﻿ Heat Transfer

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Heat Transfer

Prior knowledge

Activity: Assembling parts of a thermometer

1. Drag and drop the various parts of the thermometer on their respective positions.

2. Drag and drop the various labels at the right positions on the thermometer.

Prior knowledge

Activity: Assembling parts of a thermometer

1. Drag and drop the various parts of the thermometer on their respective positions.

2. Drag and drop the various labels at the right positions on the thermometer.

Exercise

Objectives

By the end of this topic you should be able to:

• State the modes of heat transfer
• Explain the modes of heat transfer

Heat and Temperature

Activity: In the activity that follows, the temperatures of two samples of water, of the same amount, are measured. The samples are then mixed and the temperature of the mixture is determined.

Click on the play button; then read and record the temperatures of the two samples as well as the temperature of their mixture.

Observation

Temperature of the mixture is found to be 40 degrees Celsius.

Conclusion

The temperature of water at 20 degrees Celsius rose to 40 degrees Celsius while that of water at 60 degrees Celsius dropped to 40 degrees Celsius. That is, there is exchange of heat between hot and cold water when they are mixed.

Experiment1: Measurement of Temperature

Experiment on Conductivity

The set-up below shows a metal rod being heated at one end. A pin is attached to the other end using wax. Observe what happens.

Observation

After heating the metal rod for some time, the wax melts and the nail drops down.

Explanation

Heat flows from the hot end of the metal rod to the cold end by a process called conduction.

This happens because as the rod is heated the particles in it vibrate faster and knock the neighbouring particles, causing them to vibrate more vigorously as well. This goes on till heat reaches the other end.

Drag and hold metal rod onto the flame and observe what happens to wax and nail after some time

Observation:
After sometime of heating the metal rod, the wax melts and nails drops
Explanation:
Heat moved from hot end of metal to cold end by a process called conduction

This happens because as the rod is heated the particles vibrate faster, knock the neighboring particles causing them to vibrate faster .This goes on till heat reaches the other end.

PRIOR KNOWLEDGE

• Particulate of matter
• Temperature

Activity 1

Lesson Objectives

By the end of these lesson you should be able to

• State the modes of heat transfer
• Explain the modes of heat transfer

Modes of Heat Transfer

Have you ever wondered why cooking utensils are made of generally the same material; or why we are able to feel the warmth when sitting close to a source of heat? There are different means by which heat travels, and these are referred to as modes of heat transfer. Modes of heat transfer include: conduction, convection and radiation.

Conduction

The next set-up shows rods of the same material and thickness but different lengths, heated equally. Click PLAY for the heating to start and observe the order in which the nails attached to the rods fall off.

The order in which the nails drop from the rods indicates the order of decreasing conductivity.

Observations

The nail attached to the shortest rod drops first, followed by the second shortest and so on.

The last nail to fall is the one on the longest rod.

Conclusion

Different lengths of the same material conduct heat at different rates. The shorter the length, the faster the conduction.

Convection in Liquids

<"bodytext1">In the following activity, grains of rice are poured into hot water held in a beaker.
Click on the play button and observe the movement of rice grains as heating goes on.

Observation

Rice grains move up and down, some in looped paths.

Explanation

On heating, water expands, gets less dense and rises. Cool and dense water at the top therefore moves down where it also gets heated and the process continues. That is, convectional currents are set up. This process of heat transfer is called convection. The rice grains follow the paths taken by particles of water as the latter move.

Analogy

Compare this (convection) with students fetching water to fill a tank. Each student carries water from the source, empties it into the tank and goes back to the source.

Convection in Gases

Activity: In the following activity a smoldering rug is held near the mouth of one chimney, A, in a smoke box. Observe what happens to the smoke.

Observation

Smoke moves down into chimney A and up through B.

Explanation

Air above the candle is hot and less dense; so it rises and flows out through chimney B. The cold air above A moves down with the smoke to replace it. This in turn is heated; so it also rises, bringing out the smoke. That is, convectional current has been set up in the air.

Definition of convection

Convection is the transfer of heat energy in fluids by actual movement of the fluid particles. Click on the button to observe the actual movement of gas particles.

The process through which heat travels from one place to another without use of material medium is called radiation. Heat from the Sun reaches the earth's surface by radiation.

Analogy

Consider one student filling the tank with water by directing a stream of water from a pipe as shown below.

In this case, the other students are not needed to fill up the tank; and the water does not require a material medium to flow. In the same way, heat flow by radiation does no require a material medium.

Analogy

Consider one student filling the tank with water by directing a stream of water from a pipe as shown below.

In this case, the other students are not needed to fill up the tank; and the water does not require a material medium to flow. In the same way, heat flow by radiation does no require a material medium.

Hold a lens above a piece of blackened paper at some height such that it focuses light onto the paper. Observe what happens to the paper. Repeat this procedure with white paper.

Observation

Black paper smolders and sometimes ignites into a flame but nothing happens to the white paper.

Conclusion

Black surfaces are good absorbers and good radiators of heat but white shiny surfaces are poor absorbers and poor radiators of heat.

Factors Affecting Heat Transfer

(a) Nature of material

The next animation shows rods of the same thickness and length immersed in boiling water.

Observe the order in which the nails attached to the rods fall off. This indicates the order of decreasing conductivity. Which of the rods conducts the fastest?

Conclusion

Different materials conduct heat at different rates. The order of conductivity, from the highest to lowest is: F, C, B, D, A and E.

(b)

Thickness

How does the thickness of a material affect conductivity?

Summary

Rate of conduction of heat depends on:

• Length of conductor
• Area of cross-section of the conductor
• Temperature difference
• Nature of conductor

Heat Transfer

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Heat Transfer

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Heat Transfer

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Heat Transfer

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