Heat Class 7 Notes Science Chapter 3

Hot and Cold

Hot : An object or environment is described as "hot" when it has high temperature, often significantly higher than the surrounding environment or the human body's normal temperature (37ºC or 98.6ºF).

Cold: An object or environment is described as "cold" when it has low temperature, often significantly lower than the surrounding environment or the human body's normal temperature.

Heat is the transfer of energy from a hot body. The sense of touch can be used to understand the degree of hotness or coldness of something. But the sense of touch is not reliable and cannot be always used to say how much hot anything is. 
Moreover, using the sense of touch can be risky in case of something very hot. Thus, hotness of anything is measured in terms of Temperature in a reliable way. To measure temperature a device called Thermometer is used.
Scales to measure heat : There are three scales which are often used to measure the heat , or temperature: Degree Celsius, Fahrenheit and Kelvin.
Degree Celsius: Degree Celsius is written as °C and read as degree Celsius or simply Celsius. For example 20°C; it is read as twenty degree Celsius.

Fahrenheit: Fahrenheit is written as °F and read as degree Fahrenheit. For example 25°F; it is read as twenty-five degree Fahrenheit.

Kelvin: Kelvin is written as K. For example 100K; it is read as hundred Kelvin.

Thermometer:
Thermometer is a device which is used to measure temperature.

Thermometer is made of a long narrow glass tube; with a bulb at one end. The narrow tube appears as a continuous silver line; because it is filled with mercury. Mercury is a metal which is in liquid state at room temperature and it readily expands or contracts at the slightest change in temperature. Hence, mercury is used in thermometer. 

Types of Thermometer

Laboratory Thermometer: Laboratory thermometer is used to measure the temperature. The scale of temperature is graduated generally from –10°C to 110°C over the glass tube. Each division of temperature scale is further divided into 10 parts to read fraction of temperature.

Celsius and Fahrenheit Thermometer

Clinical Thermometer: Clinical thermometer is used to measure the body temperature. The scale of temperature is graduated from 35°C to 42°C and or from 94°F to 108°F. 
The temperature of human body always remains within this range and this is the range on the clinical thermometer.The average body temperature for a healthy adult is generally around 37ºC (98.6ºF). There is a kink make near the bulb of clinical thermometer which prevents the automatic fall of mercury level.
Digital Thermometer: In digital thermometer, reading of temperature is displayed digitally as in digital watches. This is safer because no mercury is used in this. It is important to note that mercury is a highly toxic substance.
Maximum-Minimum Thermometer: Maximum – minimum thermometer is used to measure the daily temperature to prepare weather reports.

Measuring Temperature and Reading a Thermometer

• Take a clinical thermometer and hold it horizontally with reading scale towards your eye.

• Do not hold the thermometer from the bulb.

• Rotate the thermometer slightly clockwise and anticlockwise. By doing this you will see a shiny thin silvery thread.

• The end of the silvery thread shows the reading of temperature. If mercury lining ends at 37, the reading is 37°C.

• Wash the bulb end of thermometer with an antiseptic solution.

• Give two or three jerks slightly. By doing this the mercury level would fall. When it falls to 35°C or below, put it below the tongue and wait for one minute.

• Take out the thermometer and read the temperature. Temperature would be near 37°C.

• The normal body temperature is 37°C. This can differ from person to person.

Use of Laboratory Thermometer

• Take some water in a beaker.

• Take a laboratory thermometer and immerse its bulb end in water; holding it vertically. Ensure to dip whole portion of bulb end. The bulb end should not touch the bottom or side of the beaker.

• Observe the movement of rise of mercury. When it becomes stable, take the reading of the thermometer.

• Repeat this with hot water and take the reading.

Difference between Clinical and Laboratory Thermometer

Transfer of Heat

Heat is transferred by conduction, convection and radiation.

Conduction

Transfer of heat from one particle to the adjacent particle is known as conduction of heat. In solids, heat is transferred by the process of conduction. In this process, the transfer of heat takes place through adjacent molecules.

Example: When one end of an iron rod is put over flame then part which is nearer to the flame is heated first and heat is gradually transferred to the other end of the rod. This happens because particles of iron rod which are nearer to the flame receive the heat and transfers this to the adjacent particles. 
Subsequently, the adjacent particles transfer the heat to the next adjacent particles. This process continues and heat reaches to the other end of the rod. Thus, heat transfer in solid takes place through conduction.
Conductor and Insulator:
Materials which allow heat to pass through it are called conductor or good conductor of heat, such as iron, copper, aluminium, etc. All metals are good conductors of heat. Since, mercury is a metal and found in liquid state at room temperature, that’s why it is used in thermometer.

