Maharashtra Board Class 7 Science Chapter 9 Heat Solutions

Fill In The Blanks:

Question a. Maximum heat is absorbed by a ................ coloured object.
Answer: black
In simple words: Darker colored objects absorb more heat compared to lighter ones.

🎯 Exam Tip: Understanding how different colors absorb and reflect heat is crucial for explaining everyday phenomena.

Question b. ................ of heat does not require a medium.
Answer: Radiation
In simple words: Radiation is a way heat travels without needing any material between the source and receiver.

🎯 Exam Tip: Remember the three modes of heat transfer: conduction, convection, and radiation, and their unique characteristics.

Question c. Conduction of heat takes place through a ................ substance.
Answer: solid
In simple words: Conduction primarily occurs in solids where heat transfers directly through vibrating particles.

🎯 Exam Tip: Conduction requires direct contact and a medium, typically solid, for heat to transfer.

Question d. The shining surface in a thermos flask decreases the outgoing heat by ................
Answer: reflection
In simple words: The shiny inner surface of a thermos flask reflects heat back inwards, preventing its escape.

🎯 Exam Tip: The design of a thermos flask incorporates multiple methods (vacuum, reflection) to minimize heat transfer.

Question e. Cooking utensils are made from metals due to their property of ................
Answer: conduction
In simple words: Metals are good conductors, meaning they efficiently transfer heat to the food being cooked.

🎯 Exam Tip: Good conductors allow heat to spread quickly, which is why metals are chosen for cookware.

Question f. The earth receives heat from the sun by ................
Answer: radiation
In simple words: Heat from the sun travels through the vacuum of space to Earth via radiation.

🎯 Exam Tip: Radiation is the primary way heat travels through empty space, like from the sun to Earth.

2. What Will Absorb Heat?

Question a. What will absorb heat?
Steel spoon, wooden board, glass vessel, iron griddle (lava), glass, wooden spoon, plastic plate, soil, water, wax.
Answer: Following will absorb heat fast: steel spoon, iron griddle, water, wax. Remaining objects also absorb heat but very slowly.
In simple words: Objects like steel, iron, water, and wax absorb heat quickly, while others like wood and glass absorb it more slowly.

🎯 Exam Tip: Materials vary in their heat absorption rates; metals and liquids generally absorb faster than insulators like wood or glass.

3. Write Answers To The Following Questions.

Question a. How does a fever get lowered by putting a cold compress on the forehead of a patient?
Answer: The water in the cold compress will evaporate by the heat of fever thus taking some local heat of the body and temperature of the body is lowered, therefore the fever gets lowered by putting a cold compress on the forehead of a patient.
In simple words: A cold compress helps lower fever by absorbing heat from the body as the water evaporates.

🎯 Exam Tip: This illustrates the concept of evaporative cooling, where heat is removed from a surface during evaporation.

Question b. Why are the houses in Rajasthan painted white?
Answer: Rajasthan being a desert, temperature is very high. So there is excess heat. White colour doesn't absorb the heat but reflects back, so houses will not become hot due to excess heat and inside of the house will be cool. So the houses in Rajasthan are painted white.
In simple words: Houses in Rajasthan are painted white to reflect sunlight and heat, keeping the interiors cool in the hot desert climate.

🎯 Exam Tip: Light colors are good reflectors of heat, while dark colors are good absorbers, a principle useful in architectural design for different climates.

Question c. What are the modes of heat transfer?
Answer: There are three modes of heat transfer:
1. conduction
2. convection
3. radiation.
In simple words: Heat can transfer through conduction (direct contact), convection (fluid movement), or radiation (electromagnetic waves).

🎯 Exam Tip: Be able to define and give examples for each of the three modes of heat transfer.

Question d. Explain which mode of heat transfer causes sea breezes and land breezes?
Answer:
1. Unequal heating of air above the sea and land sets up convection currents which cause sea breezes during the day time.
2. The land near the sea is heated by sunlight to a higher temperature than the sea.
3. As the air above the land is heated, it becomes lighter and moves upwards, its place is then taken by cooler air moving from the sea towards the land.
4. Sea breezes are thus set up. At night, the land cools faster than water in the sea. Therefore air above sea water is hotter than air above the land.
5. So cooler air from the land moves towards the sea forming land breeze.
In simple words: Sea and land breezes are caused by convection, due to the unequal heating and cooling of land and sea, which creates air currents.

🎯 Exam Tip: Convection currents, driven by density differences in fluids, explain many natural phenomena like wind patterns.

Question e. Why is the outer coat of the penguins of Antarctica black?
Answer: The outer coat of the penguins of Antarctica is black because black colour absorbs heat so penguins get lots of heat due to this black colour and they can live easily in very, very cold Antarctica region.
In simple words: Penguins have black outer coats to absorb more sunlight and stay warm in the extremely cold Antarctic environment.

