HEAT

 CHAPTER 4     HEAT CLASS NOTE

·    Woollen clothes are obtained from animal fibers. Woollen clothes are generally dark in colour and are worn in the winter season as they protect    us from the cold environment.

· Cotton clothes are obtained from the plant fibres. The  cotton clothes  are generally light in colour and are worn in summer season so that we can feel cool.

Hot and Cold

·   We can understand if an object is hot or cold by the sense of touch. However, it can trick us sometimes. Therefore, we use a thermometer.

·  The temperature of an  object:  It  is  the  degree  of  hotness  or  coldness of an object.

·   Thermometer: It is a device that can be used to find out how hot an object is. In other words, we use a thermometer to measure the temperature of an object.

Measuring the temperature of an object using a thermometer Clinical thermometer

·  It is a device that is used to measure the body temperature of a person.

·  It is made up of a glass tube of uniform thickness.

·  The glass tube contains a bulb at one end which is filled with Mercury.

· The Mercury level in the thermometer rises up in the thread-like  portion of the

thermometer which therefore indicates the temperature of the body.

· The level of the Mercury can be measured by reading the scale given on the

thermometer.

· The scale of the thermometer records the temperature in degree Celsius, generally, 35o C to 45o C, which is the range of human body temperature.

· On an average, the temperature of the human body is around 37o C.

·The clinical thermometer has a small sharp curve (kink) present near the bulb. This

prevents the Mercury level from falling down on its own in the thermometer.

How to use the clinical thermometer?

· Firstly, wash the thermometer with an antiseptic solution.

· Before taking the temperature the thermometer is given a few jerks to bring down the

level of Mercury below 35o C.

·  Then the thermometer is placed beneath the tongue for about a minute.

·  Then you can take it out and observe the temperature reading on the thermometer.

What precautions should be taken while using a clinical thermometer?

·  Wash the thermometer before and after using it.

· Make sure the temperature of the thermometer is below 35o C before taking the

temperature.

·  Keep the thermometer straight in order to see the Mercury level precisely.

·   It should always be held with care or it can break down. You should   not touch the

bulb of the thermometer at all.

Laboratory thermometer

The laboratory thermometer is used to find out the temperature of the other objects such as water rather than human body temperature. It can measure    the temperature from -10o C to 110o C.

What precautions should be taken when using a laboratory thermometer?

Using Laboratory thermometer

·   You should always follow the same precautions as that of the clinical thermometer.

·  You should always hold the laboratory thermometer in a straight upright position without titling it.

·   The bulb of the thermometer should never touch the surface of the container in which the substance is kept.

·  However, the bulb of the thermometer should be completely immersed in the substance so that it covers the bulb from all the sides.

Other types of thermometers

Minimum-maximum thermometer:

It is a thermometer used to measure the minimum and maximum temperature of the day by weather forecasters.

 

Digital thermometer: It is sometimes difficult to handle a Mercury filled thermometer especially when it breaks and the mercury falls out. However, nowadays digital thermometers are available to use. This type of thermometer does not contain Mercury. It directly displays the correct temperature on a display screen.

The transfer of heat

· The flow of heat always takes place from and hot object to a cold object.

A.  Conduction: The process of flow of heat from a hot object to a cold object is called

Conduction. Some objects can conduct heat while others cannot.

·  Conductors: The objects that can allow the heat to flow through them are called Conductors. For Example, metals such as copper and aluminum.

·  Insulators: The objects that do not allow the flow of heat from them are called Insulators. For Example, Wood and Plastic.

    

B. Convection: 

        

· The  transfer   of   heat   in   liquids   and   gases   is   called Convection. The molecules of the liquid or gases that are near the source of the heat get heated first. 

· They become lighter due to the heat and move upwards. 

·   The colder particles being heavier take the place and this process continue until the whole liquid or the gas gets heated. 

         That is why the area above the flame of a candle always feels  hot but the area on the sides of the candle does not.

               Figure 9 Convection

      

C. Radiation: It is a process of transfer of heat in the form of waves. In radiation heat is transferring from one object to another without any medium. 

       For  Example,  the  sun's  heat  reaches  the  Earth’s  surface  through radiation. Every hot object radiates some heat into the environment. Hence, many times an object gets heated just by being near to a hot object.

               Figure 10 Radiation

 

 Sea Breeze and Land Breeze

Sea Breeze

·   The wind blowing from the sea towards the land is called Sea Breeze.

· During the daytime, the land in the coastal area gets heated due to the sun's radiation.

· The sea also gets heated, however it takes more time to get heated up than the land.

·  Hence the air above the land gets heated faster than the air above the sea.

· The hot air from the land rises above as it is lighter  and the cool air from the sea being heavier takes its place. This results in the sea breeze.

Land Breeze

· The wind blowing from the land towards the sea is called the land breeze.

·   During the night time, the land in the coastal areas gets cool down faster than the sea. The air above the sea is hotter than the air above the  land.

