Chapter 3 — Heat, temperature and sound
Temperature is a state; heat is energy in transfer
Temperature describes a body's thermal state and helps predict the direction of heat transfer. Heat is energy transferred because of a temperature difference. If a hot metal spoon is placed in cooler water, energy flows from spoon to water until they approach thermal equilibrium. The spoon does not “contain heat” in the same way a bottle contains water; its particles have internal energy, and heat names the transfer caused by the difference. This wording matters in questions that offer “temperature,” “heat” and “internal energy” as competing options.
The SI unit for thermodynamic temperature is the kelvin (K). A Celsius temperature of 0 °C corresponds to 273.15 K by definition of the scale relation. Thus K = °C + 273.15 exactly. School-level MCQs sometimes direct the candidate to use +273 for a rounded integer answer: 27 °C then becomes 300 K under that stated approximation, while the exact conversion is 300.15 K. Follow the instruction in the item and do not silently claim that 273.15 equals 273. A change of 1 °C has the same size as a change of 1 K; the zeros of the two scales differ.
The Fahrenheit scale can be related to Celsius by F = (9/5)C + 32. At 0 °C the Fahrenheit reading is 32 °F; at 100 °C it is 212 °F under standard reference conditions commonly used in school questions. Solve the conversion in the correct direction. For 37 °C, 37×9/5+32 = 98.6 °F. To reverse, C = (F−32)×5/9. The scales coincide numerically at −40: substituting C = −40 gives F = −72+32 = −40. Temperatures must be distinguished from temperature changes: an increase of 10 °C is an increase of 10 K, but it is an increase of 18 °F.
Thermal equilibrium and how a thermometer helps
Two bodies in thermal contact tend to reach a state with no net heat transfer between them. The zeroth law formalises temperature comparison: if body A is in thermal equilibrium with body C and body B is also in equilibrium with C, then A and B are in equilibrium with each other. A thermometer serves as the third body. Its reading becomes useful after it has equilibrated sufficiently with what is measured. Taking an instant reading from a cold probe placed in warm water can record the probe's transitional state rather than the water's stable temperature.
When a warmer object and cooler object exchange energy in an insulated ideal system, energy lost by one equals energy gained by the other. Their final common temperature need not be the arithmetic mean because masses and materials may differ. For equal masses of the same material and no losses, temperatures 80 °C and 20 °C do settle at 50 °C in the simplified model. If one sample has twice the mass, it contributes twice the heat capacity under the same material assumption; the final value shifts toward its starting temperature. In real experiments, the container and surroundings can also exchange heat, so an exam problem must state or imply what losses to ignore.
Specific heat capacity indicates how much energy is needed to change a material's temperature per unit mass per degree. In simple calculations Q = mcΔT, provided no phase change occurs and c is treated as constant over the interval. For a 2 kg sample with c = 500 J/(kg K) warmed by 10 K, Q = 2×500×10 = 10,000 J. The temperature difference could equally be written 10 °C; the size is the same. The product has unit J, a useful check. If an object is melting or boiling at approximately constant temperature, energy may instead change its phase, so Q = mcΔT alone does not describe that part of the process.
Conduction, convection and radiation
Conduction transfers energy through a material without the material moving as a whole from one place to another. A metal spoon in hot tea becomes warm along its handle partly through conduction. Metals often conduct heat well because of their mobile electrons; wood or plastic handles reduce heat flow in ordinary use. “Good electrical conductor” and “good thermal conductor” are related for many metals but are not identical universal categories. A question about cooking utensils may ask specifically which part should conduct heat and which part should insulate a hand.
Convection transfers energy with bulk motion of a fluid, such as air or water. Fluid warmed near a heat source can become less dense and rise, while cooler fluid moves to replace it. That circulating movement transports energy. It is not the main mechanism in a rigid solid because the solid does not flow as a bulk fluid. An exam diagram may show arrows rising above a heater and descending elsewhere; the arrows depict fluid movement. Sea and land breezes are often explained through unequal heating of land and water and the resulting air circulation; direction must be tied to the specific time and pressure pattern in the stem.
