We know that matter is composed of tiny particles called atoms and molecules. Molecules can exist freely in nature and possess all the properties of matter. The Molecules are in motion and also they have the force of attraction amongst them. Due to motion molecules have kinetic energy and due to the force of attraction, they have potential energy. When a substance is heated(or when a substance absorbs heat) molecules start vibrating faster so the kinetic energy increases. When the substance is cooled down the motion of molecules slowers down and hence the 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. It is measured in the unit joule.
In this lesson, we are going to learn:
When two bodies at different temperatures are kept in contact, heat flows from a body at high temperature to the body at low temperature. The average kinetic energy of the substance is a measure of the temperature of the body. When there is a rise in the average kinetic energy of molecules of a substance, its temperature increase, and if there is a fall in the average kinetic energy of molecules of a substance, its temperature decreases.
Keep a frying pan on a flame. The frying pan soon gets hot, because the heat passes from the flame to the pan. Now remove the pan from the flame. Gradually the pan will cool down because the heat is transferred from the pan to the surroundings. In both cases, heat flows from a hotter object to a colder object.
Experiment 1: Let say we have two objects. Object A having a temperature of 100o C and object B having a temperature of 10o C. Keep both the objects in contact with each other.
Result: Heat will transfer from object A to B until the temperature is the same in both the objects. Suppose object A drops to 50o C and the temperature of cold object B rises to 50o C. This state is known as Thermal Equilibrium. In the Thermal Equilibrium state, the heat energy is still transferred between these two objects but the net flow of heat energy is zero.
Experiment 2: Heat water in a small pan. After five minutes try to hold the handle of the pan to remove it from the flame. What do you think will happen to your hands? You will immediately take your hand off the steel handle.
Your hand will feel the hotness of the pan. The reason is some of the heat energy is transferred from the pan to your hand. Heat transfers from a hot object to a cold object if there is contact between them. In physics, we say that transfer of heat requires a medium. Thermal conduction is the movement of heat from one object to another one that has different temperature when they are touching each other. In solids, generally, the heat is transferred by the process of conduction.
Examples:
Conductors and Insulators
Do all substances conduct heat easily? You must have observed that the metallic pan for cooking has a plastic or wooden handle. You can lift a hot pan by holding it from the handle without getting hurt. The reason is different objects conduct different amounts of heat energy due to the nature of the material they are made of.
Experiment 3:
Heat water in a small pan or a beaker. Collect some articles such as a steel spoon, plastic scale, pencil, and divider. Dip one end of each of these articles in hot water. Wait for few minutes and then take out these articles one by one touching the dipped end. Enter your observation in a table:
| Article | Made of | Does the other end get hot Y/N? |
| Steel spoon | Metal | Y |
| Divider | Metal | Y |
| Scale | Plastic | N |
| Pencil | Wood | N |
The material that allows heat to pass through them easily are conductors of heat. For example, iron, steel, aluminum, copper. The materials which do not allow heat to pass through them easily are poor conductors of heat such as plastic and wood. Poor conductors are known as insulators.
The water and air are poor conductors of heat. Then, how does the heat transfer take place in these substances? Let us find out!
Experiment 4: Place your hand slightly above the fire. Be careful. Keep your hands at a safe distance from the flame so that they do not get burnt.
Result: You will feel the hotness of fire. Till now we learned that heat transfer between objects when they are in contact with each other, then what is causing our hands to feel the warmth of the fire without touching it? Reason: Fluids(liquid and gases) molecules possess kinetic energy and as we know the kinetic energy of gas depends upon heat energy or temperature. The gas molecules that are in touch with fire absorb heat energy from the fire, as a result, the kinetic energy of gas molecules increases hence they rise up and hits your hand. The hands absorb the heat energy from these molecules and you feel hot.
Let us now see how heat transfer happens in the case of liquid:
Experiment 5: Take a beaker and fill it with water and place it above the flame.
Result: When water is heated, the water near the flame gets hot. Hot water rises up as the water molecules become less dense as they absorb heat energy. The cold water from the sides moves down towards the source of heat. This water also gets hot and rises and water from the sides move down. This process continues till the whole water gets heated.
This mode of heat transfer due to the bulk motion of fluids is known as convection.
Examples:
When we come out in the sun, we feel warm. How does the heat from the sun reach us? It cannot reach us by conduction or convection as there is no medium such as air in most parts of the space between the earth and the sun. From the sun the heat comes to us by another process known as radiation. The transfer of heat by radiation does not require any medium. It can take place whether a medium is present or not.
Every object radiates heat. Our body too gives heat to the surroundings and receives heat from it by radiation. When this heat falls on some object, a part of it is reflected, a part is absorbed and a part may be transmitted. The temperature of the object increases due to the absorbed part of the heat.
Experiment 6: Take two identical metal containers, one in black and the other in white. Pour equal amounts of water in each and leave them in the mid-day sun for about an hour.
Result: Measure the temperature of water in both containers. The temperature of water in the black container is more than in the white-colored container. Black objects are good absorbers of radiations while white objects are bad absorbers or good reflectors of radiations.
Examples:
