CLASS VII

CHAPTER -4

HEAT

Activity -1

Clinical Thermometer

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Activity-2 Observe a Clinical thermometer

Work Sheet-2

1. What is the material used to make thermometer?

2. What is the liquid inside the thermometer?

3. What is the lower limit of temperature?

4. What is the upper limit of temperature?

5.What is the use of the Kink on a clinical thermometer?

6. A clinical thermometer should be used to measure the body

temperature only and not other hot objects. Give reason.

7.Don’t hold the thermometer bulb while reading it. Give

reason

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Precautions to be observed while reading a clinical thermometer

1. Thermometer should be washed before and after use, preferably with an antiseptic solution.
2. Ensure that before use the mercury level is below 35°C.
3. Read the thermometer keeping the level of mercury along the line of sight.
4. Handle the thermometer with care. If it hits against some hard object, it can break.
5. Don’t hold the thermometer by the bulb while reading it.

Work Sheet -3

1. Why should we jerk the clinical thermometer before measuring the temperature ?
2. Why does the mercury not fall or rise in a clinical thermometer when taken out of the mouth?
3. What is the use of the kink?
4. Why should we wash the thermometer before and after use?
5. Why is that the clinical thermometer has the range 35^oC to 42^oC.?
6. What is normal human body temperature in degree Celsius and Fahrenheit ?

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Digital Thermometer

There are 3 units for measuring temperature

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Degree Celsius ^oC

Degree Fahrenheit ^oF

Kelvin K

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Boiling point of water

100^oC = 212^oF

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Freezing point of water

0^oC = 32^oF

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Normal Human Body Temperature

37^oC = 98.6^oF

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Laboratory Thermometer

The precautions to be observed while reading a laboratory thermometer ��1.) The thermometer should be

kept upright not tilted.�2.) The bulb of the thermometer

should be surrounded from all

sides by the substance of

which the temperature is to

be measured.�3.) The bulb should not touch the

surface of the container.

4) Handle with care as it is made

of glass

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Maximum –Minimum Thermometer�(Six's thermometer)

Six's thermometer is a thermometer which can measure the maximum and minimum temperatures reached over a period of time, usually during a day.

Temperature

• When a body is heated its temperature increases.
• Temperature is the degree of hotness of a body.
• Temperature is measured by a thermometer.
• Degree celcius (^oC) and degree farenheit (^oF) are the commonly used units of temperature.

Why is mercury used in a thermometer

• It can be easily seen through glass.
• It is a liquid metal
• It expands uniformly over a wide range of temperature.
• It does not stick to the glass.

Modes of Transfer of Heat

• There are three mechanisms by which �thermal energy is transported.
•  1.  Conduction    
•  2.  Convection
•  3.   Radiation  

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Conduction

Conduction is the mode of transfer of heat taking place in solids by the vibration of molecules.

When the object is heated the molecules pick up heat and vibrate faster.They collide with other molecules and pass some heat to them.

When this continues transfer of heat by conduction takes place

Conditions of Conduction

Conduction takes place only if :

• The 2 bodies are in contact.
• They have different temperature levels

Conduction

A spoon in a cup of hot soup becomes warmer because the heat from the soup is conducted along the spoon.

Conduction

Bricks are poor conductors of heat.

They do not conduct the heat from outside to the inside of the house. This keeps the room cooler during hot summer

• All solid materials conduct heat, but some are better conductors than others.
• Metals are good conductors while porous materials are not.
• Styrofoam is an example of a poor conductor. It is a great insulator. Good conductors of electricity are usually good conductors of heat

• Material which pass heat are called conductors.metals are good conductors.
• Aluminium and copper are used to make cooking vessels bacause they are good conductors of heat.
• Materials which do not pass heat are called insulators.Wood,plastic,ceramic and paper are insulators.The handle of a pressure cooker is made of a type of platic because it is an insulator.

• The handle of tea kettle is covered with strips of cane or plastic because cane and plastic are insulators.They protect the hands of the user from heat.
• Pieces of cloth are used to remove a hot vessel from the stove because the cloth is an insulator and prevents burning of hand.

Convection

• Convection is the mode of transfer of heat by the streaming movement of molecules
• Convection takes place in liquids and gases

The greater the heat, the faster the molecules move.Warm molecules are lighter and they rise.Cold molecules sink.This creates a streaming motion of molecules in liquids and gases.

Convection

Heat leaves the coffee cup as the currents of steam and air rise. Convection is the transfer of heat energy in a gas or liquid by movement of currents

Room heating by Convection

• Even if the room heater is placed in one corner of the room the entire room becomes hot due to heat transfer by convection
• Heated air rises, cools, then falls.  Air near heater is replaced by cooler air, and the cycle repeats

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Convection

What if coils were at�the bottom?

