Keywords To Remember
- Heat
A form of energy that makes things warm or hot.
(Example: The Sun gives us heat.) - Temperature
A measure of how hot or cold something is.
(Example: We measure body temperature using a thermometer.) - Thermometer
A tool used to measure temperature.
(Example: A clinical thermometer is used to check body temperature.) - Clinical Thermometer
A thermometer used to measure human body temperature.
(Range: 35°C to 42°C) - Laboratory Thermometer
A thermometer used in science labs to measure the temperature of various substances.
(Range: -10°C to 110°C or more) - Conduction
The transfer of heat through solids by direct contact.
(Example: A metal spoon gets hot when kept in a hot cup of tea.) - Convection
The transfer of heat in liquids and gases by the movement of particles.
(Example: Water heats up from the bottom to the top when boiled.) - Radiation
The transfer of heat without any medium, in all directions.
(Example: Sunlight reaching the Earth.) - Conductor
A material that allows heat to pass through it easily.
(Example: Metals like iron, copper, and aluminum.) - Insulator
A material that does not allow heat to pass through it easily.
(Example: Wood, plastic, and cloth.) - Boiling Point
The temperature at which a liquid changes to gas.
(Water boils at 100°C.) - Melting Point
The temperature at which a solid turns into a liquid.
(Ice melts at 0°C.) - Expansion
When substances increase in size on heating.
(Example: Metal expands when heated.) - Contraction
When substances decrease in size on cooling.
(Example: Metal contracts when cooled.) - Heat Transfer
The movement of heat from a hot object to a cold one.
(Happens by conduction, convection, or radiation.) - Thermal Equilibrium
When two objects reach the same temperature and heat transfer stops.
(Example: A hot cup of tea cools down in a cold room.) - Degree Celsius (°C)
A unit of measuring temperature commonly used in thermometers. - Kelvin (K)
The SI unit of temperature, used mostly in scientific work.
(0°C = 273K) - Infrared Radiation
Heat energy from the sun or other sources that travels through empty space.
(Can be felt but not seen.) - Specific Heat Capacity
The amount of heat required to raise the temperature of 1 kg of a substance by 1°C.
Something To Know
A. Fill in the blanks.
- Heat is a form of __________.
Answer – energy
- The range of clinical thermometer is from __________ °C to __________ °C.
Answer – 35°C to 42°C
- Water is a __________ conductor of heat.
Answer – poor
- Land and sea breezes help to maintain the __________ of air.
Answer – temperature
- Transfer of heat, from the sun to the earth, takes place mainly through the process of __________.
Answer – radiation
B. Write True or False for the following statements.
- Heat cannot be produced by doing work.
Answer – False
- No gaps are left between the rails of railway tracks.
Answer – False
- Poor conductors of heat are also good insulators of heat.
Answer – True
- Ventilators, when provided in rooms, are located near their roofs.
Answer – True
- A material medium is required for transfer of heat by the process of radiation.
Answer – False
C. Tick (✓) the correct option.
- A copper ball at 30°C is put in a container, containing water at 30°C. In this case—
heat will get transferred from water to the copper ball.
heat will get transferred from the copper ball to water.
heat would flow first from copper ball to water and then from water to copper ball.
there would be no transfer of heat between the copper ball and water.
Answer – There would be no transfer of heat between the copper ball and water✓
- The reading of the laboratory thermometer, in the figure shown here, is—
10.2°C
-10°C
10.8°C
18°C
Answer – -10°C – ✓ (assuming the image shows mercury at the -10°C mark)
- It is not convenient to use the laboratory thermometer to measure our body temperature. This is so because—
its range is small.
as the thermometer is taken out from the mouth, the level of mercury, in it, immediately starts falling.
our body does not transfer heat to its bulb.
it takes a very long time to acquire the temperature of the body.
