Chapter Notes - Heat Class 7 Notes | EduRev

Science Class 7

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Class 7 : Chapter Notes - Heat Class 7 Notes | EduRev

The document Chapter Notes - Heat Class 7 Notes | EduRev is a part of the Class 7 Course Science Class 7.
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HEAT

TEMPERATURE AND ITS MEASUREMENT:

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 0°  of the scale at the freezing of water, and the 100°  at the boiling of water.

Themometer

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 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.

Types of Thermometers

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.

Clinical Thermometer:

Chapter Notes - Heat Class 7 Notes | EduRev

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 T from o note a reading, place the thermometer in the person's mouth. 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:

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

Laboratory Thermometers

Chapter Notes - Heat Class 7 Notes | EduRev

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:

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

Relation b/w Celsius and Fahrenheit

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

TRANSFER OF HEAT

Chapter Notes - Heat Class 7 Notes | EduRev

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: 

-Conduction 

-Convection

-Radiation.

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.

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 sea shore is always pleasant.

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 camp fire, microwave oven and a light bulb are all examples of radiation.

SEASONS AND CLOTHES

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

Summer Clothes

Chapter Notes - Heat Class 7 Notes | EduRev

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.

Winter Clothes

Chapter Notes - Heat Class 7 Notes | EduRev

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

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