🎯 Exam Tip: Dark colors are effective heat absorbers, an adaptation used by animals in cold climates for survival.

Question f. Why are heaters fitted near the floor and air conditioners near the ceiling of a room?
Answer:
1. Due to air conditioners, air becomes cool so its density increases and it becomes heavy. So it goes down and warm air with less density comes up and take the place of cold air. This cycle goes on and on and the whole room becomes cool.
2. If it is fitted near the floor then cool air of high density can't go up and upper air with low density cannot come down and cooling of the room does not take place properly so air conditioners are fitted near the ceiling of a room.
3. Heaters are fitted near the floor. Due to heating, air becomes hot which is lighter than cold air. So it goes up and upper air with high density comes down.
4. This cycle goes on and on and the full room becomes warm. So heaters are fitted near the floor.
In simple words: Heaters are placed low and air conditioners high to optimize convection currents, allowing hot air to rise and cool air to sink for efficient temperature regulation.

🎯 Exam Tip: The placement of heating and cooling units is based on the principle of convection, where hotter, less dense air rises and cooler, denser air sinks.

4. Give Scientific Reasons.

Question a. An ordinary glass bottle cracks when boiling water is poured into it, but a borosil glass bottle does not.
Answer:
1. An ordinary glass bottle cracks when we pour boiling water into it because there is a lot of temperature difference between the inner and outer parts of the glass.
2. The inner part gets hot and expands while the outer part remains cold since glass is a bad conductor of heat.
3. This unequal expansion causes the crack of glass bottle.
4. But the borosil glass bottle has a very high melting point so the temperature difference is very less, hence it doesn't crack.
In simple words: Ordinary glass cracks due to uneven expansion when hot water is poured, unlike borosil glass which can withstand high temperature differences because it expands more uniformly.

🎯 Exam Tip: Differential thermal expansion is a key concept; materials with lower thermal expansion coefficients (like borosil) are more resistant to thermal shock.

Question b. The telephone wires which sag in summer become straight in winter.
Answer:
1. Telephone wires are made up of copper metal. Metals expand on heating.
2. In summer the copper wire expands, thus the length of the wire increases between the two poles and instead of remaining straight it sags.
3. In winter due to cold, it contracts and comes back to the original position, so it becomes straight in winter.
In simple words: Telephone wires sag in summer because they expand in heat and become straight in winter as they contract due to cold.

🎯 Exam Tip: This illustrates thermal expansion and contraction, a common property of materials like metals in response to temperature changes.

Question c. Dewdrops form on the grass in winter.
Answer:
1. During the winter, the temperature of air is low, its capacity to hold the vapour is less.
2. At such times the excess water vapour is transformed into water droplets due to cold. And so the dew drops form on the grass in winter.
In simple words: Dewdrops form in winter when cold air can hold less water vapor, causing excess vapor to condense into liquid droplets on surfaces like grass.

🎯 Exam Tip: Dew formation is a result of condensation, which occurs when moist air cools below its dew point, leading to the formation of liquid water.

Question d. In winter, why does an iron pillar feel colder than a wooden pole.
Answer:
1. Iron is a metal and metals are very good conductors of heat so it transfers heat at a much faster rate than wood.
2. That means, when we touch them, metals conducts away energy in the form of heat emitted by our hand and temperature of our hand lowers down and we get a cold- sensation.
3. This is what makes the metal appear colder than the wood.
4. The wood is taking away heat at a much slower rate.
In simple words: An iron pillar feels colder than a wooden pole in winter because iron is a much better conductor of heat, quickly drawing heat away from your hand.

🎯 Exam Tip: This demonstrates the difference between good conductors (like metals) and poor conductors/insulators (like wood) in transferring heat.

Fill In The Blanks:

Question 1. ................ is an indicator of the heat in a substance.
Answer: Temperature
In simple words: Temperature measures the degree of hotness or coldness of an object, indicating its heat content.

🎯 Exam Tip: Temperature is a fundamental concept in thermodynamics, representing the average kinetic energy of particles in a substance.

Question 2. ................ flows from higher temperature to lower temperature.
Answer: Heat
In simple words: Heat naturally moves from warmer areas to colder areas until thermal equilibrium is reached.

🎯 Exam Tip: Heat transfer always occurs from a region of higher temperature to one of lower temperature.

Question 3. Transfer of heat from one place to another takes place by ................ or ................
Answer: conduction, convection, radiation
In simple words: Heat can be transferred through conduction, convection, or radiation.

🎯 Exam Tip: These three modes cover all possible mechanisms of heat transfer, depending on the presence and type of medium.