Thermal radiation is energy carried by electromagnetic waves and can travel through a vacuum. The Sun warms Earth through space largely by radiation; convection cannot transport heat across the vacuum between them. Dark, dull surfaces generally absorb and emit thermal radiation more effectively than shiny, reflective surfaces under comparable conditions. A vacuum flask limits conduction and convection by using a low-pressure gap, while reflective surfaces reduce radiative transfer. A vacuum does not block radiation by itself. A well-designed thermos addresses several routes at once; an item asking about the silvered walls is asking specifically about radiation.
Expansion and changes of state
Many substances expand when warmed because the average spacing of their particles increases. Railway tracks and bridges need allowances for temperature-dependent expansion. The statement is a trend, not an exception-free law for every material and temperature range; water has unusual density behaviour near freezing. In a heated metal ring, the hole usually expands along with the ring rather than shrinking. Imagine the hole filled with the same metal and expanding with the rest: this helps prevent a misleading “metal moves inward” intuition.
Melting turns a solid into a liquid; freezing reverses it. Evaporation is vaporisation from a liquid surface and can occur below the boiling temperature, whereas boiling involves vapour formation throughout the liquid under suitable conditions. Evaporation cools the remaining liquid because higher-energy molecules escape preferentially. Sweat can therefore help cool the skin as it evaporates, although effectiveness depends on humidity and airflow. In a phase-change question, distinguish temperature, heat supplied and state. During an idealised plateau of melting for a pure substance at fixed pressure, energy is still being transferred even though the temperature remains approximately constant.
Sound begins with vibration
A vibrating source makes nearby particles of a medium oscillate. In air, the resulting longitudinal sound wave travels as compressions and rarefactions. The air particles oscillate near their local positions while wave energy moves outward; they do not travel all the way from the source to the listener. Sound requires a medium for ordinary mechanical propagation, so it cannot cross an ideal vacuum. Light can cross a vacuum because it is electromagnetic. This is a high-value distinction when questions list sound, radio waves and sunlight together.
Frequency is the number of complete oscillations per second and is measured in hertz (Hz). Period is the time for one oscillation, so f = 1/T. A 100 Hz vibration has period 1/100 s = 0.01 s. Wavelength is the distance between corresponding points on successive waves, such as one compression to the next. Wave speed satisfies v = fλ in a uniform medium. If sound travels at an assumed 340 m/s with frequency 170 Hz, wavelength is 340/170 = 2 m. Write units to check: (m/s)/s⁻¹ = m. Use the speed specified by the question rather than assuming one exact speed for all air temperatures.
Pitch mainly tracks frequency: a higher-frequency tone is usually perceived as higher-pitched. Loudness is linked principally to amplitude and sound intensity, not to frequency alone. A high-pitched sound need not be loud. Timbre or quality helps distinguish sources with the same fundamental pitch and similar loudness because their waveforms and harmonic content differ. These distinctions are common MCQ distractors: “loudness–frequency” swaps two different properties. In a drawn wave, more cycles per same horizontal time means higher frequency; greater vertical height means greater amplitude if the vertical scale measures displacement or pressure variation.
Reflection, echo and applications
Sound can reflect from a surface. An echo is a separate reflected sound heard after the original if the delay is sufficient and conditions allow it. In a simple distance calculation, a pulse travels to the reflecting wall and back. If the speed is 340 m/s and a reflected pulse returns after 0.4 s, the total path is 136 m and the wall is 68 m away. Omitting the return trip doubles the wrong answer. A distinct human-heard echo also depends on persistence of hearing and the environment, so do not turn the simple distance formula into a guarantee that every reflection is perceived separately.
Ultrasound has frequency above the upper limit of ordinary human hearing, commonly described as above about 20 kHz. In an exam context, the important idea is that the wave is still mechanical sound and still needs a medium. Its wavelength in the same medium is shorter than that of a lower-frequency sound when speed is approximately unchanged. Echo-based methods can estimate distance by timing the reflected pulse; the same round-trip principle used for the wall applies. The book discusses this as physics, not as instructions to use any medical device.
