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Conduction – Heat Transfer via Particle Collision by Paul Hewitt Conduction is Heat Transfer through Touch Heat conduction is the transfer of heat by the direct contact of particles of matter. If you have ever held a warm mug of hot chocolate, you have experienced conduction. Conduction only occurs between two materials at different temperatures and when they are touching each other. In conduction, heat can also be transferred through materials. If you stir the hot chocolate with a metal spoon, heat is transferred from the hot chocolate, through the spoon and to your hand. Imagine placing a cold spoon into the hot chocolate. Because the molecules in the cocoa have a higher average kinetic energy than those of the spoon, the energy is transferred up the spoon into the material with lower kinetic energy. Another example of conduction might be found when you hold an iron nail in a flame. When you hold one end of an iron nail in a flame it quickly becomes too hot to hold. Thermal energy at the hot end travels along the nail’s entire length. conduction Atoms and Molecules Heat conduction occurs when the atoms and molecules in a material move easily. Every atom has electrons that are orbiting the nucleus. Metal atoms have loosely held electrons that are free to easily move in the metal. Heat conduction occurs when electrons collide inside the object being heated. Good Conductors are Metal Solids whose atoms or molecules have loosely held electrons are good conductors of heat. Metals have the loosest electrons and are excellent conductors of heat. Silver is the best, copper next, and among common metals, aluminum and then iron. If you want to transfer heat from the stove burner to the pan, you will choose a metal pan because it is a good conductor or heat. Insulators are Poor Conductors Wood, wool paper, cork, and plastic foam are poor conductors of heat. Molecules in these materials have electrons that are firmly attached to the molecules. Poor conductors are called insulators. If you want to put a hot liquid into a cup to hold, you will choose a styrofoam cup because it is not a good conductor of heat. Wood is a good insulator and is used for cookware handles. Even when a pot is hot, you can quickly grasp the wooden handle with your bare hand without harm. An iron handle of the same temperature would surely burn your hand. Wood is a good insulator even when it is red hot. Since wood is poor conductor, this explains how firewalkers can walk barefoot across burning coals without burning their feet. Air is a Great Insulator!! Air is a very poor conductor which makes it a great insulator. The coat at the left is made of down. The air spaces between the down feathers help keep you warm in the winter. The good insulating properties of such things as wool, fur, and feathers are largely due to the air spaces they contain. Porous substances are also good insulators because of their small air spaces. The thermos to the right can keep coffee hot or iced tea cold. The thermos uses a space of air between the inside and outside wall to prevent heat transfer by conduction. The air insulates the thermos to keep your liquid from changing temperature. Compare Conductors & Insulators In the example to the right, the tile feels colder than the wooden floor under the bare feet, even though both floors are at the same temperature. This is because tile is a better conductor of heat than wood is, and so heat is more readily conducted out of the foot touching the tile. Snow is also a poor conductor of heat. Snowflakes are formed of crystals that trap air and provide insulation. That’s why a blanket of snow keeps the ground warm in winter. Animals in the forest find shelter from the cold in snow banks and in holes in the snow. The snow doesn’t provide them with thermal energy – it simply slows down the loss of body heat generated by the animals. Then there are the igloos in Artic dwellings that are shielded from the cold by their snow covering. Snow patterns on the roof of a house show areas of conduction and insulation. Bare parts show where heat from inside has conducted through the roof and melted the snow. The walls and attics of homes built with wood are insulated with rock wool, fiberglass, or cellulose. Interestingly, insulation doesn’t prevent the flow of heat. Insulation simple slows down the rate at which heat flows. Even a well-insulated warm home gradually cools. Insulation merely delays the rate at which heat conducts from a warmer region to a cooler one. In winter, we wish to slow conduction from inside to outside. But on hot summer days, we wish to slow down conduction in the other direction, from outside to inside. Insulation slows conduction in either direction. Name ________________________________ Date ______ Name ________________________________ Date ______ Conduction Questions Conduction Questions 1. What is a conductor? _________________________________ 1. What is a conductor? _________________________________ ___________________________________________________ ___________________________________________________ 2. What are two good conducting materials? 2. What are two good conducting materials? _________________________________ _________________________________ _________________________________ _________________________________ 3. What is an insulator? _________________________________ 3. What is an insulator? _________________________________ ___________________________________________________ ___________________________________________________ 4. What are two good insulating materials? 4. What are two good insulating materials? _________________________________ _________________________________ _________________________________ _________________________________ 5. A book and metal spoon are both at room temperature of 75* F 5. A book and metal spoon are both at room temperature of 75* F BUT if you touch the book, it feels warmer and if you touch the BUT if you touch the book, it feels warmer and if you touch the spoon, it feels colder. How can you explain this? spoon, it feels colder. How can you explain this? ___________________________________________________ ___________________________________________________ ___________________________________________________ ___________________________________________________ ___________________________________________________ ___________________________________________________ 6. If you hold one end of a metal nail against a piece of ice, the end in your hand soon becomes cold. Does cold flow from the ice and into you hand …or …does heat flow from your hand and into the ice. ____________________________________ 6. If you hold one end of a metal nail against a piece of ice, the end in your hand soon becomes cold. Does cold flow from the ice and into you hand …or …does heat flow from your hand and into the ice. ____________________________________ Explain your thinking ________________________________ Explain your thinking ________________________________ __________________________________________________ __________________________________________________ __________________________________________________ __________________________________________________