CBSE Class 7 Science Heat Electric Current And Its Effects Worksheet

HEAT

Do you know, why?

1. The thick glass tumblers break when hot liquid is poured into them

Ans: If boiling hot water is poured in a thick glass tumbler, it cracks because glass is a bad conductor of heat. Thus, the inner surface of tumbler expands more than the outer surface. Due to this uneven expansion, the glass cracks.

2. Gaps are left between rail joints to allow for expansion

Ans: A small gap is kept at joints to allow for the expansion of tracks. If no gap is left for expansion or contraction, they will bend in summer. Thus, result in derailment of trains.

3. Slabs of ice are covered with sawdust or gunny bags

Ans: The saw dust or gunny bags contain large amount of trapped air which acts as an insulator. So, it doesn‟t allow the heat from outside to reach ice.

4. If we walk barefoot on a stone floor,it appears to be very cold but if walk on a carpet in the same room it feels warmer.

Ans: It is so because stone floor being a good conductor of heat, conducts away heat quickly from our feet. Our feet lose heat and make us feel cold. On the other hand, the carpet being a bad conductor of heat does not allow the heat of our feet to escape and hence feels warmer.

5. Solar cookers and solar water heaters are painted black from inside. Ans: This is because black surfaces are good absorber of heat.

6. Convection currents are produced inside the earth‟s crust.

Ans: Molten rocks close to the earth‟s core are hottest. It rises towards the crust. Molten rocks closer to the earth‟s crust are cooler. It is heavier and sinks. This exchange of material between core and crust create convection currents which move huge pieces of earth‟s crust known as tectonic plates. The tectonic plates move close together or farther, forming mountains and trenches.

I. Give one word for the following.

1) The degree of hotness or coldness of a body.

2) Metal used in thermometers.

3) Instrument used to measure temperature of a body.

4) Materials that allow heat to pass through them easily.

5) Materials that do not allow heat to pass through them.

I. Fill in the blanks.

1) Air is a ________ conductor of heat.

2) Solids ________ on cooling.

3) Heat is a form of energy that travels from a _______ object to a _______ object.

4) A spoon kept in a cup of hot tea gets heated by _________

5) The normal body temperature is _______ oC or _____o F.

6) ________ is the best conductor of heat.

7) Ice melts at ______ oC or ______ oF.

8) Water boils at _______oC or ______oF.

9) Black bodies ________ and emit ________ heat the best.

10) The another name of clinical thermometer is __________ thermometer.

II. State true or false.

1) Heat cannot travel in vacuum.

2) Gases expand the most while solids expand the least on heating.

3) Room heaters warm the rooms by conduction.

4) Blankets keep us warm at night in winters.

5) The wooden handle of a pressure cooker prevents conduction of heat to the hand.

III. Match the following.

              A                              B

1) Conduction                 a) day

2) Radiation                    b) solids

3) Heat                           c) night

4) Sea Breeze                d) energy

5) Land Breeze              e) vacuum

IV. Define.

i) Energy

ii) Temperature

iii) Conduction

iv) Convection

v) Radiation

V. Give reasons

i) Evaporation of sweat causes cooling.

ii) Ventilators are provided near the ceiling of a room.

iii) Clinical thermometers have a kink in the capillary tube

VI. Draw the diagram of the following:

i) Sea Breeze

ii) Land Breeze

1.What is the range of laboratory thermometer?

2.The capillary tube of a clinical thermometer has kink-
(A) To increase the expansion of mercury.
(B) So that the level of mercury does not fall as soon as the thermometer is taken out of the mouth.
(C) To use less mercury.
(D) To help us see it better.

3.Radiation
(A) Does not require a material medium.
(B) Is the process of the transfer of heat in liquids.
(C) Is the process of the transfer of heat in which heat travels in one direction.
(D) Occurs in solids.

4.Human body temperature is normally-
(A) 32°F.
(B) 0°F.
(C) 100.4°F.
(D) 98.6°F

5.The range of a clinical thermometer is-
(A) 0-100°C.
(B) 32-212°F.
(C) 0-273°C.
(D) 35-42°C.

6.The measure of degree of hotness or coldness of body is called-
(A) Heat energy.
(B) Celsius.
(C) Kelvin.
(D) Temperature.

7.The capacity to do work is called-
(A) Force.
(B) Movement.
(C) Energy.
(D) Momentum.

8.How can we measure the temperature of our body?

9.What is the temperature of a normal human body?

10.What is the use of the kink in clinical thermometer?

11.What is the range of the clinical thermometer?

12.Name a thermometer which does not use mercury as one of it's component?

13.Name the process by which heat is transferred from hotter object to the colder object.

14.What is the process by which heat of sun is able to reach us?

15.What is temperature?

16.Can we measure the temperature of 45°C from a clinical thermometer?
(A) No.
(B) Yes.
(C) Sometimes.
(D) From some clinical thermometers.