Temperature readings and temperature differences
The offset between Celsius and kelvin disappears when subtracting two temperatures: (35+273.15)−(20+273.15)=15 K. A candidate who writes “a rise of 15 °C equals a rise of 288.15 K” has added the scale offset to a difference instead of to a reading. By contrast, a reading of −10 °C is 263.15 K. If an MCQ asks for a ratio of absolute temperatures, convert the readings to kelvin first. For example, 20 °C is 293.15 K and 40 °C is 313.15 K; the second is not “twice as hot” on an absolute-temperature scale merely because 40 is twice 20. Celsius zero is a chosen reference, not an absence of thermal energy.
Thermal expansion offers another test of the same distinction. A 2 m rod lengthening by 1 mm has a final length of 2.001 m, while its change is 0.001 m. If a coefficient of linear expansion α is supplied, ΔL≈αL₀ΔT for a small temperature interval. This relation uses original length L₀ and a temperature change, not the final Celsius reading. Suppose α=1.0×10⁻⁵ K⁻¹, L₀=2 m and ΔT=50 K. Then ΔL=1.0×10⁻⁵×2×50=0.001 m, or 1 mm. The coefficient's inverse-temperature unit cancels the temperature-change unit. This is an illustrative idealised calculation; a material's actual coefficient can vary with temperature.
Energy balance in a mixture
Imagine 0.2 kg of warm water at 60 °C mixed with 0.3 kg of cooler water at 20 °C in a well-insulated container. Because both portions are the same material, their specific heat capacity cancels from the ideal energy balance. Energy lost by the warm portion is proportional to 0.2(60−T); energy gained by the cool portion to 0.3(T−20). Equating them gives 12−0.2T=0.3T−6, hence T=36 °C. The final value lies between 20 °C and 60 °C and closer to 20 °C because the cooler portion has greater mass. This intuitive check can catch a reversed temperature difference. The arithmetic mean, 40 °C, would be correct only for equal masses of the same material under the same ideal assumptions.
If the substances have different specific heat capacities, do not cancel c. Use m₁c₁(T₁−T)=m₂c₂(T−T₂) when heat exchange with the container and surroundings is ignored and no phase change occurs. A small mass of a substance with high heat capacity can exchange as much energy per degree as a larger mass with low heat capacity. The quantity mc is the sample's heat capacity in J/K; c alone is specific heat capacity in J/(kg K). Questions often swap those units. The units also tell you whether a given number applies to a whole object or one kilogram of it.
During melting or boiling, latent heat handles the energy associated with changing state at approximately fixed temperature under specified pressure. In a school model, Q=mL, with L in J/kg. For 0.5 kg of ice melting at its melting temperature, if the supplied latent heat is 3.3×10⁵ J/kg, the energy required for melting alone is 0.5×3.3×10⁵=1.65×10⁵ J. This does not include warming the resulting water above the melting temperature. A multi-stage item must be split into sensible-heating and phase-change stages. Draw a horizontal line for the phase-change plateau on a heating curve and a sloped line for a temperature rise; both consume energy.
Identify the route of heat transfer
Ask what moves across the space of interest. In conduction, energy is passed through matter by microscopic interactions without bulk flow from one side to the other. In convection, a fluid moves and carries energy. In radiation, electromagnetic waves carry energy, requiring no material medium. A steel pan warming from its base to its handle is primarily a conduction example. Water circulating in the pan is a convection example. Heat from a glowing element reaching a hand across an air gap includes radiation; air currents may also contribute. An exam item usually asks for the defining mechanism in a simplified arrangement. Real situations can involve several routes at once.
For a coastal-breeze item, identify which surface heats faster during the stated period. During a typical sunny day, land often warms faster than nearby sea water. Air above land warms and rises; cooler air near the sea moves toward land near the surface, giving a sea breeze. At night, land often cools faster and the near-surface flow can reverse to a land breeze. The words “day” and “night” therefore matter. The horizontal near-surface flow is part of a circulation loop driven by pressure differences associated with unequal heating. Local weather can modify the textbook pattern.
Insulation limits one or more routes. Trapped air in a fibrous material reduces large-scale convection, and the solid fibres often conduct relatively poorly. A shiny surface reduces radiative absorption or emission relative to a dark dull surface under comparable conditions. A double wall with a vacuum gap suppresses conduction and convection across that gap, but its supports and seals can still conduct. Real flasks slowly gain or lose energy. Naming the transfer mechanism tied to each component gives a more reliable answer than memorising the name of the flask.
