NCERT Solutions For Class 7 Science Chapter 4

NCERT Solutions for CBSE Class 7 Science: Chapter 2 – Nutrition in ...

Important Terms To Be Covered In Class 7 Science Chapter 4

1. Hot And Cold

In this part of the content focuses on the effects of heat energy in different materials with practical activities which help you to better understanding the concept. In our day to day life, we come across a number of objects. Some of them are hot, and some of them are cold. Tea is hot, and ice is cold. In winter we usually feel cool inside the house. If we come out in the Sun, due to effect of light, we feel warm. In summer, we feel hot even inside the house. How do we know whether an object is hot or cold? By using a reliable device for the measurement of the hotness of an object, is called ‘Thermometer’.

2. Measuring Temperature

In this part of the content introduces the importance of thermometer in our life. Do you know that the thermometer that measures our body temperature is called ‘Doctor’s Thermometer’ or ‘Clinical Thermometer’? And a clinical thermometer consists of a long, narrow, uniform glass tube with a bulb at one end. This bulb contains mercury which is the main source of the measurement. You will also find two types of scale on Clinical thermometer. 35 to 40 degrees in ‘Celsius scale’ and 94 to 108 degrees in ‘Fahrenheit scale’. Do you know the normal temperature of the human body is 37 degrees Celsius, and it is the average body temperature of us? You will be surprised to know that different types of thermometers are used for different purposes. As maximum and minimum thermometer are used for the weather of a day. Clinical thermometer is used for human body temperature. How do we measure the temperature of other objects?

3. Laboratory Thermometer

In this part, we will learn about the process of getting the temperature of different objects. For measuring the temperature of different objects, we use ‘Laboratory thermometer’ for them. The range of a laboratory thermometer is generally from -10 degree Celsius to 110 degrees Celsius. Remember, we do not use a clinical thermometer for measuring the temperature of other objects and also not be use laboratory thermometer for the human body. In both cases, we use a separate device to measure the temperature. We can compare the temperatures of Water from different sources.

4. Transfer Of Heat

In this part of the content helps you to understand the various methods of heat transfer with the help of different practical activities. You might have observed that frying pan becomes hot when kept on a flame. When the pan is removed from the flame, it slowly cools down. Do you know how it happens? Heat is a form of energy which spontaneously transferred from a hotter object to a colder object, called ‘Conduction’. The material which allows heat to pass through them easily is called ‘Conductors’. And those materials which do not pass heat through them are called ‘Insulators’.

5. Convection And Radiation

You will be surprised to know the facts, how water is boiling. When water is heated, the water near the flame gets hot. Hot water rises and cold water moves down towards the source of heat. This water also gets hot, and rises and water from the sides move down again. This process continues until the whole water gets heated. This mode of heat transfer is known as ‘Convection’. When we come out in the sun, we feel warm. It cannot reach us by conduction or convection. Remember, from the sun, the heat comes to us by another process known as ‘Radiation’. The motion of heat by radiation does not require any medium. When we sit in front of a room heater, we get heat by this process.

6. Clothes We Wear In Summer And Winter

You know that in summer we prefer light-coloured clothes and in winter we usually wear dark-coloured clothes. Do you know what the reason behind it is? The dark colour absorbs more heat, and therefore we feel comfortable with dark coloured clothes in winter. And light-coloured clothes reflect most of the light that falls on them and, therefore, we feel more comfortable to wear them in summer. As you know, 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 the cold surroundings. So, we feel warm. Uses of heat energy include but are not limited to: cooking, drying, heating, smoking, baking, water heating, cooling and manufacturing. Besides that, heat from the sun generates our weather patterns and gives energy to the growing green plants that provide the food and oxygen for life on Earth. Thus, the sun is the ultimate source of energy for all living organisms.

