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The study of Physics Topics has helped humanity develop technologies like electricity, computers, and space travel.
Heat Transfer: conduction, convection and radiation
To carry heat from one part of an object to its other part, or from one object to another object is called transfer of heat. For example, if we dip a steel spoon in a cup of hot tea, we find that the temperature of spoon rises and it becomes hot. Here, some of the heat contained in hot tea has been transferred to spoon which is placed in it. In this example, heat moves from hot tea to the cold spoon. We know that a utensil (like a frying pan) becomes hot when kept on the flame of a gas burner of the stove. This is because heat from the hot flame is transferred to the cold utensil. And when a hot utensil is removed from the gas burner, it cools down slowly. In this case, heat is transferred from the hot utensil to the surroundings.
In general, we can say that: Heat flows from a hot object to a cold object. In other words, heat flows from an object at higher temperature to another object at lower temperature. The flow of heat (or transfer of heat) stops when the two objects attain the same temperature. This means that no heat will be transferred from one object to another if the temperature of the two objects is the same (or equal). For example, if an iron ball at a temperature of 40°C is dropped into a bucket containing water at the same temperature of 40°C, then heat will not flow either from iron ball to water or from water to iron ball (because both, the iron ball and water, are at the same temperature of 40°C). We will now describe the various ways in which heat can be transferred from a hot object to a cold object.
Heat can be transferred from a hot object to a cold object in three different ways :
- by conduction,
- by convection, and
- by radiation (or heat rays).
In solids, heat is transferred by conduction. In liquids and gases, heat is transferred by convection. And in empty space or vacuum (having no medium like solid, liquid or gas), heat is transferred by radiation. We will now describe conduction, convection and radiation in detail, one by one. Let us start with conduction.
Conduction
If we heat one end of a metal spoon by keeping it over a gas burner, we find that its other end also becomes hot after a while (see Figure). In this case, heat is transferred from the hot end of the metal spoon to its cold end. Now, when heat is transferred from the hotter end of spoon to its colder end, there is no movement of the material of the spoon (which is metal) from one end to another. Such a transfer of heat from the hot part of a material to its colder part (without the movement of material as a whole) is called conduction.
In the above case the transfer of heat by conduction takes place within the same object (same spoon). Now, if we bring the hot end of the spoon in contact with the cold end of another spoon by touching them with each other, we will find that some of the heat of the hot spoon is transferred to the cold spoon due to which the cold spoon also becomes somewhat hot (see Figure). Such a transfer of heat from a hot spoon to a cold spoon which are in contact with each other, is also called conduction.
We can now define the process of heat transfer called conduction as follows. Conduction is the transfer of heat from the hotter part of a material to its colder part (Or from a hot material to a cold material in contact with it) without the movement of material as a whole. In all the solids, heat is transferred by the process of conduction. Here are some examples of transfer o heat by conduction from our everyday life. A cold metal spoon dipped in a hot cup of tea gets heated by conduction (In this case, hot tea acts only as a source of heat). A frying pan kept on a gas stove transfers the heat of gas burner through its metal bottom by the process of conduction. We will now describe an activity to demonstrate the process of conduction of heat.
Activity 4
To Show the Conduction of Heat
We take a flat aluminium rod and fix some small iron nails on it with the help of wax. This rod (alongwith its iron nails) is clamped to a stand as shown in Figure.
Let us heat the free end (left end) of the aluminium rod by keeping a burner below it. We will see that the iron nails attached to aluminium rod with wax start falling one by one. The nail attached nearest to the heated end of rod falls down first. And then the next ones fall. But the nail attached to the clamped end of the rod drops last of all. These observations can be explained as follows:
The burner is placed below the left end of aluminium rod. So, the left end of aluminium rod gets heated first. Now, the left end of aluminium rod is hot but the right end of rod is cold. So, heat is now transferred from the hotter left end of aluminium rod to its colder right end. As heat travels from the left side to the right side along the aluminium rod, it melts the wax which holds the nails. Due to this the nails fall down one by one. From this activity we conclude that heat is transferred from the hot end of aluminium rod to its colder end by the process of conduction.
