Chapter 24 Conduction

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Chapter 24 Conduction

  1. 1. CHAPTER 24 THERMAL HEAT TRANSFER
  2. 2. CONDUCTION
  3. 3. <ul><li>Conduction is the heat transfer through a material medium from places of higher temperature to places of lower temperature. </li></ul><ul><li>Conduction is the main method of heat transfer in solids . </li></ul><ul><li>Molecules in solids are not free to move from place to place. </li></ul><ul><li>The material that conducts heat faster and is a bad insulator . </li></ul><ul><li>Metals are good conductors of heat. </li></ul><ul><li>The material that conducts heat slower and is a good insulator . </li></ul><ul><li>Non-metals are poor conductors of heat. </li></ul>CONDUCTION
  4. 4. <ul><li>The diagram shows that </li></ul><ul><li>metal is a good conductor (heat spreads quickly through the base of the pan, warming the food rapidly). </li></ul><ul><li>wooden handle is a bad conductor (heat spreads slowly through the wooden handle so it feels cooler when it is held). </li></ul>WHAT IS BAD OR POOR CONDUCTOR?
  5. 5. CONDUCTION Molecular explanation
  6. 6. <ul><li>In metals </li></ul><ul><li>Metals are better conductors of heat than non-metals because they contain free electrons. </li></ul><ul><li>Due to these free electrons, metals are also good conductor of electricity. </li></ul><ul><li>So, heat is transferred from the hot end to the cold end with the help of these free electrons. </li></ul><ul><li>Watch: http://www.gcse.com/energy/conduction2.htm </li></ul>WHY METALS ARE BETTER CONDUCTORS THAN NON-METALS?
  7. 7. <ul><li>In metals </li></ul><ul><li>When the end P is heated, the free electrons there move faster, gaining more kinetic energy and move farther towards Q carrying the energy with them. </li></ul><ul><li>As a result, heat energy is transferred from one place to another both by the vibrations of molecules as well as by the movement of free electrons. </li></ul>WHY METALS ARE BETTER CONDUCTORS THAN NON-METALS? P Q
  8. 8. <ul><li>In metals </li></ul><ul><li>Diagram below shows heat is conducted quickly by moving electrons: </li></ul>WHY METALS ARE BETTER CONDUCTORS THAN NON-METALS?
  9. 9. <ul><li>In non-metals and solids </li></ul><ul><li>Non-metals/solids do not have free electrons. This is why they do not conduct heat as well as metals. </li></ul><ul><li>When the end P of a solid/non-metal is heated, the molecules at P vibrate faster and gain kinetic energy. </li></ul><ul><li>As a result, the neighbouring molecules are affected to vibrate faster. </li></ul><ul><li>Finally, the molecules at the end Q also vibrate faster. </li></ul><ul><li>In this way, heat is transferred from P to Q by conduction. </li></ul>WHY METALS ARE BETTER CONDUCTORS THAN NON-METALS?
  10. 10. <ul><li>In non-metals and solids </li></ul><ul><li>Diagram below shows heat is conducted slowly by atoms bumping into each other: </li></ul>WHY METALS ARE BETTER CONDUCTORS THAN NON-METALS? P Q
  11. 11. <ul><li>The molecules in a liquid are further apart than those in a solid. </li></ul><ul><li>The molecules of a gas are even further apart than those of a solid and a liquid. </li></ul><ul><li>Therefore, it is more difficult to transfer the heat energy or movement of the molecules to other molecules for liquids and gases. </li></ul><ul><li>However, heat can still move through them by convection. </li></ul><ul><li>Watch: Video – water is a bad conductor of heat http :// youtu.be/mBaaolWt3PM </li></ul>WHY LIQUIDS AND GASES ARE POOR CONDUCTORS OF HEAT?
  12. 12. <ul><li>Diagram below shows molecules of solids, liquids and </li></ul><ul><li>gases: </li></ul>WHY LIQUIDS AND GASES ARE POOR CONDUCTORS OF HEAT? SOLID LIQUID GAS
  13. 13. CONDUCTION The transfer of heat through a medium from one particle to another without any flown of the medium Different materials conduct heat at different rates Good Conductors Define as Bad Conductors Insulators Only take place in Solid Materials that conduct heat at faster rate Define as Also known as Materials that conduct heat at faster rate Define as Metals examples Aluminium Copper Iron Brass Non-metals, Most liquids (except mercury), gases examples Plastics Wood Water Air Two mechanisms Take place through Atomic or molecular molecules Free electron diffusion such as such as

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