Important Questions For Practice

1. Paheli and Boojho measured their body temperature. Paheli found hers to be 98.6 °F and Boojho recorded 37°C. Which of the following statement is true?
(a) Paheli has a higher body temperature than Boojho.
(b) Paheli has a lower body temperature than Boojho.
(c) Both have normal body temperature.
(d) Both are suffering from fever.
2. A beggar wrapped himself with a few layers of newspaper on a cold winter night. This helped him to keep himself warm because
(a) friction between the layers of newspaper produces heat.
(b) air trapped between the layers of newspaper is a bad conductor of heat.
(c) newspaper is a conductor of heat.
(d) newspaper is at a higher temperature than the temperature of the surrounding.
3. A marble tile would feel cold as compared to a wooden tile on a winter morning, because the marble tile
(a) is a better conductor of heat than the wooden tile.
(b) is polished while wooden tile is not polished.
(c) reflects more heat than wooden tile.
(d) is a poor conductor of heat than the wooden tile.
4. Shopkeepers selling ice blocks usually cover them with jute sacks. Explain why.
5. Why is it advised not to hold the thermometer by its bulb while reading it?
6. To keep her soup warm Paheli wrapped the container in which it was kept with a woolen cloth. Can she apply the same method to keep a glass of cold drink cool? Give reason for your answer.
7. You may have noticed that a few sharp jerks are given to clinical thermometer before using it. Why is it done so?
8. In a mercury thermometer, the level of mercury rises when its bulb comes in contact with a hot object. What is the reason for this rise in the level of mercury?

Answers Of Important Questions

1 (c)
2 (b)
3 (a)
4. They must use some insulating material like, sack, saw dust, newspaper, etc. to cover the ice.
5. If we hold a thermometer by its bulb, the mercury in the bulb will expand due to our body temperature.
6. Yes. Wool is poor conductor of heat.
7. The jerk to the thermometer will allow the mercury in or above the kink to flow into the bulb so that the mercury level is below normal temperature.
8. Mercury expands when heated. Hence, it rises in the capillary tube.b

Important NCERT Questions Class 7 Science Chapter 4

प्रयोगशाला तापमापी तथा डॉक्टरी थर्मामीटर के बीच समानताएँ तथा अंतर लिखिए।

समानताएँ:
दोनों थर्मामीटर का कार्य सिद्धांत समान है।
दोनों का उपयोग तापमान को मापने के लिए किया जाता है।
दोनों में एक मोटी दीवार वाली कांच की नली होती है, जो एक महीन पतली नली से जुडती है।
तापमान मापने के लिए दोनों थर्मामीटर में पारे का उपयोग किया जाता है।
दोनों में सेल्सियस और फ़ारेनहाइट अंकित हैं।

अंतर:
प्रयोगशाला थर्मामीटर का उपयोग कारखानों और प्रयोगशाला में विभिन्न वस्तुओं के तापमान को मापने के लिए किया जाता है जबकि डॉक्टरी थर्मामीटर का उपयोग केवल मनुष्यों के तापमान को मापने के लिए किया जाता है।
आम तौर पर, प्रयोगशाला थर्मामीटर में तापमान -10 ℃ से 110 ℃ तक होता है जबकि डॉक्टरी थर्मामीटर में तापमान 35 ℃ से 42 ℃ तक होता है।
आमतौर पर प्रयोगशाला थर्मामीटर में कोई अवरोध नहीं होता है जबकि डॉक्टरी थर्मामीटर में पारा बल्ब के पास छोटा अवरोध होता है।
मान लेते समय प्रयोगशाला थर्मामीटर को सीधा रखना पड़ता है जबकि डॉक्टरी थर्मामीटर झुकाया जा सकता है।

Give two examples each of conductors and insulators of heat.

Conductors: Copper, aluminum and iron.
Insulators: Wood, water and air.

Fill in the blank: The hotness of an object is determined by its __________.

The hotness of an object is determined by its temperature.

रिक्त स्थान की पूर्ति कीजिए: उबलते हुए पानी का ताप _____________ तापमापी से नहीं मापा जा सकता।

उबलते हुए पानी का ताप डॉक्टरी तापमापी से नहीं मापा जा सकता।

सर्दियों में एक मोटा वस्त्र पहनने के तुलना में उसी मोटाई का कई परतों का बना वस्त्र अधिक उष्णता क्यों प्रदान करता है? व्याख्या कीजिए।

कपड़ों की परतों के बीच हवा फंस जाती है। वायु ऊष्मा रोधी का कार्य करती है। यह परत हमारे शरीर की गर्मी को आसपास के वातावरण में जाने से रोकती है। पतले कपड़े की अधिक परतें अधिक हवा को फंसने देंगी और परिणामस्वरूप शरीर की गर्मी वातावरण में जाने की संभावना कम होगी। इसलिए सर्दियों में एक मोटा वस्त्र पहनने के तुलना में उसी मोटाई का कई परतों का बना वस्त्र अधिक उष्णता प्रदान करता है।

In places of hot climate it is advised that the outer walls of houses be painted white. Explain.

