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Heat Class 7 Notes Science Chapter 3

Introduction

  • Wollen clothes are made from animal fibers while cotton clothes are made from plant fibers.
  • We wear woollen clothes during winters to stay warm as they provide insulation and help in retaining body heat.
  • Light-colored cotton clothes are preferred during hot weather for a cooling effect.
  • Specific types of clothes are suitable for different seasons based on their properties.
  • In winter, you feel cold indoors but warm in the sun; in summer, you feel hot both indoors and outdoors.

Hot and Cold

Have you ever experienced cold inside the house and warmth in the sun, during winter? Also, feeling hot inside as well as outside the house in the summer season? 

To protect ourselves from the chilling cold, we wear clothes made of wool. These woolen clothes are obtained from wool-yielding animals such as sheep, goats, yack, etc. Wearing light-colored clothes during summer will give us a feeling of coolness.

Temperature

  • Temperature is a measure the sensation of warmth or coldness of an object, felt from contact with it. 
  • This sensation of touch gives an approximate or relative measure of the temperature. 
  • Temperature is measured in different scales, including Fahrenheit (F) and Celsius (or centigrade C)
  • The units of the Fahrenheit and Celsius scales are called degrees and are denoted by °
  • Swedish astronomer Anders Celsius devised the Celsius scale in 1742
  • He fixed the   of the scale at the freezing of water, and the 100°  at the boiling of water.

Question for Chapter Notes: Heat
Try yourself:
What is temperature?
View Solution

Measuring Temperature

  • There are different types of thermometers that measure the temperatures of different things like air, our bodies, food and many other things. 
  • There are clinical thermometers, laboratory thermometers, Galileo thermometers and digital remote thermometers
  • Among these, the commonly used thermometers are clinical thermometers and laboratory thermometers.

Thermometer

  • A thermometer is used to measure the temperature of an object - it is used to find how cold or hot the object is. 
  • Galileo invented a rudimentary water thermometer in 1593. He called this device a "thermoscope". 
  • However, this form was ineffective as water freezes at low temperatures. 
  • In 1714, Gabriel Fahrenheit invented the mercury thermometer, the modern thermometer. 
  • The long narrow uniform glass tube is called the stem of a thermometer. 
  • The small tube is called the bulb, which contains mercury. 
  • Mercury is toxic, and it is very difficult to dispose it when the thermometer breaks. 
  • So, nowadays digital thermometers are used to measure the temperature, as they do not contain mercury.

Clinical Thermometer

Heat Class 7 Notes Science Chapter 3

  • These thermometers are used to measure the temperature of the human body, at home, clinics and hospitals. 
  • All clinical thermometers have a kink that prevents the mercury from falling down rapidly so that the temperature can be noted conveniently. 
  • There are temperature scales on either side of the mercury thread, one in Celsius scale and the other in Fahrenheit scale.
  • A clinical thermometer indicates temperatures 35° c to 45° c or from to 94° F to 108°  F.
  • Since the Fahrenheit scale is more sensitive than the Celsius scale, body temperature is measured in degrees Fahrenheit only
  • A healthy person's average body temperature is between 98.6° F to 98.8° F.

Precautions:

  1. Wash the thermometer before and after use with an antiseptic solution, and handle it with care.
  2. See that the mercury levels are below the kink and don't hold the thermometer near its bulb.
  3. While noting down the reading in the thermometer, place the mercury level along the eyesight.
  4. Do not place the thermometer in a hot flame or in the hot sun.

Question for Chapter Notes: Heat
Try yourself:
Which type of thermometer is commonly used to measure the temperature of the human body?
View Solution

Laboratory Thermometers

Heat Class 7 Notes Science Chapter 3

  • These thermometers are used to measure the temperature in school and other laboratories for scientific research. 
  • They are also used in the industry as they can measure temperatures higher than what clinical thermometers can record. 
  • The stem and the bulb are longer when compared to that of a clinical thermometer. 
  • A laboratory thermometer has only the Celsius scale ranging from -10° C to 110° C.

Precautions:

  1. A laboratory thermometer doesn't have a kink.
  2. Do not tilt the thermometer. Place it upright.
  3. Note the reading only when the bulb has been surrounded by the substance from all sides.

Question for Chapter Notes: Heat
Try yourself:What is the temperature range indicated by a clinical thermometer?
View Solution

Relation between Celsius and Fahrenheit

The Celsius and Fahrenheit scales are related as: C/5 =F-32/9

Mercury is a toxic substance and difficult to dispose of if a thermometer breaks.

Modern digital thermometers, which do not contain mercury, are now available as an alternative.

Transfer of Heat

Heat Class 7 Notes Science Chapter 3

  • When an object is at different a temperature from its surroundings, then heat transfer takes place so the body and surrounding reaches the same temperature.
  • Heat transfers occurs from hotter objects to colder objects. 
  • When an object is at different a temperature from its surroundings, then heat transfer takes place so the body and surrounding reaches the same temperature. 
  • Also, heat from a hotter object is transferred to the particles of the surrounding air that are comparatively cooler. 
  • For example, when milk is boiled and the flame is off, the milk slowly transfers heat and becomes cooler.

