Heat Questions and Answers – Page 4

This question tests knowledge of all four temperature scales simultaneously. Option A (100°C, 212°F, 80°R, 373 K) is CORRECT — the boiling point of pure water at standard atmospheric pressure (1 atm) is: Celsius = 100°C (upper fixed point, 100 divisions above freezing), Fahrenheit = 212°F (upper fixed point, 180 divisions above 32°F), Reamur = 80°R (upper fixed point, 80 divisions above 0°R), Kelvin = 373.15 K ≈ 373 K (using K = C + 273 = 100 + 273 = 373). Option B (100°C, 180°F, 80°R, 373 K) is incorrect — 180°F is the DIFFERENCE between boiling and freezing on the Fahrenheit scale (212-32=180), NOT the boiling point itself. The boiling point in Fahrenheit is 212°F. Option C (0°C, 32°F, 0°R, 273 K) is incorrect — these represent the FREEZING point of water across all scales, not the boiling point. Students frequently confuse the fixed points of different scales. Option D (100°C, 212°F, 100°R, 373 K) is incorrect — 100°R is wrong. The Reamur scale has only 80 divisions (not 100), so the boiling point is 80°R. The 100 belongs to Celsius (100 divisions), not Reamur. For exams: Boiling = 100°C = 212°F = 80°R = 373K; Freezing = 0°C = 32°F = 0°R = 273K.

Normal human body temperature in Kelvin is derived from the standard Celsius value using the Kelvin conversion formula. Option A (273 K) is incorrect — 273 K corresponds to 0°C, which is the freezing point of water. A person at 273 K (0°C) would be frozen solid and clinically dead. Option B (300 K) is incorrect — 300 K = 27°C, which is approximately normal room temperature in warm conditions. While 300 K is used as a convenient approximate room temperature in many physics problems, it is not the normal human body temperature. Option C (310 K) is CORRECT — the normal human body temperature is 37°C. Converting to Kelvin: K = C + 273 = 37 + 273 = 310 K. Therefore, 37°C = 98.6°F = 310 K. The image conversion table specifically shows body temperature as 37°C = 98.6°F = 310 K. Option D (373 K) is incorrect — 373 K corresponds to 100°C, which is the boiling point of water, not human body temperature. A person at 373 K (100°C) would be at boiling water temperature — instantly fatal. For exams: Body temp = 37°C = 98.6°F = 310 K.

This is a very popular question in competitive exams related to latent heat. Option A (Steam has higher temperature than water) is incorrect — both steam and boiling water are at the same temperature: 100°C (at standard atmospheric pressure). There is no temperature difference between them. If the temperature were higher for steam, the question would be trivial and uninteresting. Option B (Steam contains more latent heat which it releases on condensing) is CORRECT — the image states: 100°C की भाप द्वारा उत्पन्न जलन 100°C के पानी की अपेक्षा अधिक गंभीर होती है क्योंकि भाप में गुप्त ऊष्मा अधिक होती है। When steam at 100°C touches the skin, it first condenses to water at 100°C, releasing the latent heat of vaporization (540 cal/g = 2260 J/g). This additional energy is transferred to the skin, causing more severe burns than boiling water at the same temperature. Boiling water only transfers its sensible heat as it cools. Option C (Steam is lighter and spreads more quickly) is incorrect — the spreading of steam has no direct relation to the severity of burns in this context. The physics explanation is entirely based on latent heat. Option D (Steam has higher pressure than boiling water) is incorrect — at 100°C and 1 atm, steam pressure equals atmospheric pressure. Steam in this context is not under

Heat transfer methods differ fundamentally in whether they require a medium and whether particles physically move. Option A (Conduction) is incorrect — conduction requires a medium but the particles do NOT physically move from their positions. In conduction, energy passes from particle to particle through vibrations, like a chain of balls passing a push without the balls themselves moving positions. Option B (Radiation) is incorrect — radiation does NOT require a medium and involves no movement of particles. Radiation travels as electromagnetic waves through vacuum. Option C (Convection) is CORRECT — convection requires a medium AND involves the actual physical movement of heated particles from one place to another. When a liquid or gas is heated, the heated portion becomes less dense, rises, the cooler denser portion sinks, and a convection current is established. The particles themselves physically move, carrying thermal energy with them. Option D (All three methods) is incorrect — only convection satisfies both conditions (needs medium AND particles move). Conduction needs medium but no particle movement. Radiation needs no medium and no particle movement.

The Sun is approximately 150 million kilometres (1.5 × 10⁸ km) from Earth, and the space between them is essentially a vacuum. Option A (Conduction) is incorrect — conduction requires direct physical contact or a connected material medium. Since there is vacuum between the Sun and Earth, conduction cannot occur. You cannot conduct heat through empty space. Option B (Convection) is incorrect — convection requires a fluid medium (gas or liquid) through which heated molecules can physically move. Since space between the Sun and Earth is a vacuum (no matter), convection is impossible. Option C (Radiation) is CORRECT — the Sun's heat reaches Earth through radiation. Radiation travels as electromagnetic waves (infrared, visible light, ultraviolet) and does not require any material medium. The Sun's radiation takes approximately 8 minutes 20 seconds to reach Earth. Option D (All three methods) is incorrect — only radiation can travel through the vacuum of space between the Sun and Earth. Conduction and convection are impossible in vacuum.

