50 |
| Glass | ~1 |