Answer – As the thermometer is taken out from the mouth, the level of mercury, in it, immediately starts falling. – ✓
- When we hold our hands close to the side of a flame, they get warmed up mainly due to—
conduction as well as convection
conduction
radiation
convection
Answer – Radiation – ✓
- The freezer compartment in a refrigerator is usually put near its top. This provides good cooling throughout the refrigerator through—
the good conductivity of air.
the radiation of heat by the food items kept inside the refrigerator.
the setting up of convection currents’, in the air inside the refrigerator.
transfer of heat by conduction and radiation.
Answer – The setting up of convection currents in the air inside the refrigerator. – ✓
D. Answer the following questions in brief.
- In which direction does the transfer of heat normally take place?
Answer – From a hotter object to a colder one.
- What is the cause of heat generation in the following situations:
(a) We apply brakes on our fast moving car.
Answer – Friction between the brake pads and wheels produces heat.
(b) People often jump up and down to feel warmer in cold weather.
Answer – Jumping increases body movement and generates heat due to muscle activity.
- State two types of physical changes that may take place when a substance is heated.
Answer – Expansion and change of state (solid to liquid, or liquid to gas).
- In what way(s) does a clinical thermometer differ from an ordinary thermometer?
Answer – A clinical thermometer has a narrow range (35°C–42°C).
It has a kink to prevent the mercury from falling back immediately.
- State the mode/different modes, of heat transfer, in the following situations.
(a) A paper cup, full of hot soup, lying on a table.
Answer – Conduction (heat from soup to cup to table).
(b) Cooking vegetables in a pan.
Answer – Conduction and convection.
(c) Melting of a chocolate bar, in the school bag, on a hot day.
Answer – Radiation.
(d) Cooking food in a microwave oven.
Answer – Radiation and convection.
- What is meant by sea breeze? When does it occur?
Answer – A sea breeze is a wind that blows from sea to land during the day due to differential heating of land and water.
E. Answer the following questions
- In the arrangement shown in the figure, pins A, B, C, and D are fixed to a circular metal loop with the help of wax.
Answer – The pins will fall in the following order:
A → B → D → C
Justification:
Heat conduction in solids takes place from the hotter end to the colder end.
The metal loop conducts heat from point A to the other points.
Since AB < AD, pin B is closer to A than pin D. So, heat reaches pin B faster than pin D.
Pin C is the farthest from point A in terms of conduction path, so it gets heated the last.
Therefore, wax at A melts first, followed by B, then D, and finally C, due to progressive conduction of heat through the metal loop.
- Give reasons for the following:
(a) Iron rims are heated red hot before ‘fixing’ them on cart wheels.
Answer – Hot iron expands to fit tightly when cooled on the wheel.
(b) A clinical thermometer has a slight bend, or kink, in its capillary tube.
Answer – Kink prevents mercury from falling immediately.
(c) A new quilt is warmer than an old one.
Answer – New quilts trap more air, acting as better insulators.
(d) A brass tumbler feels much cooler than a wooden tray on a chilly day.
Answer – Brass conducts heat better, feels colder.
(e) The bottoms of cooking utensils are often kept black.
Answer – Black surfaces absorb more heat, aiding in cooking.
- Akshit visited Rishikesh for river rafting during summer holidays. At a campsite, there were two tents, one made with a black fabric and the other with a white fabric. Which one should Akshit prefer? Give reason for the choice. Should Akshit prefer the same tent during winters?
Answer – Akshit should prefer the white tent in summer as it reflects heat.
In winter, he should prefer the black tent, as it absorbs heat.
- Explain briefly how winds are caused.
Answer – Winds are caused by differences in air pressure created by unequal heating of the Earth’s surface.
- Supriya, while doing an experiment in the Science Laboratory, kept a laboratory thermometer ‘P’ 10 cm away on one side of the flame of a candle. Her friend Riya kept a similar thermometer ‘Q’ 10 cm above the flame of the candle as shown in the figure. In which of the thermometers, ‘P’ or ‘Q’, the rise in temperature will be faster? Give reason for your answer
Answer – The rise in temperature will be faster in thermometer ‘Q’.