17.Heat of the Sun reaches to us by-
(A) Reflection.
(B) Condensation.
(C) Radiation.
(D) Sunlight.

18.Define conduction.

19.How does heat flow?

20.What will happen to a clinical thermometer if we keep it in the Sun or near a flame?
(A) It will measure the temperature.
(B) It will break.
(C) It will melt.
(D) Nothing will happen.

21.What is temperature ?

22.Name the mode of heat transfer in which heat energy flows from one body to another only if they are in contact with each other ?

23.Name the mode of heat transfer in which heat is transferred even in the absence of a medium ?

24.Temperature is the degree of or …………………..of a body.

25.The hotness of a body is determined by its…………………

26.A clinical thermometer can measure a temperature between ... °C and……………°C.

27.No medium is required for transfer of heat by the process of

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

29. is the process of transmission of heat in which heat travels directly from one place to another without the agency of any intervening medium.

30.The cold air blowing from the sea towards the land during the day is called ………………

31.The cold air blowing from the land towards the sea during night is called ………………..

32.Clothes of light colours are better………………….of heat than clothes of dark colour.

33.What is the value of the normal body temperature ?

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

35.Explain why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of cloth?

36.How does the heat travel in air? In which direction does the smoke go?

37.What are the 2 conditions for conduction to take place?

38.What is temperature? Name the device used to measure it.

39.Name the two common scales in which temperatures are usually measured ?

40.Why a clinical thermometer should not be washed in hot water ?

41.Why in places of hot climate it is advised that the outer walls of houses be painted white.

42.Why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.

43.Why do we wear woollen clothes in winter ?

44.What kind of clothes will keep us comfortable in summer ? Why ?

45.Describe the construction of a clinical thermometer

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46.What are conductors and insulators of heat? Give examples.

47.Write any three similarities between laboratory thermometer and the clinical thermometer.

48.What do you understand by clinical thermometer and laboratory thermometer?

49.differentiate between heat and temperature

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50.Suggest an activity to explain the term convection. >

51.Explain the process of conduction with the help of activity and neat labeled diagram

52.How will you read a thermometer?

53.What are the precautions to be take while reading a clinical thermometer?

54.Write short notes on :-
(a) Sea breeze .
(b) Land breeze.

55.What precautions will you observe while using a clinical thermometer ?

56.write the differences between a clinical thermometer and a laboratory thermometer ?

57.write the differences between a clinical thermometer and a laboratory thermometer ?

Q.1 Multiple choice Question

1. The clinical thermometer’s bulb consists of

a. lead

b. mercury

c. Zinc

d. None of these

2. Which of the following materials are good conductors of heat?

a. Iron

b. Copper

c. Silver

d. None of these

3. Water is a _____________ conductor of heat.

a. good

b. bad

c. both

d. None of these.

4. The process of transfer of heat in which the heated molecules of a liquid themselves carry heat from the hot to the cold region?

a. Conduction

b. Convection

c. Radiation

d. None of these

5. Sea breeze and land breezes are actually

a. Currents

b. Water currents

c. waves

d. Convection currents

Q.2 Answer the following questions

a. Why are cloudy nights warmer than clear nights?

b .How do the feathers of a bird protect it from cold ?

Q1. Fill ups:

a) ____________ is the degree of relative hotness or coldness.

b) Cooking vessels should be made up of __________ conductors.

c) Heat is a form of ___________.

d) Mercury level falls easily in a _____________thermometer.

e) S.I unit of heat is __________________.

f) Range of clinical thermometer is from __________̊C to ________̊C.

g) A kink is not provided in a ___________ thermometer.

Q2. Choose the correct option:

1) Conduction takes place in:
a) Solids only
b) Liquids only
c) Gases only
d) in solids, liquids & Gases

2) In which method of transfer of heat do the molecules travel from hot to cold portion?
a) Conduction
b) Convection
c) Radiation
d) All of the above

3) A thermos flask prevent loss or gain of heat by
a) Conduction
b) Convection
c) Radiation
d) All of the above

4) Convection occur only in
a) Solids
b) Liquids
c) Gases
d) Liquids & Gases

5) Ventilation in the room is due to:
a) Conduction
b) Convection
c) Radiation
d) All of the above

6) The method of heat transfer in a steel rod is by:
a) Conduction
b) Convection
c) Radiation
d) All of the above

Q3. Why is underside of cooking vessels painted black?

Q4. Why do we feel cold when our body sweats?

Q5. Why only mercury is used in thermometer?

Q6. Explain three modes of transmission of heat i.e. conduction, convection & radiation, with the help of their applications.

Q7. Name the following:

a) Fastest mode of transfer of heat._____________

b) Mode of transfer of heat where the medium does not get heated.___________

c) Mode of transfer of heat where no medium is required.______________

d) The metal used in the bulb of thermometer.___________________

MODULE - 1

I. Fill in the blanks:

Question (i). The liquid used in thermometer usually is____________ .
Answer: Mercury
In simple words: Mercury is a shiny liquid metal that expands uniformly with temperature.