There are three modes of heat exchange: 

1. Conduction

  • Conduction is the transfer of heat from the hotter part to the colder part of an object without the movement of its particles. 
  • Also, in conduction, heat gets transferred between substances that are in direct contact with each other. 
  • The better the conductor the more rapidly does the heat transfer take place. 
  • For example, when you pop corn in a cooker on a flame, heat is transferred from the flame to the corn by conduction.

Conductors: Materials that allow the flow of heat are called conductors. Examples of good conductors are copper, steel, silver and iron.

Insulators: Materials that do not allow the flow of heat are called insulators. Examples of insulators are wood, paper, rubber, cork, glass, Bakelite and ceramic.

2. Convection

  • Convection is the transfer of heat by the movement of particles of a medium from one place to another. It takes place only in liquids and gases.
  • Examples of convection are wind currents, the lower floor of a building is cooler than the upper floor, and water is warmer at the surface of a swimming pool or lake. 
  • Due to convection, the atmosphere at the seashore is always pleasant.

3. Radiation

  • The process in which heat flows from one object to another either through a medium or vacuum is called radiation. 
  • The heat absorbed from the surroundings by a body increases its temperature. 
  • The sun warms the earth through radiation. 
  • A campfire, microwave oven and a light bulb are all examples of radiation.
  • The coastal areas exhibit a unique phenomenon where the land heats up faster than the water during the day.
  • As a result, the air over the land becomes hot, rises, and is replaced by cooler air from the sea, known as the sea breeze.
  • At night, the process reverses as the land cools down slower than the water, leading to the land breeze where cool air moves from land to sea.
  • Heat from the sun reaches us through radiation, a process that doesn't require a medium and can occur in the absence of one.

Question for Chapter Notes: Heat
Try yourself:
What is the process of heat transfer that occurs through the movement of particles in a medium?
View Solution

Kinds of Clothes We Wear in Summer and Winter

Depending upon the season, we need to choose the clothes we wear.

Summer Clothes

Heat Class 7 Notes Science Chapter 3

  • In hot weather, white or light coloured clothes suit better, because they reflect the heat and keep the body cool, whereas black clothes retain heat. 
  • During summer, clothes made of cotton are more comfortable. 
  • Cotton clothes allow the body heat to escape. 
  • People wear loose clothes to keep cool. 
  • Loose clothes allow air to circulate below the fabric. 
  • Thus, loose clothes are more suited during summer than tight-fitting clothes.
  • Hence, summer wear should be breathable, light-coloured and loose-fitting rather than dark and tight-fitting.

Woolen Clothes Keep Us Warm in Winter

Heat Class 7 Notes Science Chapter 3

  • In cold weather, warm and thick clothes should be worn. 
  • A wool base layer helps maintain body temperature at a comfortable level in either cool or warm conditions. 
  • So, woollen garments are suitable for cold weather.
  • As wool is a very good insulator and a poor conductor of heat, it can absorb moisture without becoming wet. 
  • Woollen clothes keep the body warm and protect from the cold winds.
  • Also, the air trapped between the woollen fibres prevents the flow of heat from the body to the cold surrounding. 
  • It also prevents the cold air from coming in contact with the body. 
  • Woollen garments have excellent shape retention because of the crimp in the fibres. 
  • The crimp creates many tiny air pockets that trap the warm air of the body or form a sort of insulation from the external air. 
  • This insulating barrier of air pockets protects from the cold winds. 
  • Sweaters, mufflers, cardigans and woollen garments give protection from winter. 
  • Thus, dark, thick, woollen garments are suitable during
The document Heat Class 7 Notes Science Chapter 3 is a part of the Class 7 Course Science Class 7.
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FAQs on Heat Class 7 Notes Science Chapter 3

1. What are laboratory thermometers used for?
Ans. Laboratory thermometers are used to measure temperature accurately in laboratory settings for scientific experiments and research purposes.
2. How do hot and cold temperatures affect the transfer of heat?
Ans. Hot temperatures increase the rate of heat transfer, while cold temperatures decrease it. Heat always flows from a hotter object to a colder one.
3. Why do we wear different kinds of clothes in summer and winter?
Ans. We wear lighter and breathable clothes in summer to help keep us cool by allowing air to flow through and absorb sweat. In winter, we wear heavier and insulating clothes to trap body heat and keep us warm.
4. What is the purpose of measuring temperature accurately?
Ans. Measuring temperature accurately is important for various reasons, including maintaining safety in different environments, ensuring the efficiency of machines and equipment, and understanding chemical reactions in laboratory settings.
5. How do we use hot and cold temperatures in everyday life?
Ans. We use hot temperatures for cooking, heating water, and warming our homes, while cold temperatures are used for refrigeration, preserving food, and creating a comfortable environment during hot weather.
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