Follow sound across a change of medium
For sound in a specified medium, v=fλ. If source frequency doubles while sound still travels through the same air at approximately the same temperature, wave speed changes little and wavelength approximately halves. A 170 Hz tone with v=340 m/s has wavelength 2 m; a 340 Hz tone in the same assumed air has wavelength 1 m. The source controls frequency, while the medium largely controls propagation speed in this simplified model. A louder source does not thereby send sound through the same air faster merely because its amplitude is greater. Loudness and propagation speed describe different properties.
When sound enters a different medium, its speed and wavelength can change. Its frequency, set by the source and fixed across a stationary boundary in the basic wave model, remains the same. If a 200 Hz sound travels at 340 m/s in air, its wavelength there is 1.7 m. If it enters a material in which sound travels at 1,500 m/s, its wavelength becomes 7.5 m while frequency remains 200 Hz. The larger wavelength is not evidence that the source suddenly vibrates fewer times each second. Many MCQs reverse frequency and wavelength at a boundary, so write the three quantities in a small table before judging an option.
Intensity falls with distance from a compact source in open space under an ideal inverse-square model because the same power spreads over a larger area. Doubling distance then reduces intensity to roughly one quarter. This model assumes no strong reflections, absorption, directional focusing or nearby boundaries; a room can behave differently. The decibel scale is logarithmic, so a change in decibel value cannot be interpreted as an equal numerical change in intensity. Unless a question supplies the logarithmic relation, concentrate on the qualitative contrast: intensity concerns energy flow per area, perceived loudness is a human response, and pitch mainly concerns frequency.
Resolve echo-time distractors
If a sound pulse is emitted and its reflection returns after time t, the reflecting surface is at distance d=vt/2 in the stationary one-path model. For a delay of 0.5 s and a stated speed of 320 m/s, the pulse covers 160 m in total and the wall is 80 m away. An option of 160 m is the full out-and-back path, not the distance to the wall. Conversely, if the wall is given as 80 m away, the expected round-trip time is 2×80/320=0.5 s. Drawing an arrow to the wall and another back makes the factor of two visible.
An echo differs from reverberation. Many close reflections can blend with the original sound and prolong it, producing reverberation instead of a separately heard repeat. Absorbing materials can reduce reverberation in a hall by weakening reflections, although exact acoustic design depends on many factors. The reflected sound generally arrives weaker as energy spreads and is absorbed. The basic timing formula does not calculate echo loudness. A question about “distance by echo” asks for travel time; a question about “clear speech in a room” may ask about excessive reverberation.
One integrated worked route
Consider two synthetic tasks. First, a room warms from 20 °C to 35 °C. Its temperature change is 15 °C or 15 K, not 288 K. If a 0.5 kg sample with specific heat capacity 800 J/(kg K) undergoes that change without phase transition, it receives Q = 0.5×800×15 = 6,000 J in the simplified model. Second, a sound pulse reflects from a surface and returns in 0.6 s at a stated speed of 300 m/s. Its total travelled distance is 180 m, so one-way distance to the surface is 90 m. Both tasks require a small interpretation before calculation: use a difference in the heat formula and halve the round-trip sound path.
Recall before the questions
Without options, explain why a metal spoon warms along its handle, why the Sun can heat Earth across space, and why a vacuum flask uses both a vacuum gap and reflective surfaces. Convert 27 °C to kelvin with both exact and school-rounded offsets. Explain why a wave's pitch and loudness are not synonyms. Derive the 68 m wall distance in the echo example, including the outward and return paths. If you can state the mechanism and the unit before selecting a letter, the chapter's factual items become easier to check independently.
Figure 3. Original chapter diagram.
Practice 36 questions
Choose one answer for each item. The answer key and explanations follow this chapter set.
1. At standard atmospheric pressure, water boils at approximately what temperature on the Fahrenheit scale?
A. 180°F B. 100°F C. 212°F D. 373°F
2. The temperature of a room is 27°C. What is it on the kelvin scale (using K = °C + 273)?
A. 246 K B. 300 K C. 327 K D. 273 K
3. Which of the following statements about absolute zero is NOT correct?
A. It is 0 K B. It equals −273.15°C C. It is the temperature at which water freezes D. It equals −459.67°F
4. An echo is heard because of which behaviour of sound waves?
A. Reflection B. Refraction C. Resonance D. Interference
5. In the usual introductory sound model, which pairing of a perceived sound characteristic with its main wave property is incorrect?
