What is Convection?

Written by: Dr. Crystal Cooper • Edited by: Lamar Stonecypher
Published Aug 30, 2009 • Related Guides: Cooking

By what process is heat transferred to the boiling water you use to steam vegetables such as a corn cob, or to your room when it is heated by a radiator?

Introduction

click to enlarge

When I was a brand new research associate at the University of Maryland, I rented an apartment not far from campus. The apartment was carpeted, and had plenty of space. It had one quirk which I unfortunately did not notice in my haste to rent it. The heating vents were up high on the walls, practically at the ceiling. At night it became very cold, despite the fact that management was generous with the heat. If I closed my bedroom door, I would nearly freeze. My only recourse was to keep the door open to let the heat circulate, and this would allow me to get some sleep.

My unfortunate encounter with the heating system was due to the fact that its designers ignored the physics of heat transfer by convection.

Heat Convection Physics

click to enlarge

Air and water, like typical gases and liquids, are very poor conductors. However, they have a heat transfer mechanism through the process of convection. Basically, convection is heat transfer via fluid flow. When you watch steam rise from a cup of hot chocolate or watch vegetables boiling in water or cooking in a steamer, you are observing this effect.

Convection occurs because fluid that is heated becomes less dense, and as a result, it rises. A pot of water placed on the stove heats from the bottom up as convection currents form, rise, and are then replaced by cooler water. The cooler water then heats, and the process is repeated.

Comparison to Heat Conduction

In conduction, heat is transferred directly by two objects touching. In convection however, heat is transferred by the movement of molecules from one place to another. Unlike conduction, where electrons and atoms move very short distances, the molecules in convection move long distances. Also, whereas in conduction successive free electrons and atoms transfer heat through collisions, in convection the entire fluid mass moves.

Examples

The cold and warm water currents in the ocean are examples of convection. Your home is heated by the process of convection.

click to enlarge

The heater increases the temperature of the water, which then rises and expands. The hot water increases the temperature of the air as it circulates through radiators or floor vents. This heated air rises and is replaced by cooler air repeatedly in the convective process described earlier. It is because of convection that vents are usually placed on the floor, and why buildings with high ceilings are hard to keep warm.

Weather changes also proceed through convection. The movement of wind is a good example. Wind occurs as the sun heats the surface of the Earth and causes air to also heat and then move. Thunderstorms also contribute to air movement.

Other examples of heat convection include blood as it transfers heat to the surface of the skin, heat flow from a burning candle, and smoke as it rises from the end of a cigarette. Convection due to the heating of water that turns it to steam is the reason you need to be careful opening your bag of popcorn when you take it from the microwave.

Convection is studied by architects and engineers and physical scientists. The consequences of people involved in construction ignoring its principles may be dire, such as the inadvertent freezing of a newly hired research associate.

Next: Why do you feel cooler if you wear white clothes instead of black ones on a hot sunny day? And why is the inside of a thermos bottle shiny? Continue on to part three to find out.

References

Conceptual Physics by Paul Hewitt

Essential University Physics by Paul Wolfson

Fundamentals of Physics by Halliday, Resnick, and Walker

Physics for Scientists and Engineers by Douglas Giancoli

Resources

Heat Conduction and Convection on HyperPhysics

Humourous Student Convection Experiment Video

Charles' Law and Microwaving Popcorn

Professsor Julius Sumner Miller's Convection Demo Video