Reason:
Hot air rises upwards because it is lighter than cold air. This movement of heat through gases is called convection.
Thermometer Q is placed above the flame, where hot air rises directly.
Thermometer P is placed beside the flame, where the hot air does not move directly.
Therefore, Q receives more heat quickly through convection, leading to a faster rise in temperature.
- In the two ‘set-ups’ X and Y, shown on the next page, the wires AB and PQR are made of the same material and have equal ‘thickness’. The length of the wire AB, (in the set-up ‘X’) is equal to the diameter (= PR) of the semi-circle, formed by the wire PQR, (in the set-up ‘Y’). Pins, Pl and P2, are attached, to wires AB and PQR respectively, with the help of wax. Which of the two pins, Pl or P2, will fall off later? Give reason for your answer.
Answer – In Setup X, a straight metal rod AB is used, and a pin P1 is attached with wax near end A. A candle flame is placed beneath end B.
In Setup Y, a semicircular metal wire PQR is used, and pin P2 is fixed with wax at end P. A candle flame is placed beneath end R.
In both cases, heat is conducted through the metal from the point of heating (B in X and R in Y) to the point where the pin is attached (A in X and P in Y). The pin falls off when the wax melts due to conduction of heat.
Value Based Question
During the summer holidays, Hridyika went to visit her grandparents in their village. During her morning walks there, she observed that a girl in the village would play very good music on her home-made musical instrument. Hridyika was very much impressed by her skill and wanted to learn playing that instrument from her. She, however, felt hesitant to ask her to teach her. When she discussed her problem with her mother, she advised her to politely and keenly request that girl to teach her that skill. She told Hridiyika that just as heat always flows from a hot object to a cold object, knowledge and skill always ‘flow’ to a pupil from her/his mentor.
- State the values displayed by Hridiyika’s mother.
Answer – Respect, open-mindedness, and eagerness to learn.
- Name the three different modes of transfer of heat and give one example of each.
Answer – Conduction: Cooking in a metal pan.
Convection: Boiling water.
Radiation: Heat from the sun.
- Have a group talk in which students talk about how they learnt some skill from a mentor.
Answer – To be done in class by students in groups.
Something To Do
- Boil some water in vessel. Cover it with a plate. Remove the plate after sometime. Allow it to cool. We will see droplets of water on the surface of plate. Try to find out why, and how, these water droplets are formed.
Answer – Water droplets form due to condensation—steam touches the cool plate and condenses.
- Describe the various ways in which heat energy can enter/escape from our houses. Learn about the measures that are now being taken to reduce such heat transfers.
Answer – Heat enters/exits via windows, doors, walls, roofs.
Use of insulating materials, double-glazed windows, reflective roofs help reduce heat loss/gain.
- Take a cup of hot water/tea. Measure its temperature accurately with a thermometer. What will happen to the hot water/tea if it is left in a room for, say, one hour? Measure the temperature at fixed intervals of five minutes and record (your observations) in the given table. Using these readings, try to draw a temperature-time graph.
Answer –
S.No. | Time (minutes) | Temperature (°C) |
1 | 0 | 25 |
2 | 1 | 30 |
3 | 2 | 36 |
4 | 3 | 42 |
5 | 4 | 48 |
6 | 5 | 54 |
- Take some (moderately) hot water in a tumbler. Dip one end of a steel spoon, a plastic spoon, a glass rod, a wooden stick and a copper rod in the hot water. After waiting for few minutes (say 2-3 minutes) (carefully) touch the other end of each item. Observe which ones become hot and which ones do not. Hence, categorise them as ‘Heat conductors’ or as ‘Heat insulators’.
