As you know, heat always transfers from a hotter object to a cooler object. This changes the internal energy of both objects involved. The rate of heat flow depends on three things:

• the type of heat transfer,
• the temperature difference across which the heat flows, and
• the conductivity of the materials involved.

This is called Fourier's Equation and was first stated by French mathematician/ physicist J.B.J. Fourier in 1822. To see it in equation form, go to http://www.csounds.com/ezine/spectra  (Figure 3).

Heat transfer happens in one of three different ways:

By Conduction -- the flow of heat energy through a material without any movement of the material itself. This is the most important type of heat transfer in solids and only occurs when two objects are in direct contact with one another.

By Convection -- the transfer of heat energy by the movement of a liquid or gas, like water or air.

By Radiation -- in which radiant energy (in the form of particles or electromagnetic waves) is given off by a hot object and directly absorbed by another, cooler object. Heat from the Sun is transferred to Earth by radiation. Different colors and surfaces absorb heat differently. For example, the most heat is absorbed by a black matte (dull) surface, while the least heat is absorbed by a white shiny surface.

Understanding the different types of heat transfer is key to understanding The First Law of Thermodynamics, which states: "the change in the internal energy of a system is equal to the heat added to the system minus the work done by the system."

This Instruction is going to be mostly about the transfer of energy (heat flow) in solids.

More About Conduction
 

As we said, conduction (conductivity) is the direct flow of heat through matter. It results from the actual physical contact of one part of a body with another part of the same body -- or of one body with another.

For example, if one end of an iron rod is heated, the heat will travel by conduction to the other end. Another example is when a cooking pot sits directly on the solid surface of a hot stove.

Some materials are better conductors than others.

• Good Conductors include silver, copper, aluminum, zinc, brass, platinum, iron, nickel, tin, lead, solid rock and gold.
• Fair Conductors include charcoal, carbon, metallic ore and moist earth.
• Partial Conductors include linen, cotton, mahogany, pine, rosewood, teak, marble and the human body.
• Poor Conductors include plastics, ceramics and glass.

In general, the more dense a substance is, the better a conductor it is.

Resistance
 

Resistance is the amount of opposition offered by a substance to the passage of heat through it. It is the exact opposite of conductance.

Materials with a high thermal conductivity have a low thermal resistance and poor heat insulation qualities. These materials include copper and aluminum.

Conversely, materials with a low thermal conductivity have a high thermal resistance and good heat insulation qualities. This includes corkboard and fiberglass insulation.

Reading List From the California Dept. of Education

To read a description of this book from the California Dept. of Education, click on this address and follow the instructions:
http://www.cde.ca.gov/ci/sc/ll/ap/searchlist.asp

for Students, Parents and Teachers

Now let's do Practice Exercise 3-3 (top). Choose printer friendly or online exercises. Printer friendly version requires the Adobe Acrobat Reader 5. Click HERE to obtain a free copy.

Summary

You have now completed this Lesson and are ready to do the Problem and Test sections.

You may wish to review any or all of the topics before answering the questions that follow. You may also wish to obtain additional material from the Links in any of the Instructions before answering the questions.

Good luck!