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Heat Transfer

Anti Thermal Radiation Suit Anti-thermal radiation suit This image shows the Arcelor–Mittal Galati, the largest steel producer in Romania, where the workers at the furnace wear an anti-thermal radiation suit which protects them from heat transfer of the hot furnace at 2000°C. What is the reason for the transfer of heat? Which type of heat transfer is possible in this case? To learn more lets explore the topic.

Learning objectives

After completing the topic, the student will be able to:

  • Understand the concept of conduction by examining and investigating the conduction of heat that occurs in multiple aspects of daily life.
  • Explore heat conduction and thermal conductivity based on their applications.
  • Discuss and explore convection process in all fluids by relating to daily life scenarios.
  • Examine and explore the process of radiation and relate its properties to everyday life.
  • Explore the emission, absorption and reflection of the radiant energy and investigate their daily life applications.
  • Discuss and explore the principle of heat exchange and its real life applications.
Copper wires Copper wires Copper is a good conductor of heat and electricity. It possesses high electrical and thermal conductivity. Due to the fact that virtually all valence electrons participate in conduction. It is second to elemental silver in conductivity and very economical which is why Cu is preferred for wiring purposes.

Transfer of heat

Heat transfers from warmer to cooler things. Heat seeks its own level like water or pressure, and equalizes everywhere. If several objects with different temperatures are in contact, those that are warm become cooler and those that are cool become warmer. They tend to reach a common temperature. This equalizing of temperature occurs in three ways: conduction, convection, and radiation. Conduction usually takes place in solids, convection in liquids and gases, and no medium is required for radiation.

Conduction: If you leave the end of a metal poker in a fire for enough time, its handle will get hot. Energy is transferred from the fire to the handle by (thermal) conduction along the length of the poker. The vibration amplitudes of the atoms and electrons of the metal at the fire end of the poker become relatively large because of the high temperature of their environment. These increased vibrational amplitudes, and thus the associated energy, are passed along the poker, from atom to atom, during collisions between adjacent atoms. In this way, a region of rising temperature extends itself along the poker to the handle. Heat conduction occurs by electron and atomic collisions.

Metals are better conductors than non-metals. This is because metals have the "loosest" outer electrons, which are free to carry energy by collisions throughout the metal. They are excellent conductors of heat and electricity for this reason. Silver is the best conductor (but costly too!). Copper is next best followed by the common metals aluminium and iron. Wool, wood, straw, paper, cork, and Styrofoam, on the other hand, are poor conductors of heat. The outer electrons in the atoms of these materials are firmly attached. Poor conductors are called insulators.

Igloos Igloos Ice is a bad conductor of heat. Any heat generated from inside the igloo, usually from body heat or a small fire, stays inside. A fire inside an ice structure, won’t that melt the igloo? The warmth inside the igloo does melt the inner layer of snow and ice, but the air outside – often much colder than the air inside – freezes it back. The result is an additional sheet of ice on the interior walls, which adds to the insulation! An igloo can be big enough for one person to crawl inside, or can be a large dome enough for 20 people to live-in together.
Heat conduction

The difference in heat conduction by various materials is seen in use very often in our everyday lives. For example, look at the cooking utensils in the kitchen. The cooking pans most certainly are made up of thick–bottomed copper or iron vessels with insulating handles made up of wood or artificial plastics. Eskimos who live in the Arctic build their igloos with double walls – air in between the walls acts as a good insulator.

Similarly double layered blankets, double glazed windows make better use of the bad heat conducting properties of air. Even birds puff their feathers to keep themselves warm. Gunny bags or saw dusts are poor conductors of heat. Moreover air gets trapped inside such materials. Since air is a bad conductor of heat, ice wrapped in gunny bags and rice husk cannot gain heat from the surrounding and so does not melt fast.

Wood is a good heat insulator even when it's red hot, which is why firewalkers can walk barefoot on red–hot wood coals without burning their feet. Evaporation of moisture on wet feet can play a role in firewalking too. Although its temperature is high, relatively little heat is conducted to the feet, just as little heat is conducted by air when you put your hand briefly in a hot oven. But you get burns if you touch metal in the hot oven. Similarly, a firewalker who steps on a hot piece of metal or another good conductor will definitely burn his feet!

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