Heat Transfer - Convection Currents

What happens when air is heated?

What is Needed.

· A cold can

· Hands and mouth

· A small bowl of dish soap

What to Do.

With your mouth open, blow on your hand. Now repeat, but this time, pucker your lips and make a small hole to breathe through. Notice the difference in how it feels when your mouth is wide open compared to when you pucker. Now get an ice cold can with one end open. Dip the open end of the can into a dish of soap. Make sure that a film of soap covers the end of the can. Now turn the can upright and hold it with your hands in order to warm the can. YOU DO NOT HAVE TO SQUEEZE THE CAN to allow the heat from your hands to warm it.

What is Happening?

With your mouth open, your breath is warm. When you pucker, the air is cooler because expanding air cools. This is the opposite of what occurs when air is compressed. If you have ever compressed air with a tire pump, you may have noticed that the air and the pump became hot.

Heat travels by convection. Convection is the rising motion of heat. Heating causes molecules to move faster. The hotter the air is, the faster the air molecules are moving. Fast moving molecules tend to move apart, because of this, hot air is less dense than cold. Fluids transmit heat mainly by convection currents, which is heat transfer due to the motion of the fluid itself. Convection can occur in all fluids, being liquid or gas.

Purdue University
Department of Physics
Physics on the Road Hands-On