From heat and temperature you know that heat is energy flowing from a hotter object to a colder one. It can travel in three ways, and most real situations use more than one.
Key idea
Conduction: heat passes through touching materials, particle to particle. Convection: a warmed fluid (liquid or gas) moves and carries the heat with it. Radiation: heat travels as light, mostly infrared you can't see, even through empty space.
Why it works
In conduction, fast-jiggling particles bump their neighbors and pass energy along; metals do this especially well. In convection, warm fluid expands, becomes less dense and rises, while cooler fluid sinks to take its place, which sets up a circulating current. Radiation needs no material at all, which is how sunlight warms Earth across 150 million km of space.
Worked example: A pot of soup
Name the ways heat moves as soup heats on a stove.
- The burner heats the metal pot bottom by contact, and heat passes through it: conduction.
- Warm soup at the bottom rises and cooler soup sinks: convection.
- You can feel the glowing burner's warmth from the side: radiation.
Answer: All three
Common mistake
Thinking a metal bench feels colder because it is colder than a wooden one in the same park. Both are at the air temperature. Metal conducts heat away from your skin much faster than wood, so your skin cools faster, and that is what you feel. Check: a thermometer would read the same on both.
| Way | Needs | Example |
|---|---|---|
| Conduction | Touching materials | A metal spoon warming in hot cocoa |
| Convection | A moving liquid or gas | Warm air rising from a heater |
| Radiation | Nothing (works in space) | Sunlight warming your face |