# Heat Class 7 Notes Science Chapter 4

## CBSE Class 7 Science Notes Chapter 4 Heat

Learn from School Connect Online in this chapter we will learn about the heat. Heat is a form of energy that makes substances hot. In winter, it is our common experience that we feel cold inside the house and if we come out in front of sun rays, then we feel warm. Heat Class 7 Notes Science Chapter 4

Contents

## Ex. 4.1 – Hot and Cold

• In our daily routine, we come across several objects, out of which some are hot while other objects are cold.
• E.g:- When a frying pan kept on a burning gas stove becomes hot but the handle of the pan is cold. Even among the hot objects, some objects may be hotter than others.
• In the same manner, among the cold objects, some objects may be colder than others.
• So, if you have to decide the relative hotness or coldness of objects, then your answer will be by simply touching the objects’.
• But our sense of touch is not enough in telling us whether an object is hot or cold, so this can be understood by performing a simple activity.

## Ex. 4.2 – Measuring Temperature

The degree of hotness or coldness of the object is known as the temperature of an object. The temperature of an object is the only property that indicates which object is hot and which one is cold. A high temperature of a body indicates that it is very hot whereas a low temperature of the object indicates that it is quite cold, e.g. the temperature of boiling water is quite high, so boiling water appears to be very hot. On the other side, the temperature of melting ice is quite low. So, the ice appears to be very cold on the touch.

It is measured by using an instrument called a thermometer, which has a scale marked on it which is used to read the temperature, e.g. the scale in laboratory thermometer is marked along the length of the thermometer’s tube between 0° mark and 100° mark into 100 equal divisions. So, each division is called a degree. The temperature of an object should always be stated with its unit. So, the most common unit for measuring temperature is degree Celsius (°C).

Both the clinical thermometer and laboratory thermometer are mercury thermometers. So, when a particular amount of heat is supplied to the thermometer bulb consisting of mercury (by the hot body whose temperature is to be measured), then the mercury expands and rises in the glass tube of the thermometer. This fact is used in measuring the temperature.

## Ex. 4.3 – Laboratory Thermometer

A device which is used for measuring the temperature in a science laboratory is called a laboratory thermometer.

This thermometer is made up of a long glass tube having a thin bore. The graduation marked on the tube of a laboratory thermometer can measure the temperature from -10°C to 110°C, this is known as the range of a laboratory thermometer. Also, determine how much a small division on this thermometer reads (this is also known as the least count of the thermometer), it is because this information is required to read the thermometer correctly.

## Ex. 4.4 – Transfer of Heat

Heat flows from a hot object to a cold object or heat flows from an object at a higher temperature to another object which is at a lower temperature. This flow of heat is known as the transfer of heat, e.g. if you dip a steel spoon into a cup of hot tea, then we will find that the temperature of the spoon rises and it becomes hot. In this case, some of the heat contained in hot tea has been transferred to a spoon that is placed inside it.

When the two objects attain the same temperature, then the flow of heat stops. This means that no heat will be transferred from one object to another if the temperature of the two objects is the same.

There are three ways through which heat can be transferred from a hot object to a cold object.

• By conduction (in solid, heat is transferred by conduction)
• By convention (in liquid and gases, heat is transferred by convection)
• By radiation (in free space or vacuum, heat is transferred by radiation)

## Ex. 4.5 – Kinds of Clothes we wear in summer and winter

During hot summer days, people prefer to wear white clothes or light coloured clothes because light coloured clothes absorb less heat from the sun and hence, keep us cool and comfortable in hot weather while in the cold winter days people prefer to wear dark clothes because the dark coloured clothes absorb more heat rays from the sun and keep us warm in the winter season.

Thus, we can say that dark coloured objects absorb heat better and also emit heat better than light coloured objects. Now, let us try to study this concept on the basis of the given activity.

In the winter, we use woollen clothes. Wool is a poor conductor of heat. Moreover, there is air trapped in between the wool fibres. This air prevents the flow of heat from our body to cold surroundings. So, we feel warm.

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