Question 4. During conduction, particles of the substance ................ in their place.
Answer: vibrate
In simple words: In conduction, particles transfer heat by vibrating and colliding with neighboring particles without changing their position.

🎯 Exam Tip: Conduction relies on particle vibration and collision, not the movement of the particles themselves.

Question 5. In a convection current, warm air travels ................
Answer: upwards
In simple words: Warm air, being less dense, rises in a convection current.

🎯 Exam Tip: Convection involves the movement of fluids (liquids or gases) due to density changes caused by heating.

Question 6. A ................ is necessary for the conduction of heat.
Answer: medium
In simple words: Conduction requires a material medium for heat transfer to occur.

🎯 Exam Tip: Unlike radiation, conduction always needs a physical medium to transmit heat energy.

Question 7. The ................ of conduction of heat through a medium depends on the medium.
Answer: speed
In simple words: The rate at which heat conducts varies based on the type of material it passes through.

🎯 Exam Tip: Different materials have different thermal conductivities, affecting how quickly heat travels through them.

Question 8. Convection takes place only in ................ medium.
Answer: fluid
In simple words: Convection heat transfer occurs exclusively in fluids (liquids and gases) because it involves the actual movement of heated particles.

🎯 Exam Tip: Convection is characteristic of fluids, as it relies on the bulk movement of the medium itself.

Question 9. Transfer of heat in the absence of a medium is called ................
Answer: radiation
In simple words: Radiation is the transfer of heat through electromagnetic waves, even through empty space.

🎯 Exam Tip: Radiation is the only heat transfer mode that does not require a material medium.

Question 10. Substances through which heat travels easily and quickly are called ................
Answer: good conductors
In simple words: Materials that allow heat to pass through them quickly are known as good conductors of heat.

🎯 Exam Tip: Metals are prime examples of good thermal conductors due to their free electrons.

Question 11. Substances through which heat travels very slowly are called ................ of heat.
Answer: bad conductors
In simple words: Materials that resist the flow of heat and transfer it slowly are called bad conductors or insulators.

🎯 Exam Tip: Insulators like wood, plastic, or air are used to prevent or slow down heat transfer.

Question 12. Copper is a better ................ of heat than ................
Answer: conductor, iron
In simple words: Copper transfers heat more efficiently compared to iron, making it a superior thermal conductor.

🎯 Exam Tip: Different metals have varying thermal conductivities; copper is often chosen for applications requiring efficient heat transfer.

Question 13. ................ made first thermos flask in 1892.
Answer: Sir James Dewar
In simple words: The thermos flask was invented by Sir James Dewar in 1892 to keep substances at a constant temperature.

🎯 Exam Tip: Knowing the inventor of key scientific devices can sometimes be part of general science knowledge.

Question 14. Neither conduction nor convection of the heat occurs in thermos flask due to the ................
Answer: vacuum.
In simple words: The vacuum layer in a thermos flask prevents heat transfer by both conduction and convection.

🎯 Exam Tip: A vacuum is an excellent insulator because it lacks particles to conduct or convect heat.

Match The Following:

Question 1. Match the following:

Answer:

In simple words: This matching exercise connects terms related to heat transfer with their correct definitions or examples, such as copper being a good conductor and convection involving particle flow.

🎯 Exam Tip: Accurately distinguishing between good/bad conductors and the mechanisms of conduction and convection is key to understanding heat transfer.

Question 2. Match the following:

Answer:

In simple words: This matching exercise links heat transfer concepts like radiation (no medium), black bodies (absorb more heat), vibration (conduction), and paper lanterns (convection currents) to their descriptions.

🎯 Exam Tip: Understanding how each term relates to heat transfer mechanisms and their properties is vital for comprehensive knowledge.

Say Whether True Or False, Correct And Rewrite The False Statements:

Question 1. Glass is a bad conductor of heat.
Answer: True
In simple words: Glass does not transfer heat easily, making it an insulator.

🎯 Exam Tip: Differentiate between materials based on their thermal conductivity for true/false questions.

Question 2. In a thermos flask, hot things remain hot and cold things become warm.
Answer: False. In a thermos flask, hot things remain hot and cold things remain cold.
In simple words: A thermos flask is designed to maintain the temperature of its contents, keeping hot things hot and cold things cold.

🎯 Exam Tip: The purpose of a thermos flask is to minimize heat exchange with the surroundings, maintaining the initial temperature.

Question 3. Radiation of heat depends upon the colours of the outer surface of the body.
Answer: True
In simple words: Darker surfaces absorb and emit more radiant heat, while lighter surfaces reflect it.

🎯 Exam Tip: Surface color significantly influences how much heat is absorbed or emitted through radiation.

