ICSE Class 8 Physics Chapter 06 Heat Transfer

Heat Transfer

Theme

In both boiling and evaporation, matter changes from liquid to gas. But the two processes are quite different. When temperature of a matter increases, the particles of the matter gain energy and move with greater speed. In evaporation, the particles of the surface escape and form gas. Other particles, inside the liquid, do not have enough energy. So the process of evaporation of the liquid are at the same temperature and are involved in the process. It happens at a fixed temperature, particular to a liquid. But before change of states takes place due to supply of heat. There is another effect which is commonly observed. That is the expansion of matter. Matters in all form, except some exceptions, expand on heating. In solids, the effect is less, in liquids more, and in gases maximum. Classification of expansion into three types-linear, superficial and volume are explained with examples from daily life.

In This Chapter You Will Learn To

Compare and contrast boiling and evaporation.

Describe thermal expansion of matter.

Describe, linear, area (superficial) and volume expansion.

Compare expansivity in solids, liquids and gases.

Construct models based on scientific process.

Observe and cite multiple physical phenomena from one experiment.

Learning Objectives

Revising and revisiting previous concepts learnt by children.

Building on children's previous learning - Demonstrating points of boiling and evaporation.

Teacher's Note

Help students understand that boiling happens at a specific temperature for each liquid, while evaporation occurs at all temperatures from any liquid surface.

In This Chapter You Will Learn To (Continued)

Engaging children in tasks related to boiling and evaporation.

Explaining the difference in boiling and evaporation.

Demonstrating linear expansion, area expansion and volume expansion through conducting simple experiments for children.

Explaining expansion with the help of examples from daily life activities.

Knowing Concepts

Difference between boiling and evaporation.

Thermal expansion:

Linear expansion

Superficial expansion

Volume expansion

Compare expansivity in solids, liquid, and gases.

Examples and real world applications

Introduction

In the earlier classes you have read that matter is composed of tiny particles called the molecules. A molecule can exist freely in the nature and it possess the properties of the matter. It is very small in size (nearly 10-10 m) and cannot be seen even by a microscope. The molecules are in motion as well as they have inter-molecular forces of attraction amongst them. Due to the forces of attraction amongst them, the molecules have the kinetic energy and due to forces of attraction, they have the potential energy. When a substance absorbs heat (or the substance is heated), the motion of its molecules becomes rapid, so their kinetic energy increases. When the substance is cooled (or it gives out heat), the motion of its molecules becomes slow and so their kinetic energy decreases. The total kinetic energy of molecules of the substance is called its internal kinetic energy and the total potential energy of molecules is called its internal potential energy. The sum of internal kinetic energy and internal potential energy is called the total internal energy or heat energy of the substance. Thus, heat is the internal energy of a substance. It is measured in the unit joule (symbol J).

When two bodies at different temperatures are kept in contact, heat flows from a body at high temperature to a body at low temperature. The average kinetic energy of the substance is a measure of temperature of the body. When there is a rise in average kinetic energy of molecules of a substance, its temperature increases and if there is fall in the average kinetic energy of molecules of a substance, its temperature decreases.

Other common unit of heat is calorie (symbol cal). Where 1 cal = 4.2 J (nearly)

Effects of Heat

Heat produces mainly the following three effects:

Change in temperature of the body.

Change in state of the body, and

Change in size of the body.

Change in temperature of the body

When a body is heated, its temperature rises and when it is cooled, its temperature falls. The change in temperature of the body depends on the following two factors:

Quantity of heat imparted to (or rejected from) the body

When heat is imparted (or given) to the body, its temperature rises while if heat is rejected (or taken) from the body, its temperature falls.

Reason - On heating, the molecules begin to move faster, so the average kinetic energy of molecules increases and so the temperature rises. On the other hand, on cooling, the average kinetic energy of molecules decreases, so temperature falls.

Material of the body

Some materials rise to high temperature while some to a low temperature even when same quantity of heat is imparted to them.

Reason - Different materials have different specific heat capacity (i.e., different amount of heat required to raise the temperature of unit mass by unit rise in temperature).

Change in state of the body

Matter exists in three different states, namely solid, liquid and gas.

The process of change from one state to another at a constant temperature is called change of state.

When a solid is heated, it changes into its liquid at a fixed temperature. This process is called melting. The reverse happens when a liquid is cooled, the liquid freezes into solid at the same fixed temperature. This process is called freezing or fusion. For example, ice at 0-C on heating melts into water at 0-C, while water at 0-C on cooling freezes or fuses into ice at 0-C.

When a liquid is heated, it changes into its vapour (or gas) at a fixed temperature. This process is called vaporization or boiling. The reverse happens when vapour is cooled, the vapour condenses into liquid at the same fixed temperature. This process is called condensation. For example, water at 100-C on heating vaporizes into steam at 100-C, while steam at 100-C on cooling condenses into water at 100-C.

When a solid on heating changes directly into its vapour at a fixed temperature, the process is called sublimation. For example, camphor on heating, changes directly from solid to vapour. These vapour on cooling change directly into solid state. This process is called solidification or deposition.

The change of state from liquid to gas at all temperatures is called evaporation. Thus, evaporation differs from boiling. Boiling is at a fixed temperature while evaporation takes place at all temperatures. For example, drying of clothes is due to evaporation of water.

Teacher's Note

Create a visual diagram showing the transitions between states of matter. Students can use this to understand that melting, freezing, boiling, and condensation all occur at specific temperatures for each substance.

The heat absorbed, (or rejected) during the change of state is called latent heat or hidden heat because it is not manifested by any change in temperature. This heat when expressed for unit mass of a substance is called specific latent heat. Since temperature does not change during the change of state, there is no change in average kinetic energy of molecules of the substance but the heat absorbed (or rejected) changes the average potential energy of the molecules of the substance.

Change in size of the body

When a body is heated, it expands and upon cooling, it contracts. This change in size of the body due to heating is called thermal expansion.

Generally all solids, liquids and gases expand on heating or contract on cooling. The reason is that on heating a substance, the average inter molecular separation between the molecules of the substance increases, while on cooling it decreases.

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