Exam Tip: Remember mercury is used because it does not stick to glass walls and remains liquid over a wide range of temperatures.

Question (ii). Normal body temperature of a human is _____ C &______F.
Answer: \(37^\circ\text{C}\) & \(98.6^\circ\text{F}\)
In simple words: Normal human body temperature is fixed at 37 degrees Celsius or 98.6 degrees Fahrenheit.

Exam Tip: Memorize both values as questions frequently test conversions between these two standard scales.

Question (iii). SI unit of heat is ________.
Answer: Joule (J)
In simple words: Heat is a form of energy, so its standard International System unit is the Joule.

Exam Tip: Do not confuse heat with temperature; temperature is measured in Kelvin or Celsius, whereas heat is energy measured in Joules.

Question (iv). A reliable measure of the hotness of an object is its ____________.
Answer: Temperature
In simple words: Touch can be deceiving, so we use temperature as a dependable measure of heat intensity.

Exam Tip: Human touch is subjective and cannot reliably measure exact degrees of hotness or coldness.

II. Correct the following statements:

Question (i). Clinical thermometer can measure temperature of all objects.
Answer: Clinical thermometer can only measure human body temperature (typically between \(35^\circ\text{C}\) and \(42^\circ\text{C}\)).
In simple words: Clinical thermometers have a restricted range meant specifically for human bodies, not for boiling water or weather readings.

Exam Tip: Using a clinical thermometer for high-temperature objects will cause the bulb to break due to excessive expansion.

Question (ii). Clinical thermometer has 48°C as its upper limit.
Answer: Clinical thermometer has \(42^\circ\text{C}\) as its upper limit.
In simple words: The scale of a clinical thermometer ranges only from \(35^\circ\text{C}\) to \(42^\circ\text{C}\).

Exam Tip: Human body temperature never exceeds \(42^\circ\text{C}\), hence the scale stops there.

Question (iii). Temperature is measured in calories.
Answer: Temperature is measured in degree Celsius, Fahrenheit, or Kelvin (heat energy is measured in calories or Joules).
In simple words: Calories measure heat energy, while degrees measure the degree of hotness (temperature).

Exam Tip: Clear distinction between heat (energy) and temperature (degree of hotness) is crucial for scoring.

III. Name the various parts of the clinical thermometer drawn in the figure and answer the given question.

Question III. Name the various parts of the clinical thermometer drawn in the figure and answer the given question.
Answer: The main parts shown in a clinical thermometer are: (1) Bulb containing mercury, (2) Glass stem, (3) Capillary tube, (4) Kink or constriction just above the bulb.
In simple words: A clinical thermometer consists of a glass tube with a bulb at one end containing mercury and a narrow constriction called a kink.

Exam Tip: Label the kink clearly in diagrams as it is the most unique and important functional part of a clinical thermometer.

MODULE - 2

I. Fill in the blanks.

Question (i). In nuclear power plant, nuclear energy is first converted into _________energy and then into ___________ energy.
Answer: Heat (thermal), Electrical
In simple words: Nuclear reactions generate heat, which produces steam to turn turbines and generate electricity.

Exam Tip: Trace the energy chain step-by-step: Nuclear - Heat - Mechanical - Electrical.

Question (ii). The handle of a kettle is made up of a ______________ conductor of heat.
Answer: Poor (or bad / insulating)
In simple words: Handles are made of insulators so heat from the kettle does not burn our hands.

Exam Tip: Common materials used for handles are ebonite, wood, or thick plastic.

Question (iii). Ice is usually covered with sawdust to prevent heat gain because sawdust is ______________ conductor of heat.
Answer: A poor (bad)
In simple words: Sawdust traps air, making it a bad conductor that stops outside heat from melting the ice.

Question (iv). Heat cannot flow from ______________ body to ______________ body.
Answer: Colder, hotter
In simple words: Heat naturally flows only from a region of higher temperature to a region of lower temperature.

Exam Tip: Remember the golden rule: Heat flows spontaneously from hot to cold objects only.

Question (v). Conduction takes place in _____________ only.
Answer: Solids
In simple words: Conduction requires direct particle-to-particle contact, which is most effective in closely packed solids.

Exam Tip: Liquids and gases generally transfer heat through convection, not conduction.

Question (vi). Fluffed up cotton & woollens are poor conductors of heat because of ________ present in them.
Answer: Trapped air
In simple words: Air spaces trapped between fibers prevent body heat from escaping into the cold surroundings.

Exam Tip: Air is a poor conductor of heat; trapping it is the secret behind warm winter clothing.