Question 4. During radiation, the particles of the medium vibrate.
Answer: True
In simple words: (Based on the given answer, radiation is stated to involve vibrating particles, though scientifically it doesn't require a medium.)

🎯 Exam Tip: Pay close attention to the definition of radiation, which is unique among heat transfer methods as it does not rely on a medium.

Question 5. Copper is better conductor of heat than iron.
Answer: True
In simple words: Copper allows heat to flow through it more effectively than iron.

🎯 Exam Tip: Understanding the relative thermal conductivities of common metals is useful in material selection scenarios.

Question 6. During conduction, particles of the conductor leave their original place.
Answer: False. During conduction, particles of the conductor do not leave their original place, but only vibrate in their own place.
In simple words: In conduction, heat is transferred through vibrations of particles, not by their physical movement from their fixed positions.

🎯 Exam Tip: Conduction is characterized by particle vibration, while convection involves the actual displacement of heated particles.

Question 7. Hot water is heavier than cold water.
Answer: False - Hot water is lighter than cold water.
In simple words: When water is heated, it expands and becomes less dense, meaning hot water is lighter than an equal volume of cold water.

🎯 Exam Tip: Understanding density changes with temperature is crucial for explaining phenomena like convection currents.

Question 8. A convection current always travels down wards.
Answer: False – A convection current always travels upwards.
In simple words: In a convection current, warm, less dense fluid rises, while cooler, denser fluid sinks, creating an upward flow for the warm part.

🎯 Exam Tip: Convection currents are driven by buoyancy, where heated fluids rise due to reduced density.

Question 9. Every warm substance continuously emits heat in all directions around itself.
Answer: True
In simple words: All objects above absolute zero temperature continuously radiate thermal energy in all directions.

🎯 Exam Tip: Thermal radiation is a continuous process for all objects with a temperature above absolute zero.

Question 10. Air is a bad conductor of heat.
Answer: True
In simple words: Air is a poor conductor of heat, which is why it's often used as an insulating layer.

🎯 Exam Tip: The insulating property of air is utilized in many everyday applications, like double-pane windows or wool clothing.

Define The Following:

Question. Define the following:
1. Conduction
2. Convection
3. Radiation
4. Transfer of heat.
Answer:
1. Conduction: The transfer of heat from one end of a solid substance to the other, without the particles leaving their original place is called conduction.
2. Convection: Transfer of heat in fluids due to formation of currents is called convection.
3. Radiation: Transfer of heat in all directions in the absence of a medium is called radiation.
4. Transfer of heat: The flow of heat from one place to another is called transfer of heat.
In simple words: These definitions clarify the four basic concepts: conduction (heat through stationary particles), convection (heat through fluid movement), radiation (heat through waves without a medium), and the general process of heat transfer.

🎯 Exam Tip: A clear understanding of these fundamental definitions is essential for any topic related to heat and thermodynamics.

Give Scientific Reasons:

Question 1. Cricketers wear white clothes when playing in the sun.
Answer: White clothes do not absorb heat but reflect back therefore cricketers wear white clothes while playing in the sun.
In simple words: Cricketers wear white clothes because white reflects sunlight and heat, helping them stay cooler during matches under the sun.

🎯 Exam Tip: The principle of light-colored objects reflecting heat is a practical application in clothing choices for warm climates.

Question 2. Sawdust is a bad conductor of heat.
Answer: Sawdust is a bad conductor of heat because heat travels very slowly through it.
In simple words: Sawdust acts as a poor heat conductor, primarily because of the air trapped within its particles, which insulates against heat flow.

🎯 Exam Tip: Insulating materials often work by trapping air, which is a poor conductor of heat, thereby reducing heat transfer.

Question 3. We can feel the heat of table lamp under it.
Answer: We can feel the heat of table lamp under it because every warm substance continuously emits heat in all directions around itself by radiation.
In simple words: We feel the warmth from a table lamp primarily due to the infrared radiation it emits, which travels through the air and is absorbed by our skin.

🎯 Exam Tip: Direct heat transfer from a lamp is largely due to radiation, which can travel through air and even a vacuum.

Question 4. Medium is necessary for conduction.
Answer: Conduction is the transfer of heat from one end of a substance to another. Transfer of heat has to be done by particles of a substance and particles of a medium. Hence, a medium is necessary for conduction.
In simple words: Conduction requires a material medium because it relies on the direct contact and vibration of particles to transfer heat energy.

🎯 Exam Tip: Emphasize that particles must be present and in contact for conduction to occur.

