• Explain the three types of heat transfer: conduction, convection and radiation.
• Identify materials that are good insulators or conductors of heat.

Conduction is heat flow from one part of a solid object to another or between two objects in contact with one another. Perhaps the best way to understand this is through a discussion of molecular motion. As an object is heated, its temperature increases. By definition, temperature is the average amount of kinetic energy in a system. As the kinetic energy of the system increases, the random motion of the molecules comprising that system begin colliding more with each other as well as any objects that the system is in direct contact with. By molecules of the system colliding with an adjacent object, the system can transfer its heat and impart more kinetic energy onto the object, thus increasing its temperature as well. (Examples: touching a hot or cold object, the handle of a soup spoon being hot because heat is conducted through it from the heated liquid).

Convection is heat flow through fluid or gas movement or through a fluid or gas moving past an object. In conduction, heat flows through the material. In convection, heat flows as hot air or water flows away from a hot object. (Example: By blowing on soup, effectively the air above the soup is being swept by at an increased rate. This increase in air flow causes a decrease in the temperature of the air directly above the soup thus allowing the warm liquid to dissipate some of its heat). Convection can also be thought of in terms of the transfer of energy due to density differences. If a gas or liquid expands and becomes less dense, it therefore becomes lighter as well. Lighter materials will rise while heavier materials will fall. If a cooler material is above a warm material, the warmer material will raise though the cooler material to the surface dissipating its energy (heat) to the surrounding environment. Through this process, heat is transferred from an area of high heat to one of low heat as would be expected.

Radiative heat transfer is heat energy transferred through the movement of electromagnetic waves. Heat transfer via radiation is dependent on the temperature as well as the emissivity of the objects that are heating or cooling. Emissivity is a material property that is related to the color of the object. Radiation is the only type of energy transfer that does not require a medium to flow through. Radiation transfers energy via photons that impart their kinetic energy into a system when they strike its surface. This transfer of kinetic energy thus heats whatever substance the radiation is striking. Keep in mind that radiation can be either in the visible or invisible regions, with shorter wavelengths carrying more energy. All energy from the sun initially strikes the Earth as solar radiation.

In an engineering and scientific sense, power and energy are not the same. Energy is the ability to do some sort of work and is measured in Joules (J) in the SI system of measurement. This work can be moving something or it can be heating something. Power is the rate at which energy is used to do work or in this case transferred in the form of heat. Power is measured in Watts (W) which are defined as 1 Joule per second (J/s). The flow of heat is generally measured in units of power because it is the rate at which energy is transferred from one place to another. These concepts of energy and power can be difficult for students to grasp. For the purpose of this lesson, it is not crucial that students come away with an understanding of the difference between power and energy, but it is important that the teacher presents these concepts accurately.