Answer – Heat Conductors
Copper
Aluminium
Iron
Heat Insulators
Wood
Plastic
Wool
Explanation:
Heat conductors allow heat to pass through them easily. Metals like copper, aluminium, and iron are good conductors of heat.
Heat insulators do not allow heat to pass through them easily. Materials like wood, plastic, and wool are poor conductors and act as insulators.
Additional Questions and Answer – 01
Fill in the Blanks (with Answers)
- The SI unit of temperature is __________.
Answer – Kelvin - Heat flows from a __________ object to a __________ object.
Answer – hotter, cooler - __________ thermometer is used to measure the temperature of the human body.
Answer – Clinical - The substance that allows heat to pass through easily is called a __________ conductor.
Answer – good - The mode of heat transfer that does not require any medium is __________.
Answer – radiation - A black surface absorbs __________ heat compared to a white surface.
Answer – more - The boiling point of water is __________ °C.
Answer – 100 - __________ is the process in which heat is transferred through solids.
Answer – Conduction - The normal human body temperature is __________ °C.
Answer – 37 - A laboratory thermometer cannot be used to measure __________ temperature.
Answer – body
Multiple Choice Questions (MCQs)
- Which of the following is the best conductor of heat?
a) Wood
b) Plastic
c) Copper
d) Glass
Answer – c) Copper - The mode of transfer of heat in liquids and gases is—
a) Conduction
b) Convection
c) Radiation
d) None
Answer – b) Convection - Which object would feel colder on a winter morning?
a) A woolen blanket
b) A cotton shirt
c) A steel plate
d) A plastic tray
Answer – c) A steel plate - Which surface reflects most of the heat?
a) Black rough surface
b) White shiny surface
c) Matte black surface
d) Grey dull surface
Answer – b) White shiny surface - What is the function of the kink in a clinical thermometer?
a) To increase accuracy
b) To prevent mercury from falling back
c) To allow easy handling
d) To show high temperature
Answer – b) To prevent mercury from falling back
True or False
- Heat is measured in joules.
Answer – True - Convection occurs in solids.
Answer – False - All metals are good conductors of heat.
Answer – True - Radiation requires a medium for heat transfer.
Answer – False - Water is a better conductor of heat than copper.
Answer – False
Short Answer Questions
- What is the difference between heat and temperature?
Answer – Heat is a form of energy that causes change in temperature or state of matter, while temperature is a measure of how hot or cold an object is. - Why are cooking utensils made of metal but their handles made of wood or plastic?
Answer – Metals conduct heat well to cook food, while wood or plastic handles prevent heat transfer to protect hands from burns. - What is a convection current?
Answer – The upward movement of warm air and downward movement of cool air forming a circular pattern is called a convection current. - Why do we wear woolen clothes in winter?
Answer – Woolen clothes trap air, a poor conductor, and keep our body warm by preventing heat loss. - Name two good conductors and two insulators of heat.
Answer – Conductors: Copper, Aluminum; Insulators: Wood, Plastic
Competency Based Questions (Reasoning & Assertion)
- Assertion: Sea breeze occurs during the day.
Reason: Land heats up faster than water.
a) Both A and R are true, and R is the correct explanation of A.
b) Both A and R are true, but R is not the correct explanation of A.
c) A is true, R is false.
d) A is false, R is true.
Answer – a) - Assertion: Woolen clothes keep us warmer in winter.
Reason: Wool is a good conductor of heat.
Answer – c) A is true, R is false. - Assertion: Radiation can occur in vacuum.
Reason: Radiation does not require any medium to transfer heat.
Answer – a) - Assertion: The handle of a metal spoon remains cold when placed in hot water for a while.
Reason: Metals are poor conductors of heat.
Answer – d) A is false, R is true.
Higher Order Thinking Skill (HOTS) Questions
- Why is it not advisable to wear black clothes in summer?
Answer – Black absorbs more heat, making the body feel hotter, while light colors reflect heat and keep us cool. - Explain why desert areas are very hot during the day and very cold at night.
