Chapter 13 : Heat Transfer

Heat Transfer Introduction

In this chapter, you will understand how heat is transferred through different media from one point to another. Imagine how the temperature of water in a sauce pan rises up to 100 Degrees Centigrade when placed close to a fire place. How about the way heat from the sun reaches humans on earth?

13.1: Concept of Heat Transfer
We know that every time heat energy is supplied to an object, molecules move
faster, spread apart and finally significant expansion of the body is observed.
This is thermal expansion of a body.
However, one might wonder about how heat energy is transferred from one point to another through different media such as solids, liquids and gases. Our main focus in this section will be to extensively understand the various modes of heat transfer.

Heat Transfer in Solids
Solids are made up of closely packed molecules held together by strong intermolecular forces. These atoms vibrate about their mean positions.
Do you think it is these vibrations that facilitate heat transfer in solidso Let us carry out Activity 13.1 to demonstrate heat in solids.
Activity 13.1: Demonstrating heat transfer in solids (conduction)
What you need: retort stand or two bricks, aluminium or iron rod, wax,copper rod, brass rod, 4 drawing pins, notebooks, braille paper, stylus, slate, Perkins brailler.

What to do;
1.Make four holes on a good conductor and fix in metal rods of Aluminum, Copper, Brass and Iron.
2.Use melted wax to attach drawing pins on each rod and start the stop clock.
3.Observe and note the time when each pin falls off from the rod.
4.Compare the time in each case and identify which material conducts heat fastest and slowest.
5.What do you think will happen to the rate of conduction when you increase both the length and the cross sectional area of each rod?
6.From the above observations, conclude about the factors that affect the rate of heat transfer through solids.

The process by which heat travels through solids from a region of higher temperature to regions of lower temperature without movement of matter as a whole is referred to as conduction. This means that heat transfer only occurs when there exists significant temperature difference between the ends of a conductor.
Heat transfer in solids is due to the vibrations of atoms at a higher temperature.
Since these are held together by inter atomic forces, these vibrations are transferred to other atoms and set into vibrations as well.
In addition, heat transfer in solids most especially in metals is due to the movement of free electrons from the hot to the cold end.
Conductors can be classified as good or bad conductors of heat. Bad conductors are also referred to as insulators and these do not easily conduct heat as compared to good conductors.
Both good and bad conductors have a wide range of applications which range from being used as handle to more complex applications such as in steam engines.

Exercise 13.1
In groups, discuss the various applications of good and bad conductors in our daily life. Include diagrams where necessary and make a presentation to the rest of the class.
13.2: Heat Transfer in Fluids
From the previous section, you learnt that heat transfer through solids does not involve movement of the whole material. What do you think happens when heat is transferred through fluids such as water? Let us carry out activity 13.2 to demonstrate heat transfer by convection.

Activity 13.2: Demonstrating heat transfer in liquids
What you need: round bottom flask or glass beaker, water, straw, tripod stand, extract of coloured petals, and Heat source, notebooks, braille paper, stylus, slate, Perkins brailler

What to do;
In groups,

1. Fill the round bottomed flask with water to about two-thirds of the whole volume.
2. Place the flask ona tripod with the heat source underneath it.
3. Use a straw to place an extract of coloured petals at the bottom of the water in a round bottomed flask.
4. Observe and sketch the motion of extract of coloured petals .
5. Conclude about the way heat is transferred through liquids.
6. Suggest a suitable name that describes the motion of extract of coloured petals.
7. State any other three examples of materials where the above mode of heat transfer can take place.
8. Describe any two applications of this mode of heat transfer.

Convection is the transfer of heat through fluids from a region of high temperature to a region of low temperature with the movement of fluids as a whole.

Convection currents are the up and down movement of the liquid molecules.
When fluids are heated, they expand and become less dense and hence occupying a larger volume than they would when at a lower temperature.
Convection Currents in the Atmosphere
You have already seen that transfer of heat by convection is characterised by convectional currents. Similar convection currents are formed in the atmosphere.
In this case, warm air rises up and creates space for the cool air. The motion becomes continuous resulting into formation of a breeze.
The breeze is either a land or a sea breeze. Let us carry out Activity 13.3 to demonstrate both the sea and land breeze.
Activity 13.3: Demonstrating a sea and land breeze
What you need: Notebooks, braille paper, stylus, slate, Perkins brailler.
What to do;
In groups, Studythe Figures 13.4 (a) and (b) below and discuss the questions that follow;

1.Explain how does a land and sea breeze form.
2.Analyse the diagrams and identify when does:
(a) a land breeze form
(b) a sea breeze form.
3.Differentiate between the sea and land breeze.

13.3: Heat Transfer by Radiation
Heat transfer in a vacuum occurs by means of electromagnetic radiation. In this section, you will understand how heat travels from one point to another through vacuum and also identify some practical application of radiation.

• Activity 13.4: To investigate heat transfer by radiation
  What you need: source of heat,
  What to do:

1. Place a lit bunsen burner on a table.
2. Stand about one metre close to the burner. What do you feel.
3. More closer to the burner; what do you notice. Explain.
4. Explain the process by which you receive the heat.
5. State the application of heat by radiation.
   Radiation of Heat from Different Surfaces

All objects whether hot or cold absorb or emit radiations. Some are good emitters and absorbers while others are not. Based on their characteristics, objects have different applications in daily life. For example, wearing black clothes may not be favorable during hot days but instead a white cloth being a better option. Let us do Activity 13.5 to compare radiation of surfaces.
Activity 13.5: Comparing radiation of heat from different surfaces
What you need: 3 identical metal tin cans, thermometer, card boards with holes drilled through the centre, heat proof mat, and paint brush, notebooks, braille paper, stylus, slate, Perkins brailler
What to do:
In groups:

1. Clean the identical metal tin cans and dry them.
2. Paint one of them white and the second one black, both inside and outside.
3. Leave the third metal can shiny.
4. While placed on the heat proof mats or wooden tables fill the cans with hot water (about 700C) up to the top and cover them with cardboards.
5. Place a thermometer in each can through the hole at the center of the cardboard as shown in Figure 13.5.

6. Start the stop clock and record the temperature of water in the cans in an interval of 5 minutes for about 40 minutes. Identify which can loses heat:
7. Conclude about the way radiation of heat energy in relation to the nature of surfaces.

   (a)Fastest than the others
   (b) Slowest than the others

8. State applications of materials which lose heat at a:
   (a)Faster rate
   b)Slower rate
   Good absorbers of radiations are similarly good emitters of the radiations. This also explains why car radiators are always painted black.
   Vacuum Flask
   You should have used vacuum flasks at home mostly to keep hot tea, porridge or milk hot. Did you know that a vacuum flask also keeps cold drinks cold?
   The most important thing here is to understand the mechanism of how the flask performs its role of temperature regulation.
   In Activity 13.6, you will understand how a vacuum flask regulates temperature.

Activity 13.6: Understanding how a vacuum flask regulates the temperature
What you need: Vacuum flask, thermometer, Container, stop clock, hot water at known temperature, notebooks, braille paper, stylus, slate, Perkins brailler.
What to do;
In groups,

1. Place the vacuum flask on a table. Carefully open it and place all the parts a side.
2. Identify and name the parts A, B,C, and D of the vacuum flask in Figure 13.6.
3. Carefully pour hot water at the same temperature in both the flask and container.
   Cover the flask and the container.
4. Start the clock and measure the temperatureafter every 5 minutes for about 40 minutes.
5. Compare the rate of heat loss in the flask as compared to that in the container.
6. Explain how your answer in (5) is facilitated by parts A, B, C, and D.