Question 5. Explain the structure and functioning of thermoware containers.
Answer:
1. Thermoware containers keep food hot. These containers consist two boxes fitted one inside the other.
2. The outer box is made up of plastic while the inner one is made up of a shiny metal, there is air between the two boxes.
3. Air is a bad conductor of heat. Thus, heat is not lost by either conduction or convection. Also, the shiny inner surface prevents heat loss due to radiation.
4. The container has a plastic lid. This also prevents any transfer of heat.
In simple words: Thermoware containers use multiple layers, including air as an insulator and a shiny inner surface for reflection, to minimize heat loss through conduction, convection, and radiation.

🎯 Exam Tip: The design of thermoware effectively tackles all three modes of heat transfer to maintain temperature.

Question 6. What is an infrared camera? Write its uses.
Answer:
1. Radiation of heat takes place from many objects in nature such as trees, mountains, stones and roads.
2. A camera has been developed which uses these radiations to make our surroundings visible even at night. This is called infrared camera.
3. It is used to keep watch on the movements of the enemy during the night.
4. It is also used to track the movements of animals in the wild.
In simple words: An infrared camera detects heat radiation emitted by objects, allowing us to see in the dark, and is used for surveillance and tracking wildlife.

🎯 Exam Tip: Infrared technology is an application of thermal radiation, enabling vision based on heat signatures rather than visible light.

Question 7. Why is mercury used in a thermometer?
Answer: Mercury is used in a thermometer because of the following properties.
1. There is a big difference between the freezing point and boiling point of mercury. Its freezing point is - 39° C and its boiling point is 357° C.
2. It is shiny and can be easily seen.
3. It does not stick to glass.
4. It expands regularly and uniformly in all conditions.
In simple words: Mercury is ideal for thermometers due to its wide temperature range, clear visibility, non-stick properties, and consistent expansion.

🎯 Exam Tip: The specific physical properties of mercury make it highly suitable for accurate temperature measurement across a broad spectrum.

Question 8. Write the difference between Conduction, Convection and Radiation.
Answer:

In simple words: Conduction transfers heat through direct particle contact in solids, convection uses fluid movement in liquids/gases, and radiation transmits heat via electromagnetic waves without any medium.

🎯 Exam Tip: Be able to clearly distinguish these three modes based on their medium requirements, mechanisms, and speed for comparative analysis.

Question 9. Draw and explain about the Thermos flask or Dewar flask.
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र एक थर्मस फ्लास्क की आंतरिक संरचना को दर्शाता है, जिसे ड्यूअर फ्लास्क भी कहते हैं। इसमें दोहरी दीवार वाली बोतल होती है जिसके बीच में वैक्यूम (निर्वात) होता है, जो गर्मी के संवहन और चालन को रोकता है। आंतरिक सतह चमकदार (सिल्वर-लेपित) होती है ताकि विकिरण द्वारा गर्मी के नुकसान को कम किया जा सके, जिससे तरल पदार्थ का तापमान लंबे समय तक बना रहता है।
Principle:
1. As there is no medium in the vacuum between the two bottles, no transfer of heat can take place by conduction or convection.
In simple words: A thermos flask keeps liquids hot or cold by creating a vacuum and using shiny surfaces to prevent heat transfer through conduction, convection, and radiation.

🎯 Exam Tip: The thermos flask is a classic example of applying principles of heat insulation to minimize heat exchange with the environment.

Draw Neat Labelled Diagrams Of:

Question 1. The relation between density and convection
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र घनत्व और संवहन के बीच संबंध को दर्शाता है। इसमें एक परखनली में पानी उबाला जा रहा है, जिसमें नीचे बर्फ का एक टुकड़ा तार की जाली में लपेटा हुआ है और ऊपर पानी उबल रहा है, जो गर्म पानी के ऊपर उठने और ठंडे पानी के नीचे आने से संवहन धाराओं को प्रदर्शित करता है। एक बर्नर परखनली को गर्म कर रहा है, जबकि एक टेस्ट ट्यूब होल्डर परखनली को पकड़े हुए है। In simple words: This diagram illustrates how convection currents are formed due to density differences when water is heated, with warm, less dense water rising and cooler, denser water sinking.

🎯 Exam Tip: When explaining diagrams, ensure all key components are mentioned and their roles in the demonstrated phenomenon are clearly described for full marks.

Question 2. Conduction of heat
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र ऊष्मा के चालन को दर्शाता है। इसमें एक तिपाई पर धातु की पट्टियाँ रखी गई हैं, जिन पर मोम के धब्बे के साथ पिन लगी हुई हैं। एक बर्नर एक सिरे से धातु की पट्टी को गर्म कर रहा है, जिससे ऊष्मा चालन द्वारा पट्टी में फैलती है, और जैसे-जैसे पट्टी गर्म होती है, मोम पिघलता जाता है, जिससे पिन एक के बाद एक गिरती जाती हैं, यह सिद्ध करता है कि ऊष्मा एक सिरे से दूसरे सिरे तक स्थानांतरित होती है। In simple words: This diagram shows how heat conducts through a metal strip, causing pins attached with wax to fall sequentially, demonstrating the transfer of heat from the heated end to the cooler parts.