·    Therefore the air above the sea rises and the air from the land being cool flows towards the sea. This results in the land breeze

        Why do we wear light coloured clothes in summer?

The light coloured clothes reflect the heat back into the environment and keep us cool during the summer time.

Why do we wear dark coloured clothes in winters?

The dark colour clothes absorb the heat from the environment and keep us warm.

How do woollen clothes keep us warm?

· The woollen clothes are a bad conductor of heat.

·  Therefore they do not allow the heat from the body to move out in the environment.

·   As a result, the air present between the woollen clothes and our body becomes hot and

this makes us feel warm.

·Similarly, two thin blankets provide more heat than one thick blanket because air can be trapped between the two thin blankets.

important question-answer Chapter-4 heat

    Class - VII   Chapter 4 Heat

Q1. Fill in the blanks-

i.      The hotness of an object is determined by its temperature.

ii.      Temperature is measured in degree    Celsius.

iii.      A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of conduction.

iv.      Land breeze blows during night.

v.      Sea breeze blows during day.

vi.      Dark coloured clothes are preferred during winter.

vii.      Poor conductors are known as insulators.

Q2. True/False

i.      Temperature of boiling water cannot be measured by a clinical thermometer.

Ans- True

ii.      Medium is required for transfer of heat by the process of radiation.

 Ans- False

iii.      Clothes of dark colours absorb heat better than clothes of light colours. 

Ans- True

iv.      Light coloured clothes are preferred during summer. 

Ans- True

v.      We should hold the thermometer by the bulb while holding it. 

Ans- False

vi.      The bulb of laboratory thermometer should not touch the bottom or the sides of the container. 

Ans- True

Q3. In which direction does the smoke go? 

Ans. Smoke will go upward.

Q4. What do you mean by temperature?

Ans. A reliable measure of the hotness of an object is its temperature.

Q5. Which device is used to measure temperature? 

Ans. Thermometer is used to measure temperature.

Q6. What is that silver stuff in a thermometer? 

Ans. Silver stuff in a thermometer is mercury.

Q7. What is the unit of temperature as adopted by India? 

Ans. India has adopted the celsius scale.

Q8. What is the normal temperature of a human being? 

  Ans. The normal temperature of human body is 37°C.

 Q9. What is the range of a laboratory thermometer?

Ans. The range of a laboratory thermometer is generally from –10°C to 110°C.

 Q10. What is the use of the kink in clinical thermometer? Ans. It prevents mercury level from falling on its own.

Q11. How does heat flow from one object to another? 

Ans. Heat flows from a hotter object to a colder object.

Q12. Does transfer of heat by radiation require any medium?

Ans. The transfer of heat by radiation does not require any medium.

Q13. Do all hot bodies radiate heat? 

Ans. Yes, all hot bodies radiate heat.

Q14. How does heat transfer in liquids and gases?

Ans. In liquids and gases the heat is transferred by convection.

Q15. Will heat transfer if the temperature of two objects is the same?

Ans. Heat will not be transferred if the temperature of two objects is the same.

Q16. What should be the level of mercury in the clinical thermometer before use?

Ans. Mercury level should be below 35°C.

Q17. How does heat transfer in solids?

Ans. In    solids,   generally,    the    heat    is  transferred  by  the process of conduction.

Q18. What is the best way to clean a thermometer?

Ans. Thermometer should be washed before and after use, preferably with an antiseptic solution.

Q19. What is Conduction?

Ans. The process by which heat is transferred from the hotter end to the colder end of an object is known as conduction.

Q20. What are insulators of heat?Ans. The materials which do not allow heat to pass through them easily  are insulators of heat. Example: plastic and wood.

Q21. What are conductors of heat?

Ans. The materials which allow heat to pass through them easily are conductors of heat. Example: aluminium, iron and copper.

Q22. What is clinical thermometer?

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

Q23. Give two examples each of conductors and insulators of heat. 

Ans. Conductors – aluminium and copper

        Insulators - water and air

Q24. How does the heat travel in air?

Ans. The air near the heat source gets hot and rises. The air from the sides comes in to take its place. In this way the air gets heated.

Q25. What do you understand by heat?

Ans. Heat is a form of energy that can be transferred from one object to another or even created at the expense of the loss of other forms of energy.

Q26. What is the use of maximum-minimum thermometer?

Ans. The maximum and minimum temperatures of the previous day, reported in weather reports, are measured by maximum-minimum thermometer.

Q27. One litre of water at 30°C is mixed with one litre of water at 50°C. The temperature of the mixture will be

(a) 80°C                

(b) more than 50°C but less than 80°C

(c) 20°C                  

(d) between 30°C and 50°C. Ans. (d) between 30°C and 50°C.

Q28. Why does the mercury not fall or rise in a clinical thermometer when taken out of the mouth?

Ans. Clinical thermometer has kink near the bulb prevents mercury level from falling on its own.

Q29. Why clinical thermometer cannot be used to measure high temperatures?

Ans. Clinical thermometer  cannot be  used  to   measure high temperatures because the range of this thermometer is from 35°C to 42°C only.

