Heat Transfer Mechanisms- Convection, Conduction and Radiation.

Heat energy can be transferred by three different mechanisms:

• Convection: This involves the material moving, literally carrying the heat energy away. For example, bubbles in a boiling pot of water rise to the surface. The less dense hot water rises to the top and therefore transfers heat from the bottom of the pan to the top. The candle convection chamber you used in the lab is an example of air moving with convective currents.
• Conduction: Conduction requires a solid material and a temperature difference across it. Heat energy is transferred by moving and vibrating molecules interacting with each other. The higher the temp, the more the molecules vibrate. The rate at which heat is transferred in conduction depends on the material (metal conducts better than wood) and the size of the material. Recall which experiments displayed heat transfer by conduction. (Have you ever tried to drink coffee or cocoa from a tin cup? Why is a ceramic cup better? Answer this and then you know about conduction of heat energy.) Q/t = (kA(T₂-T₁))/thickness
• Radiation:
  • Unlike convection and conduction, radiation of heat energy from a hot body to a cooler one can take place without any material in between.
  • The object that is giving up heat energy need not be touching the object being heated.
  • All objects radiate heat.
  • The hotter the object, the more it radiates.
  • Radiation rates also depends upon the characteristics of the material including the color. A good radiator is a good absorber
  • Waves: velocity = (frequency x wavelength)
  • Energy depends on wavelength (color)
• Radiation of heat energy is how you warm yourself by standing next to a cozy woodstove. You are radiating to the stove, but the stove is radiating more energy to you because it is hotter. Therefore more energy is transferred to you that you to the stove and you warm up.
• Radiation of heat energy from the sun is responsible for life on Earth.
• The Earth is heated from the sun, but the Earth also radiates back to the sun during the day and into space at night. Keeps a thermal equilibrium.

To better understand the three mechanisms for heat energy transfer, imagine you were charged with doing things to lower the heating bill (weatherize) an ancient cathedral in Bath, England.

The first thing you should do is repair all those fancy windows and doors. They are a source for drafts, where warm air leaks to the outside or cold air infiltrates the cathedral. This is heat energy transfer by convection, because warm air is moving, taking heat energy with it. Studies show that cutting down air leakage is the most cost-effective form of weatherization.

The next thing to consider is insulating the ceilings, walls and floors of the cathedral. Insulation cuts down on heat energy transfer out of the building via conduction. As warm air typically accumulates near the ceiling (why is that?), a greater difference in temperature exists between the ceiling and outdoors than elsewhere in the building. That is the place to start adding insulation.

After you have caulked (cut drafts) and insulated ye ole cathedral, its time to think of extreme measures. For example, you might want to paint the outside of the cathedral black so that it better absorbs heat energy from the sun. Why is this extreme? Well, it really is the least cost-effective thing you can do. Heat loss or gain by radiation is usually a small factor affecting a building heat budget. Besides, whether this helps or not really depends on the temperature outdoors. During summer you may end up overheating the cathedral. And then where would they sing evensong?

All that aside, conservation of a building's thermal energy can be thought of as another energy source. Because a weatherized building uses less energy for heating, that saved energy can be used elsewhere. Conservation of thermal and other forms of energy has an added benefit. It is 100% efficient!

Heat Storage and Heat capacity