Cool air would remain at the bottom. As cool air is heavier it remain at the bottom only and the upper part will remain warm.

Sea Breeze & Land Breeze

During the day the land becomes warm faster than the sea. The warm air above the land rises creating a low pressure over the land.The cool air from the sea blows towards the land causing sea breeze

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At night the land cools down faster than the sea.Therefore the sea remains warm.The air above the sea is also warm.The warm air above the sea rises.The cool air from the land blows towards the sea causing land breeze.

During the day sea breeze occurs

During the night land breeze occurs

On a warm day the warm air inside the room rises and stays near the ceiling of the room.

As soon as the ceiling fan of a warm room is switched on we get warm air because the warm air staying near the ceiling is circulated.

Moderate Temperatures Near Large Bodies of Water

Warm land is cooled during the day, while cooler land is warmed at night.

Radiation

Radiation is the mode of transfer of heat from a hot body to a cold body without an intervening medium

Radiation is Electromagnetic waves that directly transport heat energy through space(infra-red rays). Sunlight is a form of radiation that is radiated through space to our planet without the aid of fluids or solids

Radiation

• The sun transfers heat through 150 million kilometres of space. Because there are no solids touching the sun and our planet, conduction is not responsible for bringing heat to Earth. Since there are no fluids (like air and water) in space, convection is not responsible for transferring the heat. Thus, radiation brings heat to our planet.

Factors affecting the amount of heat absorbed by a body by radiation

• Distance from the Source of heat
• Colour of the body.

Tanker lorries carrying petrol and LPG are painted white because white colour reflects heat radiations.If they were painted with black paint the tanker would absorb heat and explode.

Umbrella with silver coating reflects heat radiations and the user feels less hot while walking in the sun.

People wear white or light coloured dresses in summer because they reflect light radiations ,so people won’t feel hot.

People wear black or dark coloured clothes in winter because they absorb heat and keep warm

Conceptual Questions and Topics

In warm places the outer walls of houses are painted white. White colour reflects heat radiations and keep the interior cool

 Absorption and Emission of  Radiation

• A dark coloured body absorbs heat quickly while a light coloured body does not. So a dark coloured body gets hot more easily than a light coloured one.

Energy out = Energy in � Emitted energy/Incident energy = Emissivity = e.  

Black Bodies

• Summer clothing:  white reflects radiant energy better than black. Until equilibrium is reached, white stripes on roads are at a lower  temperature than unpainted asphalt.�
• Wrap an ice-cube in black cloth and another in aluminum foil and place both in the sunshine.  What will happen?�

How much does the human body radiate?�

Body temperature = 37 C = 37 +273 = 310 K, �Estimate surface area A = 1.5 m2  

     e = 0.70��H = e sA T4�    = (0.70)(5.67 x 10-8)(1.5 m2)(310)4�    = 550 watts (5 light bulbs)�The sun provides about 1000 watts per square �meter at the Earth's surface.  30 % is reflected by human skin.  700 watts is absorbed per square meter.  

Why is the pupil of the  �eye black? 

Pipes in Solar Panels are Painted Black

Artificial Satellites

Highly reflecting metal foil keeps�inside temperature low.

The Working of a Thermos

A thermos flask minimise the loss of heat and keeps the contents hot.

The glass is a bad conductor of heat which prevents the loss of heat by conduction.

The space inside the glass walls is evacuated.This prevents convection.

The silvered surface of the vacuume bottle prevents loss of heat by radiation

Conceptual Questions

• 1. The sun goes down, snow falls on cement playground and on an asphalt road. Why does the snow on the road melt sooner than on the cement?��2.  Two different materials at the same temperature have different emissivities. Which one glows the brightest?
• 3.  Why are fireplace pokers made of iron and not copper?
• 4.  Some animals have hair which is composed of solid tubular strands, while  others have hollow, air-filled tubes.  Where would one more likely find  the latter animal:  In cold climates, or warm?�

Conceptual Questions

• 5. Steel reinforcement bars add stability to concrete walls.  Do they also enhance the insulating value of concrete?  ��6.  Two identical cups of cocoa are sitting on a table.  One has a metal spoon in it and one does not.  After five minutes, which cup is cooler?��7.  In Alaska, a lack of snow allowed the ground to freeze down to a depth of about one meter, causing buried water pipes to freeze and burst.  Why did a lack of snow lead to this situation?
• 8.  Several days after the end of a snowstorm, the roof of a house is completely covered with snow, another house's roof has no snow.  Which house is  probably better insulated?

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