II. Correct the following statements

Question (i). The upper limit of laboratory thermometer is 60°C.
Answer: The upper limit of a typical laboratory thermometer is \(110^\circ\text{C}\).
In simple words: Laboratory thermometers measure higher temperatures up to 110 degrees Celsius or more.

Exam Tip: Laboratory thermometers usually cover a wide range from \(-10^\circ\text{C}\) to \(110^\circ\text{C}\).

Question (ii). Laboratory thermometer has kink.
Answer: Laboratory thermometer does not have a kink (clinical thermometers have a kink).
In simple words: Laboratory thermometers lack a kink so that mercury level can fall freely on its own when removed from a source.

Exam Tip: The absence of a kink means the reading drops immediately when taken out of the substance.

Question (iii). To read the temperature of an object, we take out the laboratory thermometer from it.
Answer: To read the temperature of an object, we must observe the laboratory thermometer while its bulb is still immersed in the substance.
In simple words: Because it has no kink, the reading changes as soon as it is removed, so you must read it while inside.

Question (iv). In conduction, heat is transferred from the colder end to the hotter end.
Answer: In conduction, heat is transferred from the hotter end to the colder end.
In simple words: Heat always travels downhill from hot to cold.

Exam Tip: Never write that heat flows from cold to hot in natural conduction.

III. Which energy transformation takes place in following cases:-

Question (i). In thermal power station.
Answer: Chemical energy (of coal/fuel) to Heat energy to Mechanical energy to Electrical energy.
In simple words: Burning fuel creates heat to generate steam, which rotates turbines to produce electricity.

Exam Tip: List the stages clearly in order: Chemical - Heat - Mechanical - Electrical.

Question (ii). In electric bulb.
Answer: Electrical energy to Light energy and Heat energy.
In simple words: Electricity flowing through the filament causes it to glow and heat up.

Exam Tip: Mention both light and heat since bulbs also get very hot during operation.

Question (iii). In steam engine.
Answer: Heat energy to Mechanical energy.
In simple words: Steam pressure pushes pistons to do mechanical work and move the engine.

Question (iv). In rubbing of palm together.
Answer: Mechanical energy to Heat energy.
In simple words: Friction from rubbing hands converts physical movement into warmth.

Exam Tip: Friction always converts mechanical energy into thermal energy.

IV. Name the process involved in following activities -

Question (i). Heating of pan and then boiling of water in it.
Answer: Conduction (heating of pan) and Convection (boiling of water).
In simple words: The pan heats by direct particle contact, while water circulates and boils through convection currents.

Question (ii). Sea breeze and land breeze.
Answer: Convection
In simple words: Movement of air due to uneven heating of land and sea creates convection currents.

Exam Tip: Breezes are natural convection currents in the atmosphere.

Question (iii). Circulation of air through ventilators.
Answer: Convection
In simple words: Warm air inside the room rises and escapes through ventilators, drawing cooler fresh air in through windows.

Exam Tip: Hot air is lighter and rises, which is the foundational principle of convection.

Question (iv). Level of mercury rises when we keep clinical thermometer under our tongue.
Answer: Conduction
In simple words: Heat transfers directly by contact from the body to the mercury bulb through conduction.

Exam Tip: Direct physical contact between the tongue and the glass bulb allows heat transfer via conduction.

V. Answer the following questions -

Question (i). At what temperature, Celsius & Fahrenheit scale shows equal values?
Answer: Let the temperature be \(x\). Using the conversion formula:
\(\frac{C}{5} = \frac{F - 32}{9}\)
Substitute \(C = F = x\):
\(\frac{x}{5} = \frac{x - 32}{9}\)
\(9x = 5(x - 32)\)
\(9x = 5x - 160\)
\(4x = -160\)
\(x = -40^\circ\)
Thus, both scales show equal values at \(-40^\circ\).
In simple words: At minus 40 degrees, both Celsius and Fahrenheit thermometers read the exact same number.

Exam Tip: Memorize the answer \(-40^\circ\) as it is a frequent numerical question in examinations.

Question (ii). What are the values of freezing point & boiling point of water on Celsius scale?
Answer: Freezing point of water = \(0^\circ\text{C}\); Boiling point of water = \(100^\circ\text{C}\).
In simple words: Water freezes at 0 degrees Celsius and boils at 100 degrees Celsius under standard pressure.

Exam Tip: These two fixed reference points define the Celsius temperature scale.

Question (iii). Convert both the values in Fahrenheit.
Answer: For freezing point (\(0^\circ\text{C}\)):
\(F = \frac{9}{5}C + 32 = \frac{9}{5}(0) + 32 = 32^\circ\text{F}\)
For boiling point (\(100^\circ\text{C}\)):
\(F = \frac{9}{5}(100) + 32 = 9 \times 20 + 32 = 180 + 32 = 212^\circ\text{F}\)
In simple words: On the Fahrenheit scale, water freezes at 32 degrees and boils at 212 degrees.