Answer – Sand heats up quickly during the day and cools rapidly at night, causing extreme temperature changes. - A steel glass and a plastic cup both contain hot tea. Which one will feel hotter to hold and why?
Answer – The steel glass will feel hotter because metal is a good conductor of heat and quickly transfers heat to the hand.
- Long Answer Questions (50–60 words)
- Describe the working of a clinical thermometer and how it is different from a laboratory thermometer.
Answer – A clinical thermometer is designed to measure human body temperature. It has a kink in the capillary tube that prevents mercury from falling back. Its range is from 35°C to 42°C. Unlike laboratory thermometers, it is more sensitive and not used for measuring very high or low temperatures. - How do land and sea breezes occur? Explain with reasons.
Answer – During the day, land heats faster than water, making air above land rise and cool sea air move in to take its place—this is a sea breeze. At night, land cools faster than water, so warm air from the sea rises and cool land air moves in—this is a land breeze.
Additional Questions And Answers – 01
- Fill in the Blanks (with Answers)
- The SI unit of temperature is __________.
Answer – Kelvin - Heat flows from a __________ object to a __________ object.
Answer – hotter, cooler - __________ thermometer is used to measure the temperature of the human body.
Answer – Clinical - The substance that allows heat to pass through easily is called a __________ conductor.
Answer – good - The mode of heat transfer that does not require any medium is __________.
Answer – radiation - A black surface absorbs __________ heat compared to a white surface.
Answer – more - The boiling point of water is __________ °C.
Answer – 100 - __________ is the process in which heat is transferred through solids.
Answer – Conduction - The normal human body temperature is __________ °C.
Answer – 37 - A laboratory thermometer cannot be used to measure __________ temperature.
Answer – body
- Multiple Choice Questions (MCQs)
- Which of the following is the best conductor of heat?
a) Wood
b) Plastic
c) Copper
d) Glass
Answer – c) Copper - The mode of transfer of heat in liquids and gases is—
a) Conduction
b) Convection
c) Radiation
d) None
Answer – b) Convection - Which object would feel colder on a winter morning?
a) A woolen blanket
b) A cotton shirt
c) A steel plate
d) A plastic tray
Answer – c) A steel plate - Which surface reflects most of the heat?
a) Black rough surface
b) White shiny surface
c) Matte black surface
d) Grey dull surface
Answer – b) White shiny surface - What is the function of the kink in a clinical thermometer?
a) To increase accuracy
b) To prevent mercury from falling back
c) To allow easy handling
d) To show high temperature
Answer – b) To prevent mercury from falling back
- True or False
- Heat is measured in joules.
Answer – True - Convection occurs in solids.
Answer – False - All metals are good conductors of heat.
Answer – True - Radiation requires a medium for heat transfer.
Answer – False - Water is a better conductor of heat than copper.
Answer – False
- Short Answer Questions
- What is the difference between heat and temperature?
Answer – Heat is a form of energy that causes change in temperature or state of matter, while temperature is a measure of how hot or cold an object is. - Why are cooking utensils made of metal but their handles made of wood or plastic?
Answer – Metals conduct heat well to cook food, while wood or plastic handles prevent heat transfer to protect hands from burns. - What is a convection current?
Answer – The upward movement of warm air and downward movement of cool air forming a circular pattern is called a convection current. - Why do we wear woolen clothes in winter?
Answer – Woolen clothes trap air, a poor conductor, and keep our body warm by preventing heat loss. - Name two good conductors and two insulators of heat.
Answer – Conductors: Copper, Aluminum; Insulators: Wood, Plastic
- Competency Based Questions (Reasoning & Assertion)
- Assertion: Sea breeze occurs during the day.
Reason: Land heats up faster than water.
a) Both A and R are true, and R is the correct explanation of A.
b) Both A and R are true, but R is not the correct explanation of A.
c) A is true, R is false.
d) A is false, R is true.