The requirement of a medium distinguishes radiation from the other two heat transfer methods. Option A (Conduction) is incorrect — conduction absolutely requires a material medium. It works by direct contact between particles, so without a medium (like in vacuum), conduction cannot occur at all. Option B (Convection) is incorrect — convection requires a fluid medium (liquid or gas). Convection works through the physical movement of fluid molecules carrying heat from one place to another. Without a medium, convection is impossible. This is why there are no convection currents in space. Option C (Both conduction and convection) is incorrect — as explained above, both conduction and convection require a material medium. Option D (Radiation) is CORRECT — radiation does not require any medium for heat transfer. The image states: निर्वात में ऊष्मा का संचरण किस विधि द्वारा होता है — विकिरण विधि (Radiation) द्वारा (ऊष्मा का संचरण बिना किसी माध्यम के प्रकाश की वेग से सीधी रेखा में होता है). Radiation travels in the form of electromagnetic waves at the speed of light (3 × 10⁸ m/s) in a straight line. The Sun's heat reaches Earth through 150 million km of space (vacuum) by radiation. For exams: Radiation = no medium needed = can travel through vacuum = travels at speed of light.

The heating of Earth's atmosphere is a classic example tested in competitive exams. Option A (Conduction) is incorrect — air is a poor conductor of heat. The atmosphere cannot primarily heat itself by conduction because air molecules are not closely packed, making conduction negligible over large scales. Option B (Radiation) is incorrect — while the Sun heats the Earth's surface by radiation, the heating of the atmosphere itself (air above the surface) is not primarily by radiation from the surface directly. Option C (Convection) is CORRECT — Earth's atmosphere is heated primarily by convection. The sequence is: (1) Sun's radiation heats Earth's surface. (2) The surface heats the air in contact with it by conduction and radiation. (4) Cooler air from above sinks to replace it. (5) This continuous cycle of rising warm air and sinking cool air is convection. This creates all weather patterns, winds, monsoons, and atmospheric circulation. Option D (All of the above) is incorrect — while all three modes play some role, the image clearly identifies convection as the primary method of atmospheric heating.

Heat transfer in different states of matter follows different dominant mechanisms. Option A (Conduction) is incorrect — while conduction can occur in liquids and gases, the image specifically states that conduction of heat in liquids and gases by this method is very poor (द्रवों व गैसों में ऊष्मा का संचरण इस विधि द्वारा बहुत कम होता है). This is because in liquids and gases, molecules are not tightly packed like in solids, making direct particle-to-particle vibration transfer inefficient. Option B (Convection) is CORRECT — heat in liquids and gases primarily transfers by convection. The image states: गैसों व द्रवों में ऊष्मा का संचरण किस विधि द्वारा होता है — संवहन विधि (Convection) द्वारा. In convection, heated portions of fluid become less dense, rise up, and cooler portions sink, creating convection currents that transfer heat throughout the fluid. Option C (Radiation) is incorrect — radiation can occur through liquids and gases, but it is not the primary mechanism of heat transfer in fluids. Radiation is the primary mechanism in vacuum or when no medium is present. Option D (All three equally) is incorrect — the three modes are not equally dominant in liquids and gases. Convection is overwhelmingly dominant in fluids.

Conduction is one of the three methods of heat transfer and has specific characteristics. Option A (Actual movement of heated molecules occurs) is incorrect — this describes CONVECTION, not conduction. In convection, heated fluid molecules actually move from one place to another, carrying heat with them. Option B (No medium is required) is incorrect — this describes RADIATION. Radiation is the only method that does not require a material medium and can travel through vacuum. Conduction definitely requires a material medium (solid, liquid, or gas). Option C (A medium is required and particles do not move from their positions) is CORRECT — conduction requires a material medium and works by transferring energy from one particle to its neighbor through vibrations, without the particles physically moving from their positions. The image specifically states: ऊष्मा के संचरण के लिए माध्यम की आवश्यकता होती है and माध्यम के तत्व कणों का स्थान परिवर्तित नहीं होता। Conduction is most efficient in solids (especially metals) and least efficient in liquids and gases. Option D (Heat travels at speed of light) is incorrect — this describes radiation. Electromagnetic radiation (including heat radiation) travels at the speed of light (~3 × 10⁸ m/s) in vacuum, but this is a property of radiation, not conduction.

Thermal insulators are materials through which no heat conduction takes place at all (or negligibly so). Option A (Water) is incorrect — water is listed as a good conductor of heat in the study material. Water has relatively high thermal conductivity compared to most non-metals and is used as a coolant in engines and nuclear reactors. The human body also contains water and is listed among conductors. Option B (Aluminium) is incorrect — aluminium is a metal and all metals are good conductors of heat. Aluminium is extensively used in heat exchangers, cookware, and aerospace applications due to its high thermal conductivity. Option C (Ebonite) is CORRECT — ebonite (hard vulcanized rubber) is a thermal insulator. The image specifically classifies thermal insulators (ऊष्मारोधी) as materials through which heat conduction does not occur at all. Examples given include ebonite (एबोनाइट) and asbestos (एस्बेस्टस). Ebonite is also an excellent electrical insulator. Option D (Iron) is incorrect — iron is a metal and therefore a good conductor of heat. For exams: Ebonite and Asbestos are the key examples of thermal insulators asked in SSC, Railway, and UPSC exams.