Question (i). Radiation of heat does not require any ______________.
Answer: Medium
In simple words: Heat radiation can travel easily through empty space (vacuum) without needing any material medium.

Exam Tip: This is why heat from the Sun reaches Earth through the vacuum of space.

Question (ii). Dull black surfaces are ______________ radiators and ______________ heat absorbers.
Answer: Good, good
In simple words: Black and dull surfaces both emit and absorb heat much better than shiny or light surfaces.

Exam Tip: Good absorbers are always good emitters of thermal radiation.

Question (iii). Heat can not travel through convection in____________
Answer: Solids (or vacuum)
In simple words: Convection requires actual movement of fluid particles, which cannot happen in rigid solids.

Question (iv). Heat from the sun reaches us by the process of ______________.
Answer: Radiation
In simple words: Since there is vacuum between the Sun and Earth, heat travels across space via radiation waves.

Exam Tip: Remember conduction and convection need a medium, but radiation does not.

Question (v). In solid, heat is transferred by ______________ while in liquid and gases heat is transferred by ______________.
Answer: Conduction, convection
In simple words: Solids conduct heat molecule by molecule, while fluids circulate heat through convection currents.

Exam Tip: Link solids with conduction and fluids (liquids/gases) with convection.

II. Select conductors and insulators from following -
Coin, air, paper, water, wood, iron, nail

Question II. Select conductors and insulators from following - Coin, air, paper, water, wood, iron, nail
Answer: Conductors: Coin, iron, nail (water is also a poor/moderate conductor compared to air/wood, but metals are primary conductors).
Insulators: Air, paper, wood.
In simple words: Metals like iron and coins let heat pass through easily, while wood, paper, and air block heat flow.

Exam Tip: Metals are generally good conductors of heat, whereas non-metals (except graphite) and gases are insulators.

EXTRA QUESTIONS

I. Give reasons

Question (i). Clinical thermometer bulb explodes when placed under hot water.
Answer: Hot water has a temperature much higher than \(42^\circ\text{C}\). The mercury expands rapidly and fills the bulb completely, exerting immense pressure on the glass walls, causing the bulb to burst.
In simple words: High heat makes mercury expand beyond capacity in a small clinical bulb, breaking the glass.

Exam Tip: Always emphasize that the temperature exceeds the maximum limit of the clinical thermometer.

Question (ii). If we hold clinical thermometer by its bulb, reading changes.
Answer: The heat from our fingers is transferred to the mercury bulb via conduction, causing the mercury inside to expand and rise up the tube.
In simple words: Body heat from your fingers warms the mercury, pushing the reading up incorrectly.

Question (i). Name two conditions that must be satisfied for the heat transfer by conduction.
Answer: 1. The two objects must be in direct physical contact.
2. There must be a temperature difference between the two objects.
In simple words: Conduction needs touch and a difference in temperature between the bodies.

Exam Tip: Contact and temperature gradient are the two essential prerequisites for conduction.

Question (ii). A spoon is dipped in to a cup of hot tea. Name the process by which the spoon absorbs the heat from the tea cup.
Answer: Conduction
In simple words: Heat passes directly from the hot tea into the metal spoon through touch.

Exam Tip: Solid-to-solid or liquid-to-solid contact transfer of heat is conduction.

Question (iii). Why doesn’t a clinical thermometer contain marking above 42°C?
Answer: The normal human body temperature never exceeds \(42^\circ\text{C}\) even in severe fevers, so markings above this limit are unnecessary.
In simple words: Human body temperature does not go beyond 42 degrees Celsius.

Exam Tip: Clinical thermometers are specifically designed for the physiological limit of human body temperature.

III. Answer the following questions -

Question (i). At what temperature Celsius & Fahrenheit scale shows equal values?
Answer: \(-40^\circ\) (Derived as \(x = -40\)).
In simple words: Both Celsius and Fahrenheit thermometers show the same value at minus 40 degrees.

Exam Tip: Keep this value memorized for quick objective answers.

Question (ii). What are the values of freezing point & boiling point of water on Celsius scale?
Answer: Freezing point = \(0^\circ\text{C}\); Boiling point = \(100^\circ\text{C}\).
In simple words: Water freezes at 0 degrees and boils at 100 degrees Celsius.

Exam Tip: These are standard standard reference points for water temperature.

Question (iii). Convert both the values in Fahrenheit.
Answer: Freezing point in Fahrenheit = \(32^\circ\text{F}\); Boiling point in Fahrenheit = \(212^\circ\text{F}\).
In simple words: Water freezes at 32 degrees Fahrenheit and boils at 212 degrees Fahrenheit.

Exam Tip: Use the conversion formula \(F = \frac{9}{5}C + 32\) to derive these values.

