Heat Transfer by Convection

Heat Transfer by Convection

This mode of heat transfer is seen in liquids and gases in which the molecules are less densely packed and are free to move. Thus, convection is the primary mode of heat transfer in fluids.

Let us see the following example and learn how heat is transferred by convection.
The bottom of a vessel containing water is heated. After sometime, the water at the top of the vessel also becomes hot. How does this happen?
Convection-of-Heat

Fig. Hot water (red) rises up and cold water (blue) falls to the bottom.

If a vessel containing water is kept on a flame, the water at the bottom of the vessel gets hot, and as hot water is lighter than cold water, it rises up carrying the heat energy with it (Fig.). The colder and denser water on the top falls to the bottom of the vessel and gets heated in its turn. This process continues and results in a circulating stream of hot and cold water. As a result, the whole water in the vessel gets heated.
This method by which heat is transferred by mass movement of the liquid or gas itself is called convection.

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Practical Applications of Convection

The principle of convection of heat can be used in many applications. When you want to warm a fluid, you should warm it from below. If a room has to be warmed with a room heater, the heater should be placed at a lower level, so that the room gets warmed evenly. Similarly, an air conditioner used for cooling a room should be placed at a higher level.
In earlier times, windows and ventilators were designed and placed so that the buildings were kept cool in summers. Next time you visit old buildings, look out for these special features.

Sea Breeze and Land Breeze
Convection-of-Heat-2
Convection plays a major role in maintaining a moderate temperature in places near the sea. Land masses (beach, coastal town, or city) heat up much faster than water bodies (sea, ocean) during daytime, and cool down much faster during the night. This difference in temperature sets up a wind pattern.
During the day, the air above land rises as it is warm and cooler air from over the sea flows in to take its place. This gives rise to a sea breeze (sometime in the afternoon) which cools the land. In the night, as land cools down much faster than the sea, the cooler air over land flows out to take the place of warmer air over the sea which rises, setting up a land breeze.

Activity

Aim: To show that heat transfer in water is due to convection
Materials needed: A glass beaker made of heat-resistant glass/glass test tube, candle, match box, potassium permanganate crystals/artificial food colouring, and a spoon
Method:

    1. Fill the beaker to the half way mark with clear water.
    2. Take a pinch of the potassium permanganate crystals/food colouring and drop it gently to the bottom of the beaker.
    3. Let it settle for a few minutes.

Convection-of-Heat-1
Observation: You will see that the water at the bottom of the beaker is coloured, but the water at the top is quite clear. Light the candle and pick up the beaker very gently and hold it on top of the candle flame. You will see columns of coloured liquid rise up in the beaker.
Conclusion: This is due to convection. Water molecules at the bottom get heated and rise up, carrying the heat energy with them.

Heat Transfer by Conduction

Heat Transfer by Conduction

We have seen in earlier examples that metal objects heat up when they come in contact with hot solids or liquids. For example, a stainless steel spoon becomes hot when left in a hot liquid. How do you think this happens?
The heat from the hot liquid moves to the container and from there to the stainless steel spoon.

How Does Conduction Occur?
To understand this, we need to learn what matter is made up of. We know that matter is made up of tiny particles.
You learnt in earlier classes that in solids, the particles are very tightly packed and they cannot move around freely. However, they can vibrate about a fixed point.
particles-of-solid-vibrate-fixed-point

Fig. The particles of a solid vibrate about a fixed point

Let us take the example of a solid rod being heated at one end by a candle flame.
A-solid-rod-being-heated
Heat increases the vibrations of the particles at that end of the rod which is in contact with the flame. These particles collide (bump) with each other and in the process transfer their energy to their more slowly moving neighbours further away from the flame. This makes them vibrate faster and they, in turn, pass the vibration on to their neighbours, even further away from the flame, and so on. In this manner, heat energy is transferred from one particle to the next, although each individual particle remains in its original location. Therefore, we can say that conduction is the mode of heat transfer from a region of lower temperature within a body, or when two bodies are in contact, and it takes place by collisions between neighbouring atoms and molecules of the body.

In case of solid metals or liquid metals, heat is transferred by freely moving electrons.
Conduction is the primary mode of heat transfer through a solid. Conduction of heat energy can occur within a body or between two bodies when they are in contact with each other. However, the rate of conduction of heat is different in different materials. Also, some materials allow heat energy to flow through them easily while others do not.

