Now that you understand how conduction transfers themal energy between two objects that are touching--a cold spoon left in a hot cup of tea, for example--we can move on to another question. How does blowing on that cup of tea cool it down?
Watch this video to find out how convection causes heat energy to move through liquids and gases. As you watch the segment, pay special attention to the difference between conduction and convection. You might want to check the definitions of specific terms as you go.
Stop the animation at any time, watch it again, or read the transcript.
| Keyboard Shortcut | Action |
|---|---|
| Space | Pause/Play video playback |
| Enter | Pause/Play video playback |
| m | Mute/Unmute video volume |
| Up and Down arrows | Increase and decrease volume by 10% |
| Right and Left arrows | Seek forward or backward by 5 seconds |
| 0-9 | Fast seek to x% of the video. |
| f | Enter or exit fullscreen. (Note: To exit fullscreen in flash press the Esc key. |
| c | Press c to toggle captions on or off |
Place your hand above a steaming cup of tea and what do you feel? Yep. Thermal energy, aka heat. And the reason you feel that heat is a type of energy transfer called convection.
Convection is the transfer of thermal energy through liquids and gases. You may know it better by the adage “hot air rises”, but as gas and liquids get hot, the space between molecules expands, the substance becomes less dense, and the particles rise. As they do, they lose energy (cool) and sink back down. This creates a nice circulation pattern of thermal energy.
The defining feature of convection is that energy is transferred along with the molecules. The particles don't stay put, they actually move from place to place and take their energy with them.
Convection is the way in which ice melts (air transfers heat to ice). It’s why water comes to a rolling boil (circulation of water in pot), why steam rises, and air poppers produce a perfect bowl of popcorn. Because what goes up will eventually lose thermal energy-- and must come down.
In the space below give two examples of heat transfer by convection in a liquid or gas. For each example, be sure to describe what is happening to the motion of the molecules.