In places of hot climate it is advised that the outer walls of houses be painted white because white colour reflects most of the heat that falls on it or we can say it absorbs less amount of heat. Therefore, the temperature inside the house does not increase very much.

7th Science Chapter 4 Important Questions Set – 1

How Do We Know Whether An Object Is Hot Or Cold?

By using thermometer, we can determine the temperature of any object.
Generally, thermometer is a device that measures the temperature or hotness of an object.

Why Our Touching Sense Is Not Reliable?

Our touching sense is not reliable because we can’t say the temperature of any object by touching. We just feel the flow of any temperature either that is hot or cold. The other reason is that our senses do not indicate the accurate flow of temperature by the skin. We just feel the hotness or coldness by our skin.
A classic experiment proves that our senses are not reliable
If we put one hand in a bowl of very warm water and the other hand in a bowl of cold. After keeping the hands in the two mugs for 2–3 minutes, put both the hands simultaneously in mix water or room temperature water mug. we cannot decide whether an object is hot or cold. Sometimes it may deceive us.

What Is Temperature?

Temperature is that thermal condition (hotness) of a substance which determines whether heat is applied to the other bodies or not.

What Is Clinical Thermometer?

The thermometer that measures our body temperature is called a clinical thermometer.

Thermometer

Temperature is measured by a device called thermometer. In other words, a thermometer is an instrument for measuring or showing temperature that indicate how hot or cold is something.

How Do You Read Any Clinical Thermometer? Discuss.

Wash the thermometer, preferably with an antiseptic solution. Hold it firmly and give it a few jerks. The jerks will bring the level of mercury down. Ensure that it falls below 35°C. Now place the bulb of the thermometer, under your tongue.
After one minute, take the thermometer out and note the reading. This is your body temperature. The temperature should always be stated with its unit, °C (the Celsius).
About Division: The temperature difference indicated between the two bigger marks. Suppose the bigger marks read one degree and there are five divisions between them. Then, one small division can read 1/5 =0.2 degree Celsius.

What Is Normal Temperature Of Human Body?

The normal temperature of human body is 37°C. This temperature is stated with its unit.

Is The Body Temperature Of Every Person 37 Degree Celsius?

The temperature of every person may not be 37°C. It could be slightly higher or slightly lower. Actually, the normal temperature is the average body temperature of a large number of healthy persons.

Why Clinical Thermometer Is Designed To Measure The Temperature Of Human Body Only?

The clinical thermometer is designed to measure the temperature of human body only because the temperature of human body normally does not go below 35 degree Celsius or above 42 degree Celsius. That is the reason that this thermometer has the range 35 degree Celsius to 42 degree Celsius.

Construction Of Clinical Thermometer

A clinical thermometer consists of a long, narrow, uniform glass tube. It has a bulb at one end. This bulb contains mercury. And a kink near the bulb. Outside the bulb, a small shining thread of mercury can be seen. A clinical thermometer reads temperature from 35°C to 42°C. The other scale with the range 94–108 degrees is the Fahrenheit scale (°F).

7th Science Chapter 4 Important Questions Set – 3

How Would You Measure The Temperature Of Any Day?

The maximum and minimum temperatures of any day, reported in weather reports. Temperature of any day is measured by a thermometer called the maximum -minimum thermometer.

What Is Laboratory Thermometer?

A laboratory thermometer is a tool used in laboratories to measure the temperature of an object with high accuracy. It can be partially or fully immersed in the substance being measured.

Why Does The Mercury Not Fall Or Rise In A Clinical When Taken Out Of The Mouth?

The mercury does not fall or rise in a clinical thermometer when taken out of the mouth because of the presence of kink. It prevents mercury level from falling on its own.

Now A Days, We Are Using Digital Thermometer. Why?

There is a lot of concern over the use of mercury in thermometers. Mercury is a toxic substance and is very difficult to dispose of if a thermometer breaks. These days, digital thermometers are available which do not use mercury. Digital thermometers are designed to be more accurate because it consists digital circuit board which converts the temperature into numerical values with decimals.