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Conductors and Insulators

If you observe the utensils in your kitchen, you will notice that though most of them are made of metals, their handles are made of wood or hard plastic.
Why aren’t the handles made of metal as well? This is because metals get heated whereas materials like plastic and wood do not. It would be difficult to hold the handles made of metal while cooking.

conduction
Substances that conduct heat easily are called conductors. For example, metals are conductors of heat.
Substances such as wood, straw, clay, rubber, glass, and Bakelite (a kind of plastic) do not conduct heat very well and are called insulators.
Air and water are insulators. Some materials such as wool, fur, and bird feathers are insulators because they trap air between their fibres.

Practical Applications of Conduction

Both conductors and insulators are useful to us, depending on our needs. We use good conductors (metals) to make cooking utensils, and heat-resistant plastic to make the handles for these vessels.
We wear woollen clothes in winter because wool is a bad conductor of heat. Thus, it helps in retaining body warmth. The wool fibre has a series of curls and these are called ‘crimps’.
These crimps create small air pockets. The greater the number of crimps, the greater is the number of air pockets which can hold and trap air.
Thus, a thin layer of air is created which insulates us from the cold weather and also prevents body heat from escaping into the surroundings.
cooking pan

Fig. Cooking Pan

Activity

Aim: To demonstrate that the rate of conduction of heat is different for different materials Materials needed: Rods of equal diameters and equal lengths made of aluminium, copper, and iron (you will find these in your school lab), two wooden slabs, small metal balls, wax, and flame
Method:

    1. Take two rods at a time, say, the copper and aluminium rods.
    2. Glue the metal balls on to the rods at equal distances using wax.
    3. Arrange the rods as shown in the figure.
    4. Light the burner and make your observations.
    5. Repeat the experiment with iron and aluminium rods.

conduction-2

Observation: You will see that the balls drop fastest for copper, then aluminium and slowest for iron.
Conclusion: Of the three metals, the rate of conduction is highest for copper and the lowest for iron.

What is Heat

HEAT – A FORM OF ENERGY

Have you stirred hot tea or milk with a stainless steel spoon? Next time you do this, leave the spoon in the hot liquid for a little while. Now, touch the handle of the spoon. What do you notice? The handle of the spoon would have got a little warm. Why did the handle of the spoon become warm?
When there is a difference in temperature between two bodies, a certain type of energy flows between them. This energy is called heat energy.

When this heat energy flows into a body, it warms the body. When it flows out of the body, it cools the body. In other words, when a hot body and a cold body are in contact, the hot body loses heat energy while the cold body gains heat energy.

The point to note is that, left to itself, heat energy flows from a region of higher temperature to a region of lower temperature and never in the other direction (Fig.). Heat flow continues till the temperatures of both the hot and the cold body become the same (whose value will be somewhere in between the temperatures of the hot body and the cold body).
Direction-of-flow-of-heat-energy

Fig. Direction of flow of heat energy

Measuring Heat
Just as we measure length in centimetres and metres, heat energy (and any other form of energy for that matter) is measured in calories or joules. In SI system (which you know is the international standard for the system of units), heat energy is measured in joules. The symbol used to represent ‘joule’ is ‘J’.

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Preventing Loss of Heat
A thermos flask (Fig.) is a very good example of how heat loss by all three modes of heat
transfer, namely, conduction, convection, and radiation is minimized. Let us see how this is done.
thermos-flask

Fig. Diagrammatic representation of a thermos flask.

Heat loss due to conduction is minimized by using insulating materials (like plastic) for the outer casing and the cap of the thermos flask. The inner jar is a double-walled bottle made of glass or stainless steel. The space between the two walls is a vacuum, so that heat loss due to conduction is minimized.
This also reduces heat loss due to convection as there are no air molecules to carry the heat away. Heat loss due to radiation is minimized by making the surface of the jar highly reflective, so that heat radiations are reflected back into the jar

Activity

Aim: To show that heat energy flows from a hot body to a cold body Materials needed: A coin, tongs, boiling water, cold water, and two glasses

Method:

    1. Take two glasses and fill them to three-fourths of the capacity with cold water.
    2. Boil water in a vessel and drop a coin in the boiling water. Leave it for some time so that the coin gets really hot. Ask an adult to use the pair of tongs to pick up the hot coin and drop it in one of the glasses filled with water.
    3. After about 2 minutes, dip your finger in the two glasses, first in the glass without the coin and then in the glass with the coin.

HEAT-ENERGY-1

Observation: You will observe that the water in the glass in which the hot coin was dropped will be warmer.
Conclusion: Heat energy has been transferred from the hot body (coin) to the cold body (water).