The transfer of heat by the process of conduction takes place only in solids. This can be explained as follows: In solids, the particles are closely packed together. During conduction, heat is transferred from particle to particle by means of back and forth vibrations of the particles (caused by the heat energy). The particles of a solid remain at their fixed positions. There is no actual movement of the particles of the solid from its hotter end to the colder end during the conduction of heat through it.
Good Conductors of Heat and Poor Conductors of Heat
Some materials conduct heat easily whereas other materials do not conduct heat easily. So, on the basis of conduction of heat, all the materials are classified into two groups :
- Good conductors of heat, and
- Poor conductors of heat.
‘Good conductors’ of heat are sometimes called just ‘conductors’ of heat. Poor conductors of heat are also called insulators (of heat). This classification will become more clear from the following activity.
Activity 5
We take a beaker and fill it half with hot water. Let us take two spoons, one made of metal (such as steel) and the other made of plastic. Place the metal spoon and the plastic spoon in the beaker containing hot water as shown in Figure. After about two minutes, we touch the top ends of both the spoons with our hand, one by one. We will find that the top end of metal spoon feels quite hot but the top end of plastic spoon does not feel hot. This is because heat from the hot water flows easily through the metal spoon and reaches its other end. But the heat does not flow easily through the plastic spoon due to which its other end remains almost cold. This activity tells us that metal spoon is a good conductor of heat whereas plastic spoon is a poor conductor of
heat (or bad conductor of heat). We say that plastic is an insulator of heat.
Those materials which allow heat to pass through them easily are called good conductors of heat (or just ‘conductors’ of heat). All the metals are good conductors of heat. For example, the metals such as silver, copper, aluminium, iron and mercury are all good conductors of heat. Though all the metals are good conductors of heat, some metals are better conductors of heat than others.
For example, silver metal is the best conductor of heat. Copper is one of the best conductors of heat. Aluminium is also a very good conductor of heat. Metal alloys (such as brass, steel and stainless steel) are also good conductors of heat. Copper is a much better conductor of heat than stainless steel. All the objects made of metals and metal alloys are also good conductors of heat. Since metals and their alloys are good conductors of heat, therefore, if one end of an object made of metal (or metal alloy) is heated, then its other end becomes hot very quickly. Stone (marble, etc.) and tiles are also quite good conductors of heat.
Those materials which do not allow heat to pass through them easily are called poor conductors of heat. The materials which are poor conductors of heat are called insulators. So, a poor conductor of heat is an insulator of heat. Some of the examples of poor conductors of heat (or insulators of heat) are : plastic, wood, paper, cloth, leather, cotton, wool, thermocol, rubber, asbestos, clay, bricks, cork, cane, bamboo, straw, sawdust, glass, fibreglass, water and air. In general liquids are poor conductors of heat and gases are very poor conductors of heat. For example, water is a poor conductor of heat and air is a very poor conductor of heat.
In fact, air is a very good insulator of heat. The materials which trap air (such as cotton, wool, fur, feathers, fibreglass and plastic foam) are very poor conductors of heat. In other words, materials having trapped air in them are excellent insulators of heat. If one end of an object made of a poor conductor material (or insulator material) is heated, then its other end does not become hot. It either remains almost cold or becomes only slightly hot.
Uses of Good Conductors and Poor Conductors of Heat
We make use of good conductors of heat as well as of poor conductors of heat (or insulators of heat) in many ways in our daily life. Good conductors are used to carry heat quickly where it is wanted. Poor conductors (or insulators) are used to stop heat from going where it is not wanted. Some of the examples of the use of good conductors and poor conductors of heat are given below.
The cooking utensils are made of metals (or metal alloys). This is because metals (and metal alloys) are good conductors of heat which transfer the heat from the gas stove quickly to the food placed inside the utensil (through their bottom) (see Figure). The cooking utensils are provided with handles made of plastic (or wood). This is because plastic (or wood) is a poor conductor of heat (or insulator of heat) (see Figure). A plastic (or wooden) handle prevents the heat from hot cooking utensil reaching our hand so that we can lift the hot cooking utensil safely from its handle (without the risk of burning our hand). For example, a frying pan is made of aluminium metal or stainless steel because aluminium metal and
stainless steel are good conductors of heat which conduct the heat from gas burner of stove quickly through their bottom to the food kept inside [see Figure (a)]. The handle of a frying pan is made of plastic (or wood) because plastic (or wood) is a poor conductor of heat or insulator of heat. The plastic handle (or wooden handle) does not conduct the heat of hot frying pan to its other end so that we can hold even the hot frying pan from its handle safely [see Figure (a)]. Some stainless steel frying pans are provided with copper be toms.