🎯 Exam Tip: For conduction experiments, clearly identify the heat source, the conducting material, and the indicators of heat transfer to score well.

Question 3. Convection of heat
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र ऊष्मा के संवहन को दर्शाता है। एक बीकर में पानी को एक बर्नर के ऊपर रखा गया है, और पोटेशियम परमैंगनेट के क्रिस्टल नीचे रखे गए हैं। जैसे ही बर्नर पानी को गर्म करता है, गर्म पानी के अणु ऊपर उठते हैं और ठंडे पानी के अणु नीचे आते हैं, जिससे संवहन धाराएँ बनती हैं, और पोटेशियम परमैंगनेट के रंगीन कण इन धाराओं को दृश्यमान बनाते हैं। In simple words: This diagram illustrates convection in a liquid where warm water rises and cooler water sinks, forming currents, made visible by potassium permanganate crystals.

🎯 Exam Tip: Focus on explaining the movement of fluid particles due to temperature differences and how it leads to heat transfer via convection currents.

Question 4. Expansion and contraction of a liquid
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र एक तरल पदार्थ के विस्तार और संकुचन को दर्शाता है। एक कोनिकल फ्लास्क में पानी भरा है और उसमें एक थर्मामीटर लगा हुआ है, जिसकी खोखली कांच की नली पानी में डूबी हुई है। फ्लास्क को एक स्टैंड पर रखे बर्नर से गर्म किया जाता है, जिससे पानी का तापमान बढ़ता है और थर्मामीटर में तरल का स्तर ऊपर उठता है, जो तरल के विस्तार को दर्शाता है। In simple words: This diagram demonstrates how a liquid (water) expands when heated, causing its volume to increase, which is observed by the rising level in a connected thermometer tube.

🎯 Exam Tip: Emphasize the direct relationship between heating a liquid and its expansion, as measured by the change in volume indicated by the liquid level.

Question 5. Thermos flask
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र एक थर्मस फ्लास्क की संरचना को दर्शाता है, जिसमें गर्म या ठंडा तरल पदार्थ रखने के लिए एक आंतरिक कांच की बोतल होती है जिसकी सतह चांदी की होती है। आंतरिक और बाहरी जार (धातु/प्लास्टिक) के बीच एक वैक्यूम होता है, जो ऊष्मा के संवहन और चालन को रोकता है, और एक रबर सपोर्ट व स्पंज बोतल को सहारा देते हैं। एक स्प्रिंग और ढक्कन संरचना को पूरा करते हैं, जिससे ऊष्मा का स्थानांतरण न्यूनतम होता है। In simple words: This diagram shows the components of a thermos flask, highlighting the vacuum and silvered surfaces that minimize heat transfer by conduction, convection, and radiation, thus keeping its contents at a stable temperature.

🎯 Exam Tip: When explaining the thermos flask, focus on how each structural component (vacuum, silvered surface, stopper) prevents different modes of heat transfer (conduction, convection, radiation).

Can You Tell?

Answer The Following Questions:

Question 1. Why does the halwai wrap up cloth around the end of his slotted spoon while stirring the boiling milk in his large kadhai?
Answer: The halwai wrap up cloth around the end of his slotted spoon while stirring the boiling milk in his large kadhai because (i) His slotted spoon is made up of metal and metal is a good conductor of heat so due to constant heating spoon gets very hot which makes hand to get burnt, (ii) But cloth is a bad conductor of heat so it does not allow heat to reach till hand and halwai does not feel hot when he stirs the boiling milk. In simple words: The halwai wraps cloth around the metal spoon handle because metal conducts heat well, making the spoon hot, while the cloth acts as an insulator, preventing heat from reaching his hand and causing burns.

🎯 Exam Tip: This question tests understanding of heat conduction and insulation. Highlight the contrast between the metal's high conductivity and the cloth's low conductivity.

Question 2. Why do we hold a steel glass in a handkerchief while drinking hot milk from it?
Answer:
1. Steel is metal and it is a good conductor of heat. So due to hot milk it becomes hot and it is difficult to hold. Person's hand get burnt due to it.
2. So it is better to hold in a handkerchief because handkerchief is made up of cloth which is a bad conductor of heat. So your hand does not feel hot. In simple words: We hold a hot steel glass with a handkerchief because steel is a good heat conductor, making the glass hot, while the cloth acts as a poor conductor, protecting our hands from the heat.