IV. Write short answers for the following questions:

Question (i). Give two points of difference between convection and radiation of heat.
Answer: 1. Convection requires a material medium (liquid or gas) to transfer heat, whereas radiation does not require any medium and can travel through a vacuum.
2. In convection, heat transfer takes place due to the actual movement of fluid particles, whereas in radiation, heat travels in the form of electromagnetic waves.
In simple words: Convection needs moving fluid particles, while radiation travels as waves through empty space.

Exam Tip: Emphasize the requirement of a medium and the mode of transfer in your answer.

Question (ii). How does a blanket keep us warm in winter?
Answer: Woolen blankets trap a layer of air between themselves and our body. Since air is a poor conductor of heat, it prevents body heat from escaping into the cold surroundings, keeping us warm.
In simple words: The blanket traps air, which acts as an insulator preventing heat loss from your body.

Question (iii). Why do we feel hot when we stand near a fire?
Answer: We feel hot because heat from the fire reaches us through radiation, which transfers thermal energy directly through the air without needing a medium.
In simple words: Heat travels directly from the fire to your body through thermal radiation.

Exam Tip: Direct heat sensation at a distance is always due to radiation.

Question (iv). Why do we prefer light coloured clothes in summers and dark coloured clothes in winters?
Answer: Light-colored clothes reflect most of the heat radiation falling on them, keeping us cool in summer. Dark-colored clothes absorb most of the heat radiation, keeping us warm in winter.
In simple words: Light colors reflect heat to keep you cool, while dark colors absorb heat to keep you warm.

Question (v). Write any two examples which are based on convection current.
Answer: 1. Formation of sea breeze and land breeze.
2. Circulation of air through room ventilators or exhaust fans.
In simple words: Land-sea breezes and rising warm air in rooms are classic examples of convection currents.

Question (vi). Several days after the end of a snowstorm, the roofs of a house gets completely covered with snow, another house has no snow on its roof. Which house is better insulated & why?
Answer: The house with snow on its roof is better insulated. This is because good insulation prevents heat from escaping from inside the house to the roof. Since heat does not escape, the roof stays cold and the snow remains unmelted.
In simple words: If snow stays on the roof, it means no heat is leaking from inside, showing the house has great insulation.

Question (i). Ventilators are provided near the ceiling.
Answer: Warm air inside a room is lighter and rises up towards the ceiling. Ventilators are placed near the ceiling so that this hot, stale air can easily escape out of the room.
In simple words: Hot air rises and goes out through high ventilators.

Question (ii). The bottom of cooking utensils are coloured black.
Answer: Black surfaces are good absorbers of heat. Painting the bottom black ensures maximum absorption of heat from the flame, allowing food to cook faster.
In simple words: Black bottoms absorb maximum heat from the stove to cook food quickly.

Exam Tip: Good absorption property of black surfaces improves heating efficiency.

Question (iii). Cotton is used in quilts.
Answer: Cotton fibers trap a large amount of air inside them. Since air is a poor conductor of heat, it prevents our body heat from escaping, keeping us warm.
In simple words: Cotton traps insulating air pockets that block heat loss.

Question (iv). Thermocol is used to make ice boxes to carry ice.
Answer: Thermocol is a bad conductor (insulator) of heat and contains trapped air pockets, which prevent outside heat from entering the box and melting the ice.
In simple words: Thermocol blocks outside heat from reaching the ice inside.

Exam Tip: Insulating properties prevent heat gain from the external atmosphere.

Question (v). Radiators of cars & air conditioners are painted black.
Answer: Black surfaces are good radiators and emitters of heat. Painting them black helps radiate away unwanted heat quickly and efficiently.
In simple words: Black paint helps radiate heat away rapidly to keep engines and systems cool.

Exam Tip: Good absorbers are also good radiators of heat.

VI. Draw the diagram for land breeze and sea breeze and answer the questions that follow:

Question (i). Why does land breeze occur during night-time?
Answer: During the night, the land cools down faster than the sea. The air above the sea is warmer and rises, creating a low pressure area. Cooler air from the land blows towards the sea, creating a land breeze.
In simple words: Land cools faster at night, so cooler air moves from land toward the warmer sea.

Exam Tip: Mention differential cooling of land and sea during night hours.

Question (ii). Why does sea breeze occur during daytime?
Answer: During the day, the land heats up much faster than the sea. The hot air above the land rises, and cooler air from the sea rushes in to take its place, forming a sea breeze.
In simple words: Land heats faster during the day, causing cool air to blow from the sea to the land.

Exam Tip: Always associate sea breeze with daytime and land breeze with night-time.

QUESTION BANK

I. Define:

Question (i). Heat
Answer: Heat is a form of energy that flows from a body at a higher temperature to a body at a lower temperature due to a temperature difference.
In simple words: Heat is thermal energy that moves from hot objects to cold objects.

Exam Tip: Define heat strictly as a form of energy.