The Working Method Of Laboratory Thermometer

Take some tap water in a beaker or a mug. Dip the thermometer in water so that the bulb is immersed in water but does not touch the bottom or the sides of the container. Hold the thermometer vertically. Observe the movement of mercury in the thermometer. Wait till the mercury thread becomes steady. Note the reading. This is the temperature of water at that time.
When we take the thermometer out of water, the level of mercury begins to fall. This means that the temperature must be read while the thermometer is in water.

7th Science Chapter 4 Important Questions Set – 4

What Is Heat?

The word heat to mean something that feels warm, but science defines heat as the flow of energy from a warm object to a cooler object. Heat energy is the result of the movement of tiny particles called atoms, molecules or ions in solids, liquids and gases.
In other words, heat is a form of energy which spontaneously passes to its surroundings.

It Is Advised Not To Hold The Thermometer By Its Bulb While Reading It. Explain, Why?

We are advised not to hold the thermometer bulb while reading it, as the level of mercury increases from the actual reading due to our body temperature.

How Does Heat Transfer Take Place In A Iron Rod? Discuss.

Take a rod or flat strip of a metal, say of aluminium or iron. Fix a few small wax pieces on the rod. These pieces should be at nearly equal distances. Clamp the rod to a stand. If you do not find a stand, you can put one end of the rod in between bricks.
Now, heat the other end of the rod. These wax pieces begin to fall. Heat is transferred from the end nearest to the flame to the other end. The process by which heat is transferred from the hotter end to the colder end of an object is known as conduction. In solids, generally, the heat is transferred by the process of conduction.

What Do You Mean By Conductors And Insulators?

The materials which allow heat to pass through them easily are conductors of heat. For examples, aluminium, iron and 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.

Sea breeze: During the day, the land gets heated faster than the water. The air over the land becomes hotter and rises up. The cooler air from the sea rushes in towards the land to take its place. The warm air from the land moves towards the sea to complete the cycle. The air from the sea is called the sea breeze.
Land breeze: To receive the cooler sea breeze, the windows of the houses in coastal areas are made to face the sea. At night it is exactly the reverse. The water cools down more slowly than the land. So, the cool air from the land moves towards the sea. This is called the land breeze.

7th Science Chapter 4 Important Questions Set – 5

How Does Any Water Heated Through The Convection Process?

Take a steel or aluminium container with some water. When water is heated, the water near the flame gets hot. Hot water rises up. 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 moves down. This process continues till the whole water gets heated. This mode of heat transfer is known as convection.

Why Are You Advised To Use An Umbrella When You Go Out In The Sun?

An umbrella guards against the sun’s ultraviolet rays to keeps away from sunburn. When we go out in the sun, its ultraviolet rays directly reach us that makes various skin diseases or makes hot.
To avoid further damage from the sun, you are advised to use an umbrella while going out.

Discuss Why Wearing More Layers Of Clothing During Winter Gives Us Warmer Than Wearing Just One Thick Piece Of Clothing.

The air gets trapped between two layers of warm clothes. Air acts as insulator of heat. This layer prevents our body heat to escape in the surroundings. More layers of thin clothes will allow more air to get trapped and as a result we will not feel cold. For wearing more layers of clothing during winter keeps us warmer than wearing just one piece of clothing.

For Setting Curd, As Small Amount Of Curd Is Added To Warm Milk. The Microbes Present In The Curd Help In Setting If The Temperature Of The Mixture Remains Approximately Between 35-Degree C To 40-Degree C. At Place Where Room Temperature Remains Much Below The Range, Setting Of Curd Becomes Difficult. Suggest A Way To See Curd In A Such A Situation.

For the setting of curd at places where temperature is below room temperature, the container in which curd is to be made, must be kept in a thermally insulated cover or it can be wrapped either by a woollen material or a jute sack so that temperature is maintained for the setting of curd.
The container can also be kept in the sun or near the gas stove while cooking food for the setting of curd.

Light Coloured Clothes in Summer

Light coloured clothes reflect most of the heat that falls on them and, therefore, we feel more comfortable wearing them in the summer. Dark surfaces absorb more heat and, therefore, we feel comfortable with dark coloured clothes in the winter.

Details: Parent Category: student Corner; Category: Class 7 Science; Published: 19 May 2020; Hits: 6

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