The reason for this is that copper metal is a much better conductor of heat than stainless steel (due to which it can transfer the heat of gas stove burner to the food kept inside much more quickly).
Mercury metal is used in making thermometers because it is a good conductor of heat. The handle of a soldering iron is made of wood (or plastic) because wood (or plastic) is a poor conductor of heat. The wooden (or plastic) handle does not conduct the heat of hot soldering iron to our hand so that we can hold the hot soldering iron safely from its handle. An electric iron is also provided with a plastic handle so that when electric iron gets hot, its plastic handle (being a poor conductor of heat) remains cold and can be held safely with our hand. Table mats are usually made of poor conductors (or insulators) such as plastic or cork to protect the table tops from hot dishes.
Why Do We Wear Woollen Clothes in Winter
We wear woollen clothes in winter when it is cold outside. Woollen clothes keep us warm during cold winter days. This happens as follows : Wool is a poor conductor of heat due to which woollen clothes stop the flow of heat front our warm body to the cold surroundings. Moreover, there is air trapped in-between the fibres of the woollen clothes. Air is a very poor conductor of heat. So, the air trapped in fibres of woollen clothes also stops the flow of heat from our warm body to the cold surroundings. Since our body does not lose its heat, we feel warm on wearing woollen clothes in winter. A woollen blanket also keeps us warm on a cold winter night in a similar way.
If we are given the choice in winter of using either one thick woollen blanket or two thin woollen blankets joined together, then we should prefer two thin woollen blankets joined together. This is because the two thin woollen blankets joined together will have a layer of air trapped in-between them. And this extra layer of trapped air (being a very poor conductor of heat) will prevent our body heat from going away to the cold air more efficiently and hence keep us more warm. The single, thick woollen blanket does not have this extra layer of air in it. In fact, wearing more layers of clothes keeps us warmer in winter than wearing just one thick piece of clothing. This is because more layers of clothes trap more air in them and air being a very poor conductor of heat, prevents our body heat from going away to cold surroundings.
The feathers of birds keep them warm in cold weather due to air trapped in them. Air being a very poor conductor of heat, prevents the body heat of birds from escaping to the cold surroundings and hence keeps the birds warm in cold weather. Similarly, the animals having fur (such as polar bear) keep warm in cold weather because their fur traps a lot of air (which is an excellent insulator of heat).
During the hot summer days, we use electricity to run fans, coolers and air conditioners to keep our house cool. And in cold winter days, we use electricity to run room heaters or burn fuels like coal (or charcoal) to keep our house warm. In this way, a lot of energy is used up to protect us from outside heat or cold and keep us comfortable in summer as well as in winter. It is now possible to construct houses (and other buildings) in such a way that they are not affected much by the heat or cold outside.
This can be done by constructing the outer walls of houses (or other buildings) by using hollow bricks (which contain trapped air). Since the hollow bricks contain a very poor conductor of heat air’ trapped in them, the walls made of hollow bricks will neither allow outside heat to come in during summer nor allow inside heat to go out during winter. This will save a lot of electrical energy as well as other fuels (which are otherwise used in keeping houses cool or warm).
Sometimes two things which are at the same temperature feel like they are at different temperatures: one being cold and the other being warm. This happens because some things are good conductors of heat whereas others are poor conductors of heat. Here is an example.
During winter season, a metal object kept in a room feels very cold to touch but a wooden object in the same room feels warmer to touch. This can be explained as follows : Metal object is a good conductor of heat. So, when we touch the metal object, it conducts away heat from our hand quickly. And by losing heat, our hand feels cold. On the other hand, the wooden object (being a poor conductor of heat) does not allow the heat of our hand to escape and hence feels warmer to touch.