Question (ii). Temperature
Answer: Temperature is the degree of hotness or coldness of an object, which determines the direction of heat flow.
In simple words: Temperature measures how hot or cold something is.

Exam Tip: Temperature is the effect, while heat is the cause (energy).

Question (iii). Thermometer
Answer: A thermometer is a device used for measuring the temperature of objects or substances accurately.
In simple words: An instrument used to measure temperature is called a thermometer.

Exam Tip: Mention clinical and laboratory thermometers as primary examples.

II. What are different scales for measuring temperature?

Question II. What are different scales for measuring temperature?
Answer: The three main temperature scales are: (1) Celsius scale (\(^\circ\text{C}\)), (2) Fahrenheit scale (\(^\circ\text{F}\)), and (3) Kelvin scale (\(\text{K}\)).
In simple words: Temperature is commonly measured in Celsius, Fahrenheit, and Kelvin scales.

Exam Tip: Kelvin is the SI unit of temperature in scientific applications.

III. Relation between Celsius scale and Fahrenheit scale.

Question III. Relation between Celsius scale and Fahrenheit scale.
Answer: The relation between Celsius (\(C\)) and Fahrenheit (\(F\)) scales is given by the formula:
\(\frac{C}{5} = \frac{F - 32}{9}\)
In simple words: The formula connects Celsius and Fahrenheit readings so you can convert from one to the other.

Exam Tip: Memorize this formula accurately for numerical conversion problems.

IV. Define least count of thermometer.

Question IV. Define least count of thermometer.
Answer: The least count of a thermometer is the smallest measurement or value that can be accurately read on its scale.
In simple words: The smallest value measurable between two graduation marks on a thermometer is its least count.

Question V. What is role of kink in a clinical thermometer?
Answer: The kink prevents the mercury level from falling down automatically on its own when the thermometer is removed from the patient's mouth, allowing accurate reading of body temperature.
In simple words: The kink stops mercury from falling back so you can read the temperature after taking it out.

Exam Tip: This is a crucial function tested in exams regarding clinical thermometers.

VI. What precautions should be observed while using s clinical thermometer.

Question VI. What precautions should be observed while using s clinical thermometer.
Answer: 1. Wash the thermometer before and after use, preferably with antiseptic solution.
2. Ensure mercury level is below \(35^\circ\text{C}\) before taking a reading.
3. Handle it with care; if it hits a hard object, it can break.
4. Do not hold the thermometer by the bulb while reading.
In simple words: Wash it, shake down the mercury below 35 degrees, and handle it carefully.

Question VII. State 2 similarities and 2 difference between clinical and laboratory thermometer.
Answer: Similarities:
1. Both use mercury as the thermometric liquid.
2. Both are used to measure temperature based on thermal expansion.
Differences:
1. Clinical thermometer has a kink, whereas a laboratory thermometer does not have a kink.
2. Clinical thermometer has a limited range (\(35^\circ\text{C}\) to \(42^\circ\text{C}\)), whereas a laboratory thermometer has a wider range (typically \(-10^\circ\text{C}\) to \(110^\circ\text{C}\)).
In simple words: Both use mercury, but clinical has a kink and a short range for human bodies, while lab has no kink and a wide range.

Exam Tip: The presence/absence of a kink and temperature range are the most important differences.

VIII. Convert:

Question (i). \(25^\circ\text{C}\) to \(^\circ\text{F}\)
Answer: \(F = \frac{9}{5}C + 32\)
\(F = \frac{9}{5}(25) + 32\)
\(F = 9 \times 5 + 32 = 45 + 32 = 77^\circ\text{F}\)
In simple words: 25 degrees Celsius is equal to 77 degrees Fahrenheit.

Exam Tip: Substitute \(C = 25\) directly into the standard formula.

Question (ii). \(86^\circ\text{F}\) to \(^\circ\text{C}\)
Answer: \(\frac{C}{5} = \frac{F - 32}{9}\)
\(\frac{C}{5} = \frac{86 - 32}{9} = \frac{54}{9} = 6\)
\(C = 6 \times 5 = 30^\circ\text{C}\)
In simple words: 86 degrees Fahrenheit converts to 30 degrees Celsius.

am Tip: Subtract 32 first, divide by 9, and then multiply by 5.

Question (iii). \(10^\circ\text{C}\) to \(^\circ\text{F}\)
Answer: \(F = \frac{9}{5}(10) + 32\)
\(F = 9 \times 2 + 32 = 18 + 32 = 50^\circ\text{F}\)
In simple words: 10 degrees Celsius equals 50 degrees Fahrenheit.

Exam Tip: Simplify the fraction \(\frac{9}{5} \times 10\) first to make calculations easy.

Question (iv). \(95^\circ\text{F}\) to \(^\circ\text{C}\)
Answer: \(\frac{C}{5} = \frac{95 - 32}{9} = \frac{63}{9} = 7\)
\(C = 7 \times 5 = 35^\circ\text{C}\)
In simple words: 95 degrees Fahrenheit is equal to 35 degrees Celsius.