Answer – a) - Assertion: Woolen clothes keep us warmer in winter.
Reason: Wool is a good conductor of heat.
Answer – c) A is true, R is false. - Assertion: Radiation can occur in vacuum.
Reason: Radiation does not require any medium to transfer heat.
Answer – a) - Assertion: The handle of a metal spoon remains cold when placed in hot water for a while.
Reason: Metals are poor conductors of heat.
Answer – d) A is false, R is true.
- Higher Order Thinking Skill (HOTS) Questions
- Why is it not advisable to wear black clothes in summer?
Answer – Black absorbs more heat, making the body feel hotter, while light colors reflect heat and keep us cool. - Explain why desert areas are very hot during the day and very cold at night.
Answer – Sand heats up quickly during the day and cools rapidly at night, causing extreme temperature changes. - A steel glass and a plastic cup both contain hot tea. Which one will feel hotter to hold and why?
Answer – The steel glass will feel hotter because metal is a good conductor of heat and quickly transfers heat to the hand.
- Long Answer Questions (50–60 words)
- Describe the working of a clinical thermometer and how it is different from a laboratory thermometer.
Answer – A clinical thermometer is designed to measure human body temperature. It has a kink in the capillary tube that prevents mercury from falling back. Its range is from 35°C to 42°C. Unlike laboratory thermometers, it is more sensitive and not used for measuring very high or low temperatures. - How do land and sea breezes occur? Explain with reasons.
Answer – During the day, land heats faster than water, making air above land rise and cool sea air move in to take its place—this is a sea breeze. At night, land cools faster than water, so warm air from the sea rises and cool land air moves in—this is a land breeze.
Additional Questions And Answers – 02
Fill in the Blanks
- Heat is a form of ________ that is transferred due to a temperature difference.
Answer: Energy
- Transfer of heat always takes place from a body at a ________ temperature to a body at a ________ temperature.
Answer: Higher, lower
- The method of heat transfer in solids, where molecules vibrate without moving, is called ________.
Answer: Conduction
- Liquids and gases transfer heat through ________, involving the actual movement of molecules.
Answer: Convection
- The mode of heat transfer that does not require a material medium is called ________.
Answer: Radiation
- The range of a clinical thermometer is from ________°C to ________°C.
Answer: 35, 42
- Mercury in a thermometer expands due to the phenomenon of ________ expansion.
Answer: Thermal
- Metals are ________ conductors of heat, while wood and plastic are ________ conductors.
Answer: Good, bad
- The normal human body temperature is close to ________°C.
Answer: 36.9
- During the day, the breeze that flows from the sea to the land is called a ________ breeze.
Answer: Sea
Multiple Choice Questions (MCQs)
- What happens to the temperature of a body when heat is added to it?
- a) It decreases
- b) It remains the same
- c) It increases
- d) It becomes zero
Answer: c) It increases
- Which method of heat transfer is most significant in solids?
- a) Convection
- b) Radiation
- c) Conduction
- d) Expansion
Answer: c) Conduction
- What is the range of a typical laboratory thermometer?
- a) 35°C to 42°C
- b) -10°C to 110°C
- c) 0°C to 50°C
- d) -20°C to 100°C
Answer: b) -10°C to 110°C
- Why are cooking utensils often made of metals?
- a) They are bad conductors of heat
- b) They are good conductors of heat
- c) They do not expand on heating
- d) They contract on heating
Answer: b) They are good conductors of heat
- What happens to the air above the land during the day in coastal areas?
- a) It cools down and sinks
- b) It heats up and rises
- c) It remains unchanged
- d) It moves towards the poles
Answer: b) It heats up and rises
- Which of the following is a bad conductor of heat?
- a) Copper
- b) Aluminium
- c) Wood
- d) Iron
Answer: c) Wood
- How does the heat from the sun reach the earth?