Q30. Explain land breeze.

Ans. At night, 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.

 Q31. What is the concern associated with the use of mercury thermometer?

Ans. 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.

Q32. Explain the construction of clinical thermometer.

Ans. A clinical thermometer consists of a long, narrow, uniform glass tube. It has a bulb at one end. This bulb contains mercury. Outside the bulb, a small shining thread of mercury can be seen.

Q33. In places of hot climate it is advised that the outer walls of houses be painted white. Explain.

Ans. In places of hot climate it is advised that the outer walls of houses be painted white because light color reflects most of the heat that falls on them and thus keep the house cool.

Q34. Is it possible to construct buildings that are not affected much by heat and cold outside?

Ans. This can be done by constructing outer walls of buildings so that they have trapped layers of air. One way of doing this is to use hollow bricks, which are available these days.

Q35. What do you mean by maximum-minimum thermometer?

Ans. Different types of thermometers are used for different purposes. The maximum and minimum temperatures of the previous day, reported in weather reports, are measured by a thermometer called the maximum - minimum thermometer.

Q36. Why clinical thermometers range from 35°C to 42°C?

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

Q37. Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.

Ans. Wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing because air get trapped in between the layers of clothing and being a bad conductor of heat, prevents the flow of heat from our body to the cold surroundings.

Q38. When we come out in the sun, we feel warm. How does the heat from the sun reach us?

Ans. It cannot reach us by conduction or convection as there is no medium such as air in most part of the space between the earth and the sun. From the sun the heat comes to us by another process known as radiation. The transfer of heat by radiation does not require any medium.

Q39. In summer we prefer light-coloured clothes and in winter we usually wear dark-coloured clothes. Why is it so?

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

Q40. What is sea breeze?

Ans. 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.

Q41. Two thin blankets joined together are usually warmer than one thick blanket. Give reason.

Ans. Two thin blankets joined together are usually warmer than one thick blanket because air gets trapped between the layers of blankets and being a bad conductor of heat, prevents the flow of heat from our body to the cold surroundings.

Q42. How water gets heated when kept on flame?

Ans. 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.

Q43. An iron ball at 40°C is dropped in a mug containing water at 40°C.

The heat will

(a)  flow from iron ball to water.

(b)  not flow from iron ball to water or from water to iron ball.

(c)  flow from water to iron ball.

(d)  increase the temperature of both.

Ans. (b) not flow from iron ball to water or from water to iron ball.

Q44. A wooden spoon is dipped in a cup of ice cream. Its other end

(a)  becomes cold by the process of conduction.

(b)  becomes cold by the process of convection.

(c)  becomes cold by the process of radiation.

(d)  does not become cold.

 Ans. (d) does not become cold.

Q45. Stainless steel pans are usually provided with copper bottoms. The reason for this could be that

(a)  copper bottom makes the pan more durable.

(b)  such pans appear colourful.

(c)  copper is a better conductor of heat than the stainless steel.

(d)  copper is easier to clean than the stainless steel.

Ans. (c) copper is a better conductor of heat than the stainless steel.

Q46. State the similarities and differences between the laboratory thermometer and the clinical thermometer.

Ans. Similarities

i.      Both thermometers consist of a long, narrow, uniform glass tube.

ii.      Both have a bulb at one end. This bulb contains mercury.

iii.      Both have Celsius scale.

Differences

i.      A clinical thermometer reads temperature from 35°C to 42°C whereas the range of a laboratory thermometer is generally from –10°C to 110°C.

ii.      A clinical thermometer has a kink in it whereas there is no kink in laboratory thermometer.

 Q47. What are the precautions that need to be observed while reading a clinical thermometer?

Ans. Precautions to be observed while reading a clinical thermometer

i.      Thermometer should be washed before and after use, preferably with an antiseptic solution.

ii.      Ensure that before use the mercury level is below 35°C.

iii.      Read the thermometer keeping the level of mercury along the line of sight.

iv.      Handle the thermometer with care. If it hits against some hard object, it can break.

v.      Don’t hold the thermometer by the bulb while reading it. 

Q48. State the precaution to be observed while using a laboratory thermometer.

Ans. Precaution to be observed while using a laboratory thermometer

i.      Handle the thermometer with care. If it hits against some hard object, it can break.

ii.      Thermometer should be kept upright not tilted.

iii.      Bulb should be surrounded from all sides by the substance of which the temperature is to be measured. The bulb should not touch the surface of the container.

iv.      Do not move the thermometer while measuring the temperature of the substance.

v.      Read the temperature of the object when the thermometer is in the substance.

Q49. Why can't we use a laboratory thermometer to measure human body temperature?

Ans. We can’t use a laboratory thermometer to measure human body temperature because the range of a laboratory thermometer is high generally from –10°C to 110°C while the normal body temperature of human body is 37°C. Moreover, a laboratory thermometer does not have a kink, so the mercury falls on its own upon removing it from the body orifice. Thus, it does not give accurate temperature of the human body.