Since metals are the good conductor of heat that’s why kitchen utensils are made of metals or alloys of metals (mixture of two or more elements with at least one being a metal).

Materials which do not allow heat to pass through them are called bad conductor or poor conductor of heat. They are also called insulators. Example: rubber, wood, plastic, etc. This is the cause that handles of frying pan or other kitchen utensils are made of plastic.

Convection

The transfer of heat because of movement of the molecules of the medium; via mass transfer; is called convection or convection of heat.

Water and air are bad conductors of heat. But they do become hot, in spite of being bad conductors. Heat transfer in fluids takes place through convection.
Convection in water: When water is heated in a pan, the particles of water which are near the source of heat; get heated first. Because of heating, they become light; and rise in water. The gap which is created because of rise of hot particles is filled cold particles of water from the surrounding area. 
Thus a cyclical movement of particles begins and ends up heating the whole water of the pan. The cyclical movement in fluids because of heating is called convection current.

Convection in air: Air gets heated because of convection; the way water gets heated. Air near the source of heat gets heated and rises above. This leaves a gap; which is filled by the colder air from the surrounding. The convection current thus starts in air which results in heating up of air.

When you place your palm above a flame you will feel the hotness of the flame. But when you will place your palm below the flame the area will be colder. This shows how the colder air from below moves up; due to convection current.

Land and Sea Breeze

Sea Breeze: In coastal areas, the breeze that moves from sea surface to the land is called sea breeze. This happens because, during daytime, land gets heated more quickly than water. As a result, warm air from land rises up; leaving a gap. 
To fill that gap, colder air from the ocean surface rushes towards the land. This phenomenon continues and a continuous flow of cold air keeps coming towards the land. This gives rise to the phenomenon which is called the sea breeze. Because of this, people living in coastal areas prefer to live in a sea facing house.

Land Breeze: In coastal areas, the breeze which moves from land towards the sea is called land breeze. At night, the land cools down more quickly than the ocean surface. This makes the air over the water surface warmer than air over the land surface. 
Warmer air over the water surface rises in the air and air from the land rushes towards the water surface to fill the gap. This phenomenon continues which creates a flow of air from land to the sea. This phenomenon is called land breeze.

Radiation

All hot bodies emit heat by the process of radiation.It can take place whether a medium is present or not. Sunlight comes to the earth because of radiation.

One can feel the heat of bonfire by standing around it. We get warmth from the room heater because of radiation.

Kinds of Clothes We Wear in Summer and Winter

When heat falls over an object some of the heat is absorbed by the body and some of the heat is reflected. The temperature of an object increases because of absorption of heat.

In conventional room heater you can see the reflector attached with it. 
The reflector of the room heater reflects the heat towards the person sitting or standing near the room heater.

Reflection is the cause that umbrella is used to protect from heat of the sun in summer. Dark color absorbs more heat while light color reflects most of the heat. That’s why wearing light colored clothes is preferred in summer, dark colored clothes are preferred in winter.

Dark clothes absorb more heat and keep one comfortable in winter. On the other hand, light clothes reflect most of the heat and keep one comfortable in summer.

Now-a-days many kitchen utensils come in black color, since utensils of black color absorb more heat and thus cooking becomes faster.

Woolen Clothes keep us Warm in Winter

Woolen clothes are used in winter season. Wool is a poor conductor of heat. In addition to it; air gets trapped in woolen fiber to further increase the poor conductivity of wool. This prevents the radiation of heat of our body to the surrounding and prevents the cold from surrounding to affect our body. Thus, wearing woolen cloth makes one comfortable in winter season.

Important Terms:

• Temperature: The reliable measure of hotness or coldness of anything.
• Thermometer: A device to measure the temperature.
• Degree Celsius: Unit of measurement of temperature.
• Clinical Thermometer: A device to measure the temperature of human body.
• Laboratory Thermometer: Used to measure the temperature in laboratory.
• Maximum-Minimum Thermometer: Use to measure the temperature of climate.
• Conduction: Mode of transfer of heat in solids.
• Convection: Mode of transfer of heat in liquids and air.
• Radiation: Mode of transfer of heat without medium.
• Conductor: Materials that allow the flow of heat through them.
• Insulator: Materials that do not allow the flow of heat through them.
• Land breeze: Breeze that flows from land surface to sea surface in summer nights.
• Sea breeze: Breeze that flows from sea surface to water surface in summer days.