- a) Conduction
- b) Convection
- c) Radiation
- d) Expansion
Answer: c) Radiation
- What is the normal human body temperature in Fahrenheit?
- a) 32°F
- b) 98.4°F
- c) 212°F
- d) 72°F
Answer: b) 98.4°F
- Why are light-colored clothes preferred in summer?
- a) They absorb more heat
- b) They reflect more heat
- c) They conduct heat better
- d) They expand more
Answer: b) They reflect more heat
- What causes the formation of land and sea breezes?
- a) Conduction in solids
- b) Convection currents in air
- c) Radiation from the sun
- d) Thermal expansion of water
Answer: b) Convection currents in air
True/False Questions
- Heat flows from a body at a lower temperature to a body at a higher temperature.
Answer: False (Heat flows from a higher temperature to a lower temperature)
- Temperature is an indicator of the degree of hotness or coldness of an object.
Answer: True
- Convection is the primary method of heat transfer in solids.
Answer: False (Conduction is the primary method in solids)
- Mercury in a thermometer contracts when heated.
Answer: False (Mercury expands when heated)
- The clinical thermometer has a kink to prevent the mercury from falling back after use.
Answer: True
- Black objects absorb heat better than white objects.
Answer: True
- Water is a good conductor of heat.
Answer: False (Water is a poor conductor of heat)
- The range of a laboratory thermometer is typically from 35°C to 42°C.
Answer: False (It is typically from -10°C to 110°C)
- Convection currents contribute to the formation of winds and storms.
Answer: True
- Digital thermometers are preferred over mercury thermometers because they are mercury-free.
Answer: True
Short Questions and Answers
- What is heat?
Answer: Heat is a form of energy transferred between bodies due to a temperature difference.
- How does conduction transfer heat in solids?
Answer: In conduction, heat is transferred through solids as molecules vibrate and pass energy to neighboring molecules without moving themselves.
- What is the role of the kink in a clinical thermometer?
Answer: The kink prevents the mercury from falling back after measuring body temperature, allowing for a convenient reading.
- Why are metals used in cooking utensils?
Answer: Metals are good conductors of heat, allowing quick transfer of heat from the flame to the food for efficient cooking.
- What causes thermal expansion in materials?
Answer: Thermal expansion occurs when materials expand on heating due to increased molecular motion.
- How does convection occur in liquids?
Answer: In convection, heated liquid molecules become lighter, rise, and are replaced by cooler, heavier molecules, creating a cyclic movement.
- Why are white clothes preferred in summer?
Answer: White clothes reflect more heat, absorbing less, keeping the wearer cooler in summer.
- What is the least count of a measuring instrument?
Answer: The least count is the smallest quantity an instrument can measure, determined by its calibration.
- How does radiation differ from conduction and convection?
Answer: Radiation transfers heat without a material medium, unlike conduction and convection, which require a medium.
- What causes sea breezes during the day?
Answer: During the day, land heats up faster than the sea, causing the air above the land to rise, and cooler air from the sea moves in, creating a sea breeze.
Competency-Based Questions (Reasoning and Assertion)
- Assertion (A): Heat always flows from a body at a higher temperature to a body at a lower temperature. Reason (R): Heat is a form of energy that transfers due to a temperature difference.
- a) Both A and R are true, and R is the correct explanation of A.
- b) Both A and R are true, but R is not the correct explanation of A.
- c) A is true, but R is false.
- d) A is false, but R is true.
Answer: a) Both A and R are true, and R is the correct explanation of A.
- Assertion (A): Metals are used to make cooking utensils. Reason (R): Metals are good conductors of heat.
- a) Both A and R are true, and R is the correct explanation of A.
- b) Both A and R are true, but R is not the correct explanation of A.
- c) A is true, but R is false.
- d) A is false, but R is true.
Answer: a) Both A and R are true, and R is the correct explanation of A.
- Assertion (A): Water pipes burst in very cold places. Reason (R): Water contracts when cooled below 4°C.
