Introduction to Heat Transfer: Conduction Part Two

A Simple Basic Equation

A Heated Slab

A simple equation to measure heat conduction is derived by considering a uniform slab of material with a thermal conductivity k, an area A, and a thickness or width L. The slab is heated so that transfer occurs from its hotter side to its cooler side. How quickly the transfer is made is dependent upon thickness of the slab, as well as the material it is made from.

The equation is

1) H = -kA (ΔT/L) = -kA(Thot – Tcold)/L.

For SI units, H is in watts. The minus sign indicates that flow of heat is from a warmer object to a colder one.

Examples of Heat Transfer

Styrofoam is a good insulator because it traps pockets of air and other gases inside of it. Your fur or goose down coat keeps you warm in the winter because fur and feathers also trap air pockets. Wool and fiberglass also operate on this same principal, which is why wool socks are so effective, and why fiberglass is favored for insulating houses.

Girl Playing In Snow

Clothes in general trap air pockets, and this is why they are such good insulators. In the winter, the trapped air helps keep you warm. If the air weren’t trapped, then it would move. The warm air that is heated by your body would leave to be replaced by colder air, and you would subsequently feel chilled.

Heat conduction also explains why a tile floor feels colder on bare feet than a rug or a wooden floor. The tile, wood, and rug are actually at the same temperature, but tile is a better conductor than either of them. It draws heat away from your foot more quickly than a wooden floor. The rug is even better, because it does not conduct heat away but rapidly warms up to the temperature of your foot.

What heat transfer process causes the water that you use to steam your vegetables to boil, or causes a radiator to heat your room? We will examine this in part three.

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

Image Credits

Girl in down coat by Phaewilk

Resources

Video of A Heat Conduction Trick With A Water Balloon

Professor Julius Sumner Miller’s Heat Conduction Demos Video

This post is part of the series: Introduction to Heat Transfer

This is a series that describes the three methods of heat transfer. Parts one and two explain what heat transfer is, and the method of conduction. Part three discusses convection. In part four, thermal radiation is described.
  1. What is Thermal Conduction?
  2. Thermal Conduction Examples
  3. What is Convection?
  4. What is Thermal Radiation?
  5. Simple Heat Transfer Experiments: Conduction