Exam Tip: Ensure arithmetic subtraction and division are checked before final multiplication.

IX. What are three methods of transfer of heat?

Question IX. What are three methods of transfer of heat?
Answer: The three methods of heat transfer are: (1) Conduction, (2) Convection, and (3) Radiation.
In simple words: Heat transfers through conduction, convection, and radiation.

Exam Tip: Memorize these three fundamental modes of thermal transfer.

X. Define:

Question (i). Conduction
Answer: Conduction is the process by which heat is transferred from the hotter end to the colder end of an object without the actual movement of particles from their positions.
In simple words: Heat transfer through direct contact in solids is conduction.

Exam Tip: Highlight that particles vibrate in place without migrating during conduction.

Question (ii). Conductors of heat
Answer: Materials that allow heat to pass through them easily are called conductors of heat (e.g., metals like iron, copper, aluminum).
In simple words: Materials that let heat flow through them quickly are conductors.

Exam Tip: All metals are good heat conductors.

Question (iii). Insulators of heat
Answer: Materials that do not allow heat to pass through them easily are called insulators or bad conductors of heat (e.g., wood, plastic, air, wool).
In simple words: Materials that block or resist heat flow are insulators.

Exam Tip: Wood, paper, and air are classic examples of thermal insulators.

XI. Define:

Question (i). Convection
Answer: Convection is the process of heat transfer in fluids (liquids and gases) where heated particles become lighter, rise up, and cooler particles move down to take their place, forming convection currents.
In simple words: Heat transfer through actual movement of fluid particles is convection.

Exam Tip: Emphasize actual particle movement in liquids and gases.

Question (ii). Sea breeze
Answer: The movement of cool air from the sea toward the land during daytime is called sea breeze.
In simple words: Daytime cool wind blowing from sea to land is sea breeze.

Exam Tip: Sea breeze occurs during the day.

Question (iii). Land breeze
Answer: The movement of cool air from the land toward the sea during night-time is called land breeze.
In simple words: Night-time cool wind blowing from land to sea is land breeze.

Exam Tip: Land breeze occurs during the night.

XII. Define Radiation and give its examples.

Question XII. Define Radiation and give its examples.
Answer: Radiation is the transfer of heat energy from a hot body to a cold body through space or medium in the form of waves without heating the intervening medium.
Examples: (1) Heat from the Sun reaching Earth, (2) Warmth felt when standing near a room heater or bonfire.
In simple words: Heat travelling in waves without needing a medium is radiation (like warmth from the Sun).

Exam Tip: Mention that radiation requires no medium.

XIII. Give reason:

Question (i). Slabs of ice are covered with sawdust or gunny bags.
Answer: Sawdust and gunny bags contain trapped air, which is a poor conductor of heat. This prevents outside heat from reaching the ice and melting it.
In simple words: Insulation from sawdust and trapped air stops ice from melting quickly.

Question (ii). Why are ventilators provided at high height and windows at low height?
Answer: Warm air inside a room is lighter, rises up, and escapes through high ventilators. Cooler, fresh air enters through low windows to replace it, establishing a continuous convection current for proper ventilation.
In simple words: Hot air goes out high ventilators while cool air enters low windows due to convection.

Question (iii). Why are bottom surfaces of utensils made black?
Answer: Black surfaces absorb heat efficiently. Painting the bottom black ensures maximum absorption of heat from the stove flame so that food cooks faster.
In simple words: Black absorbs maximum heat to cook food quickly.

Exam Tip: Good absorption of heat by black surfaces is the key reason.

Question (iv). Why do birds puff up their feathers in winters?
Answer: By puffing up their feathers, birds trap a large amount of air between their feathers and body. Since air is a poor conductor of heat, it prevents body heat from escaping into the cold environment.
In simple words: Puffing feathers traps air, which insulates their bodies against cold weather.

Exam Tip: Trapped air provides vital thermal insulation for animals and birds.

Question (v). Why wearing more layer of clothing during winter keeps us warmer than wearing just one piece of cloth.
Answer: Multiple layers of clothing trap layers of air between them. Since air is a bad conductor of heat, these trapped air layers act as extra insulation barriers, preventing body heat from escaping more effectively than a single layer.
In simple words: Multiple layers trap multiple pockets of air, giving better insulation against cold.

Exam Tip: Emphasize multiple trapped air spaces between clothing layers.

Question (vi). In place of hot climate it is advised that the outer walls of houses be printed white. Why?
Answer: White and light-colored surfaces reflect most of the heat radiation falling on them from the Sun. Painting outer walls white keeps the interior cool by preventing excessive heat absorption.
In simple words: White paint reflects sun rays and keeps houses cool in hot regions.

Exam Tip: Reflection of thermal radiation is the primary scientific reason.