- a) Both A and R are true, and R is the correct explanation of A.
- b) Both A and R are true, but R is not the correct explanation of A.
- c) A is true, but R is false.
- d) A is false, but R is true.
Answer: c) A is true, but R is false. (Water expands when cooled below 4°C, causing pipes to burst.)
- Assertion (A): Convection currents cause land and sea breezes. Reason (R): Air above the land heats up faster than the air above the sea during the day.
- a) Both A and R are true, and R is the correct explanation of A.
- b) Both A and R are true, but R is not the correct explanation of A.
- c) A is true, but R is false.
- d) A is false, but R is true.
Answer: a) Both A and R are true, and R is the correct explanation of A.
- Assertion (A): Black objects absorb more heat than white objects. Reason (R): Black objects reflect more radiant energy than white objects.
- a) Both A and R are true, and R is the correct explanation of A.
- b) Both A and R are true, but R is not the correct explanation of A.
- c) A is true, but R is false.
- d) A is false, but R is true.
Answer: c) A is true, but R is false. (Black objects absorb more radiant energy, while white objects reflect more.)
High Order Thinking Skill (HOTS) Questions and Answers
- Why does a clinical thermometer have a kink in its capillary tube?
Answer: The kink prevents the mercury from falling back after measuring body temperature, allowing the reading to be taken conveniently without the mercury level dropping immediately.
- How do convection currents contribute to global wind patterns?
Answer: Uneven heating of the earth causes air at the equator to rise, creating low pressure. Cooler air from higher latitudes moves in, setting up convection currents that drive global wind patterns, like trade winds.
- Why do houses with tin roofs become very hot in summer?
Answer: Tin is a good conductor of heat, quickly transferring heat from the sun to the interior. Unlike insulators like bricks, tin does not shield the house, making rooms with tin roofs very hot in summer.
- How does the property of thermal expansion help in fitting a metal rim on a cart wheel?
Answer: The metal rim, slightly smaller than the wheel, is heated to expand due to thermal expansion. It slips onto the wheel easily, and upon cooling, it contracts, gripping the wheel firmly for a secure fit.
- Why do thermos flasks keep drinks hot or cold for a long time?
Answer: Thermos flasks have a double-walled structure with a vacuum or air layer, which is a poor conductor of heat. Their shining surfaces also absorb and radiate little heat, minimizing heat transfer and keeping drinks hot or cold.
Long Questions and Answers (50-60 words)
- Explain how conduction transfers heat in solids, with an example.
Answer: Conduction transfers heat in solids as molecules vibrate and pass energy to neighboring molecules without moving. For example, when one end of a metal rod is heated in a flame, the heat travels to the other end, making it hot, because metals are good conductors of heat.
- Describe the process of convection in liquids with an example.
Answer: Convection in liquids occurs when heated molecules become lighter, rise, and are replaced by cooler, heavier molecules. For example, in a flask of water with potassium permanganate crystals, heating causes colored streaks to rise and fall, showing convection currents as the water heats up uniformly.
- How does radiation differ from conduction and convection in heat transfer?
Answer: Radiation transfers heat without a medium, as radiant energy travels through space, like heat from the sun to earth. Conduction and convection require a medium—conduction transfers heat through vibrating molecules in solids, while convection involves the movement of molecules in liquids and gases, creating currents.
- Why are blankets effective in keeping us warm during winter?
Answer: Blankets trap a layer of air, a poor conductor of heat, between themselves and the body. This trapped air prevents body heat from escaping easily through conduction or convection, keeping us warm by reducing heat loss to the colder surroundings during winter.
- Explain the formation of sea breezes and their effect on coastal climates.
Answer: During the day, land heats up faster than the sea, causing air above the land to rise. Cooler air from the sea moves in, creating a sea breeze. This breeze helps maintain a more uniform temperature in coastal areas, leading to an equitable climate throughout the day and night.