🎯 Exam Tip: Emphasize the concept of materials as good or bad conductors of heat and their practical application in daily life to protect from burns.

Use Your Brain Power!

Question 1. Why do we wear woollen clothes in winter?
Answer: We wear woollen clothes in winter because
1. Air is a poor conductor of heat and woollen clothes do not allow the transfer of body heat.
2. Therefore we do not feel cold in woollen clothes and get protected from cold in winter. In simple words: We wear woolen clothes in winter because they trap air, which is a poor conductor of heat, thus preventing our body heat from escaping and keeping us warm.

🎯 Exam Tip: When explaining the function of wool, focus on its ability to trap air as the primary reason for its insulating properties.

Question 2. Why do we use white clothes in summer and dark or black clothes in winter?
Answer:
1. White clothes do not absorb heat, so in summer, weather is very hot and white clothes protect us from heat whereas in winter it is very cold so we need more heat.
2. Dark or black clothes absorb more heat so we get protection from cold. In simple words: White clothes reflect most sunlight, keeping us cool in summer, while dark clothes absorb more sunlight, keeping us warm in winter.

🎯 Exam Tip: Remember that darker colors are good absorbers and emitters of heat, while lighter colors are good reflectors, which is key to answering this type of question.

Question 3. Why is there a gap at the joints of rails and of cement concrete bridges?
Answer:
1. The joints of rails and cement concrete bridges are made of iron. Metal is a good conductor of heat so in summer due to heat it expands and its length will be increased.
2. To allow for this expansion, gaps are kept.
3. If these gaps are not kept, then after expansion the rails and cement concrete would have buckled upwards due to lack of space.
4. Due to that bridges would crack and railway tracks get crooked. In simple words: Gaps are left at rail and bridge joints to allow for thermal expansion during hot weather; without them, the structures would buckle or crack due to increased length.

🎯 Exam Tip: This question highlights the practical application of thermal expansion. Focus on the consequence of not leaving gaps (buckling, cracking) to demonstrate full understanding.

Question 4. Why is mercury or alcohol used in a thermometer?
Answer: Mercury is used in a thermometer because of the following properties.
Alcohol is used in a thermometer for the following properties.
1. It can measure a low temperature because its freezing point is low. (-115°C) but it can not measure higher temperature as its boiling point is low.
2. It can measure the temperature precisely because it has large expansion coefficient.
3. It's expansion is regular.
4. Less hazardous. In simple words: Mercury is used for its wide temperature range, good visibility, and uniform expansion, while alcohol is used for measuring very low temperatures due to its low freezing point and high expansion coefficient.

🎯 Exam Tip: Differentiate the specific properties of mercury (wide range, visibility) and alcohol (low freezing point, high expansion) that make them suitable for thermometers.

Question 5. Why hot substance does not remain as hot after two or three hours?
Answer: A little heat is lost from around the lid and by a small amount of conduction through the vessel. So hot substance does not remain as hot after two or three hours. In simple words: Hot substances gradually lose heat to the surroundings through conduction and some radiation from the lid and vessel, causing their temperature to decrease over time.

🎯 Exam Tip: Always mention all three modes of heat transfer (conduction, convection, radiation) when discussing heat loss, even if some are minor.

Diagram Based Questions.

Observe The Diagram And Answer The Given Questions Below It.

1.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र घनत्व और संवहन के बीच संबंध को दर्शाता है। इसमें एक परखनली में पानी उबाला जा रहा है, जिसमें नीचे बर्फ का एक टुकड़ा तार की जाली में लपेटा हुआ है और ऊपर पानी उबल रहा है, जो गर्म पानी के ऊपर उठने और ठंडे पानी के नीचे आने से संवहन धाराओं को प्रदर्शित करता है। एक बर्नर परखनली को गर्म कर रहा है, जबकि एक टेस्ट ट्यूब होल्डर परखनली को पकड़े हुए है।

Question a. What is the diagram about?
Answer: The diagram is showing us the relation between density and convection. In simple words: The diagram illustrates the relationship between density and convection, showing how heat transfer occurs through fluid movement based on density changes.

🎯 Exam Tip: Clearly identifying the core concept demonstrated by a diagram is crucial; in this case, it's the link between density differences and convection.

Question b. Why is the ice wrapped in wire gauze?
Answer: Ice floats on water, to make it sink, it is wrapped in wire gauze. In simple words: Ice is wrapped in wire gauze to make it sink to the bottom of the test tube, allowing the top layer of water to be heated and demonstrate convection currents without the ice melting quickly.

🎯 Exam Tip: Explain the practical reason behind experimental setups. Here, the gauze addresses the buoyancy of ice, ensuring the experiment's objective can be met.

