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Transmission of Heat

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Contents


Introduction

Different materials in the surroundings such as iron, copper, wood, paper, water, milk, air respond differently when exposed to heat. This is due to the modes by which heat is transferred. In our daily life we use these modes to cook food, to prepare tea, to keep our body and house warm in winter season etc. This unit will deal with the different modes by which heat energy is transferred.

Objectives

  1. Teachers will be able to state what is transmission of heat and different modes of transmission.
  2. Teachers will be able to differentiate the behaviour of metals and non metals under heating.
  3. Teachers will be able to explain the difference between good conductor and bad conductor.
  4. Teachers will be able to explain the mechanism of heating through conduction, convection and radiation.
  5. Teachers will be able to perform the activity showing transmission of heat by conduction.
  6. Teachers will be able to site examples of different modes of transmission.
  7. Teachers will be able to explain the physical phenomena in connection with transmission of heat.

Modes of Transmission:

Conduction:

Think of a frying pan set over an open stove. The fire's heat the pan from the bottom and gradually heat the upper part of the pan.

Conducton








Activity-1

Objective: To illustrate the mode of transfer of heat in metals.

Material required: Iron rod, candle, 4-5 nails, a clamp stand
Activities:
Activity-1
  • Coat the iron rod with wax
  • Fix nails on the iron rod.
  • Fix the wax coated rod on the stand.
  • Heat the free end of rod with the help of candle.
Observation:
  • As the iron rod gets heated nails fall down one by one as wax melts due to heat.
Inference:
  • Heat is transferred from free end to the other gradually through the iron rod.
Explanation:
  • As the iron rod gets heated its molecules transfer the heat energy to neighbouring molecules


Definition: Conduction is the process by which heat energy is transmitted through collisions between neighboring molecules without actual displacement of particles.

Assessment:

Predict how other materials would conduct heat using a similar apparatus. For example, if you used aluminum, ceramic, and glass rods, how iron, wood, and fiberglass rods might behave. Justify your answer.

Categorize the materials as good and bad condutors of heat.

Convection:

Consider what happens to the water in a pot as it is heated over an open camp stove.

conduction

The water at the bottom of the pot heats up first. This causes it to expand. Since the warmed water has a lower density than the water around it, it rises up through the cooler, dense water. At the top of the pot, the water cools, increasing its density, which causes it to sink back down to the bottom. This up and down movement eventually heats all of the water. The continual cycling of the fluid is called a convection current.

Convection currents are found in many places and on many scales, from huge convection currents in the atmosphere, oceans, and even in the earth's interior to smaller convection currents found in a cup of hot cocoa or a fish tank. Convection also occurs in the air as up and down motions of air moleclures. Heat gained by the lowest layer of the atmosphere from sun or conduction is most often transferred by convection.

Convective motions in the atmosphere are responsible for the redistribution of heat from the warm equatorial regions to higher latitudes and from the surface upward.

    
Water Breeze
    
Cloud Formation
    
Land Breeze

These two examles show that heat energy is transferred from one part to another with the actual displacement of molecules, as the molecules near the source of heat energy get heated they move to the area where temperature is comparatively low and after transfering the energy by collissions with other molecules they move again towards the source of energy.

Activity-2

Objective: To illustrate the mode of transfer of heat in liquid.

Material required: Flask, Tripod stand, Wire guage, Candle, Potasium permagnet
Activities:
Activity-2
  • Fill ¾ th of the flask with water
  • Dorp some grains of potasium permagnet in the water
  • Heat the flask as shown in the figure
Observation:
  • As the water gets heated colored currents of water start from the bottom to top of the flask and moves down after a while to the bottom again.
Inference:
  • Heat is transferred from bottom to top by the actual movement of the molecules and transfer the heat energy there and again comes down to the bootm after cooling.
Explanation:
  • Heat is transferred from one part to the another by actual movement of molecules of water.


Convection:Convection is the transfer of heat by the actual movement of the heated material.

Assessment

List the materials in which heat is transferred through the convection.

Radiation

Keep a vessel of water for boiling. Hold your hand close to the vessel. You will feel the warmth of the vessel.

Let us think about how we get heat energy from the Sun. As the Sun rises we receive light and heat, we feel warm in the sunlight, water and the earth get heated during the day time.The Sunlight travels through nothingness! Just think of it! The sun transfers heat through 93 million miles of space. Because there are no solids (like a huge spoon) touching the sun and our planet, conduction is not responsible for bringing heat to Earth. Since there are no fluids (like air and water) in space, convection is not responsible for transferring the heat.

image:Sunrad.jpg    image:HeatRadiation.jpg    image:Radiation.jpg

How is the heat transmitted in this case? The process cannot be conduction, as there is no direct contact. The process cannot be convection because there are no material particles moving from the heat source towards the hand.  The invisible way of transmission of heat is known as radiation.

Activity-3

Objective: To illustrate the mode of transfer of heat in space.

Material required: Bowl, Silver foil, Cotton, GI wire,
Activities:
Activity-3
  • Wrap the bowl with silver foil
  • Set the GI wire as shown in figure
  • Attach cotton at the center
  • Put the bowl facing sunlight
Observation:
  • The cotton start burning after a while


Inference:
  • Heat is transferred from the Sun to the cotton to burn it.
Explanation:
  • Heat is transferred from the Sun to the earth without medium.


                                                       Mechanism of Heat Transmission

    

Summary and Assessment

Summary&Assessment.ppt

Collaborative Learners

S.N. Salam

Sandhya

R. K. Tiwari

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