Conduction (heat transfer through direct contact)
The process of thermal energy transfer without any flow of the material medium.

There are two types of conduction:

• molecular vibration
• free electron diffusion
  

MOLECULAR VIBRATION (occurs in ALL solids)

• Particles in matter are kept in constant random motion by the energy they possess.
• The higher the temperature, the higher the average kinetic energy.

• Regions with greater molecular kinetic energy will pass their thermal energy to regions with less molecular energy through direct molecular collisions.

When an object is heated, the particles on the hotter side of the substance gain energy and vibrate more rapidly than those on the colder side. When a particle that is vibrating vigourously collides with its less rapidly vibrating neighbour, part of its kinetic energy is transferred to the neighbouring particle and makes it vibrate more. Through the continuous collisions of these neighbouring particles, energy is transferred from the hotter side of an object to the colder side. When thermal equilibrium is reached within the whole substance, there is no net flow of thermal energy between the both sides.

• Conduction is most effective in solids.

1. Solids are the best conductors of heat followed by liquids and then gases.
2. The particles in a solid are very closely packed and their positions are more or less fixed relative to each other. The force of attraction between adjacent particles is strong and therefore making heat transfer by collision is very efficient.
3. The particles in liquid are able to slide about each other, which means that the force of attraction between the particles is not as strong as the particles in solids. Thus, liquids are usually poor conductors of heat.
4. In a gas, the particles are far apart, making energy transfer by collision very inefficient. Thus gases, like air, are very poor conductors of heat.
   

FREE ELECTRON DIFFUSION (occurs in METALS only)

• Different materials conduct heat at different rates.

-Substances that allow thermal energy to move easily through them are called conductors.
-Substances that do not allow thermal energy to move through them easily are called insulators

• Metals like copper and aluminium are good conductors of heat because they contain many free electrons which move randomly between the atoms or molecules. Free electrons in metals gain kinetic energy and move faster as a result. These fast-moving electrons then diffuse or spread into the cooler parts of the metal. In the process, they collide with the atoms in the cooler parts of the metal and transfer their kindetic energies to them, in addition to the molecular vibrations between the atoms or molecules. Therefore free electrons are effective in transferring heat by collision.
• Metals tend to feel cold as they conduct heat away from your hand. As such, we perceive the heat that is leaving our hand as cold.
• Non-metals like glass, wood and polystyrene are usually poor conductors of heat, or good insulators, because they do not have free electrons to help transfer heat. Therefore, non metals are poor conductors of heat as they can only rely on the collision of atoms or molecules to transfer heat.

Molecular vibrations is a slow process while free electron diffusion is a fast process.
As such, metals heat up faster as they have 2 mechanisms of conduction occuring at the same time- molecular vibrations and free electron diffusion. Also, in metals, free electron diffusion is the main mechanism, which is faster.



APPLICATIONS OF CONDUCTION


-Uses of GOOD conductors of heat
If thermal energy has to be transferred quickly through a substance, good conductorsof heat such as metals are used.

1. Cooking utensils like kettles, saucepans and boilers are usually made of aluminium or stainless steel where direct heating is involved.
   
   
   
2. Soldering iron rods are made of iron with the tip made of copper, as copper is a much better conductor of heat than iron.
   
   
   
3. Heat exchangers, such as those used in a large laundry facility help save energy.
   
   
   

-Uses of BAD conductors of heat (good insulators)
Insulators are very useful if we want to minimise loss of thermal energy, or prevent thermal energy from being transferrred quickly.

1. Handles of appliances and utensils like saucepans, kettles, teapots, irons and soldering iron rods are made of wood or plastics which are poor conductors of heat. In this way, the hot utensil or iron can be picked up without scalding our hands.
   
   
   
2. Table mats are usually made of cork so that hot kitchenware can be placed on them without damaging the table-top.
   
   
   
3. Sawdust is used to cover ice blocks because of its good insulating property.
   
   
   
4. Wooden ladles are very useful for stirring or scooping hot soup and also for scooping rice that has just been cooked.
   
   
   
5. Woollen clothes are used to keep people warm on cold days by trapping air.
6. Fibreglass, felt and expanded polystyrene foam which trap large amounts of air are employed as insulators in the walls of houses, ice boxes and refrigerators. Double-glazed windows have air trapped between two panes of glass, which reduce thermal energy transfer through windows.
   click to enlarge! (:
   
   
   
   
   
   
   

   Experiment 2 (:
   
   1. Coat the parts of the rods that are on the outside of the tank evenly with melted wax.
   2. Pour boiling water into the bath, so that the ends of the rods are submerged.
   3. Record the length of wax that melts in a given interval of time for each of the four rods.
      
   • In this experiment, thermal energy is transferred from a region of higher temperature to a region of lower temperature.
   • The wax on the rods melts as thermal energy is transferred from the boiling water (hot end) towards the colder end of the rods.
   • But the length of melted wax on each of the four rods are different- shortest length of unmelted wax for the copper rod and longest length of unmelted wax for the wooden rod, as seen in the diagram.
   • Therefore we can conclude that thermal energy flows through the material of the rods without any flow of the material itself, which is conduction.
     
   • We can also conclude that different materials conduct heat at different rates. Since the length of unmelted wax for the copper rod is the shortest and the length of unmelted wax for the wooden rod is the longest, it can be concluded that copper is a good conductor of heat and wood is a poor conductor of heat (good insulator).
   The reason why metals melt the wax faster than the non-metals is because of the free electron diffusion stated above.
   
   
   
   Experiment 3 (:
   
   1. Place an ice cube at the bottom of a test-tube and use a gauze to trap the ice and prevent it from floating.
   2. Fill the test tube with water till its almost full.
   3. Heat the test-tube at the upper end, as shown in the diagram below.
   4. Observe the water being heated and the ice below it.
      
      
   • When the water is being heated, we will realise that the water at the upper end of the test-tube soon starts to boil, while the ice melts very slowly.
   • This shows that the rate of thermal energy transfer by conduction from the water at the top to the bottom of the test-tube is extremely slow, which means that water is a bad conductor of heat. As such, heat cannot travel efficiently to the ice.
   • Also, as the fire is at the top, the convection current will only be found at the top too. As such, the water that is heated will remain at the top. This is because when the water molecules expand, the density decreases and the water will rise while the cooler water, which has a higher density would sink to the bottom. Thus the ice melts very slowly.
     
   
   
   
   link: Conduction & Convection
   The applet on the left shows the conduction in a solid bar and the applet on the right shows convection in a gas. For the left applet, above the bar is a graph showing the temperature at each point on the bar. Note you can start, stop and pause. You can turn off the heat applied to the end of the bar. For the right applet, the bottom slider controls the position of the heat source. You can turn the heat off and on by clicking in the box. As the particles heat up, they change color from blue to red.
   
   

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