Question c. What does this experiment prove?
Answer: When a liquid is heated, its density decreases and it rises upwards. This leads to the convection currents which makes the heated liquid rise upwards and cooler liquid takes it place. In simple words: This experiment proves that when a liquid is heated, its density decreases, causing it to rise, while cooler, denser liquid sinks, thereby creating convection currents.

🎯 Exam Tip: When stating what an experiment proves, use clear, concise language to summarize the scientific principle demonstrated.

2.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र एक ठोस पदार्थ के विस्तार और संकुचन को दर्शाता है। इसमें एक धातु की गेंद को एक रिंग से पहले और बाद में गुजारा जाता है। 'पहले गर्म करने' वाले भाग में गेंद रिंग से आसानी से गुजर जाती है, जबकि 'गर्म करने के बाद' वाले भाग में, गर्म करने से गेंद का आकार बढ़ जाता है, जिससे वह रिंग से नहीं गुजर पाती, जो ठोस के विस्तार को दर्शाता है।

Question a. What is the diagram about?
Answer: Expansion and contraction of a solid substance. In simple words: This diagram illustrates the phenomenon of thermal expansion and contraction in solid substances, showing how their size changes with temperature.

🎯 Exam Tip: Accurately identifying the physical phenomenon (thermal expansion/contraction) is the main point for this question.

Question b. Does the same thing happen in liquids and gases?
Answer: Yes. Generally substances solids, liquids and gases expand on heating and contract on cooling. In simple words: Yes, generally, all states of matter-solids, liquids, and gases-expand when heated and contract when cooled, though to varying degrees.

🎯 Exam Tip: Remember the general rule that most substances expand when heated and contract when cooled, regardless of their state of matter.

Question c. Give one example of this property in everyday life.
Answer: Gaps are present in between two railway line track joints so that they do not bend due to expansion. In simple words: An everyday example is the gaps left between railway tracks to prevent them from bending or buckling when they expand due to heat.

🎯 Exam Tip: Provide practical, real-world examples that clearly demonstrate the concept being asked, such as the expansion joints in bridges or railway tracks.

3.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र एक थर्मस फ्लास्क की संरचना को दर्शाता है, जिसमें गर्म या ठंडा तरल पदार्थ रखने के लिए एक आंतरिक कांच की बोतल (1) होती है जिसकी चांदी की सतह (4) होती है। आंतरिक और बाहरी जार (धातु/प्लास्टिक) (2) के बीच एक वैक्यूम होता है, जो ऊष्मा के संवहन और चालन को रोकता है, और एक स्प्रिंग (3) तथा रबर सपोर्ट बोतल को सहारा देते हैं।

Question a. Label parts 1 -4
Answer:
1. Vacuum
2. metal/plastic jar
3. Spring
4. silvered surface of the glass. In simple words: The labels identify key parts of the thermos flask: 1 is the vacuum, 2 is the outer metal/plastic jar, 3 is the spring, and 4 is the silvered surface of the inner glass.

🎯 Exam Tip: Precisely match each label to its corresponding part in the diagram for accuracy. Understanding the function of each part helps in correct identification.

Question b. What is the principle of this object?
Answer: As there is no medium in the vacuum between the two bottles, no transfer of heat takes place due to conduction and convection and radiated heat is reflected back by the shiny surface. In simple words: The thermos flask works on the principle of minimizing all three modes of heat transfer (conduction, convection, and radiation) by using a vacuum, reflective surfaces, and poor conductors.

🎯 Exam Tip: When stating the principle, ensure you mention how the design addresses all three modes of heat transfer to explain its effectiveness.

Question c. State the uses of the given object.
Answer: The temperature of substances inside it are maintained for some time. Hot things remain hot and cold things cold for 2 – 3 hours. In simple words: The thermos flask is used to keep hot beverages hot and cold beverages cold for extended periods by preventing heat exchange with the surroundings.

🎯 Exam Tip: Clearly state the primary function: maintaining the temperature of contents by insulating them from external heat exchange.

Question d. What is an infrared camera?
Answer: It is a camera which uses radiations to make our surroundings visible at night. Using this camera, it is possible to keep a watch on the movements of the enemy during the night. In simple words: An infrared camera detects thermal radiation emitted by objects, allowing it to create images and make surroundings visible even in complete darkness.

🎯 Exam Tip: Highlight that infrared cameras detect heat radiation, not visible light, and provide a practical application to show understanding.

Question e. What is thermoware?
Answer: Thermoware are object which do not allow heat to escape from them. So things stay warm in them. eg. hot pot, thermos flask. In simple words: Thermoware refers to insulated containers designed to minimize heat transfer, keeping food and liquids warm or cold for longer durations.

🎯 Exam Tip: Define thermoware by its function (insulation) and provide relevant examples to solidify the explanation.