A. Loudness – frequency B. Pitch – frequency C. Loudness – amplitude D. Quality (timbre) – mixture of overtones
6. Using the school approximation K = °C + 273, arrange these temperatures in increasing order: A. 212°F; B. 273 K; C. −40°C; D. 37°C.
A. B, C, D, A B. C, D, B, A C. C, B, D, A D. C, B, A, D
7. Assertion (A): Woollen clothes keep us warm in winter because wool generates heat of its own.
Reason (R): Air trapped between the fibres of wool is a poor conductor of heat.
Select the correct option.
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
8. Which instrument is used to measure very high temperatures, such as inside a furnace?
A. Hygrometer B. Manometer C. Lactometer D. Pyrometer
9. In a thermos (vacuum) flask, the silvered inner walls mainly reduce heat transfer by:
A. Conduction B. Convection C. Radiation D. Evaporation
10. "If two bodies are each in thermal equilibrium with a third body, they are in thermal equilibrium with each other." This is the:
A. Zeroth law of thermodynamics B. First law of thermodynamics C. Second law of thermodynamics D. Third law of thermodynamics
11. Match List I (process) with List II (change of state) and select the correct code.
List I:
A. Sublimation
B. Deposition
C. Condensation
D. Vaporisation
List II:
1. Gas to solid
2. Solid to gas
3. Liquid to gas
4. Gas to liquid
A. A-1, B-2, C-4, D-3 B. A-2, B-1, C-4, D-3 C. A-2, B-1, C-3, D-4 D. A-2, B-4, C-1, D-3
12. Match the everyday event in List-I with the change of state in List-II.
List-I: A. Frost forming on a cold window pane B. A camphor tablet vanishing without leaving any liquid C. Steam turning into water droplets D. Ice turning into water
List-II: 1. Condensation 2. Melting 3. Sublimation 4. Deposition
A. A-3, B-4, C-1, D-2 B. A-4, B-3, C-1, D-2 C. A-4, B-3, C-2, D-1 D. A-1, B-3, C-4, D-2
13. A traditional glass clinical thermometer for human body temperature is commonly marked over which approximate range?
A. 35°C to 42°C B. 0°C to 100°C C. −10°C to 110°C D. 20°C to 60°C
14. One calorie of heat is approximately equal to:
A. 1 J B. 4.18 J C. 0.24 J D. 41.8 J
15. Taking 1 calorie = 4.18 J, how much heat, in joules, is needed to raise the temperature of 200 g of water by 15°C? (1 calorie raises the temperature of 1 g of water by 1°C.)
A. 3,000 J B. 717.7 J C. 1,254 J D. 12,540 J
16. A temperature of 50°C is equal to how many degrees Fahrenheit?
A. 90°F B. 132°F C. 122°F D. 112°F
17. At what temperature do the Celsius and Fahrenheit scales show the same reading?
A. 0° B. −40° C. 40° D. −32°
18. Aquatic life survives in lakes whose surface has frozen in winter because:
A. Ice is less dense than water, floats and insulates the water below B. Ice is denser than water and sinks to the bottom C. Water below ice is heated by the Sun through the ice D. Fish produce enough heat to prevent freezing
19. At high altitudes, water boils at a temperature below 100°C because:
A. Atmospheric pressure is higher there B. Air contains more oxygen there C. Gravity is stronger there D. Atmospheric pressure is lower there
20. Steam at 100°C causes more severe burns than boiling water at 100°C because steam:
A. Has a higher temperature than boiling water B. Has a higher specific heat capacity C. Releases its latent heat of vaporisation on condensing D. Is denser than water
21. Water is used as a coolant in car radiators mainly because it has:
A. High specific heat capacity B. Low boiling point C. High density D. Low thermal expansion
22. Woollen clothes keep us warm in winter mainly because:
A. Wool produces heat of its own B. Wool is a good conductor of heat C. Wool absorbs heat from the surroundings by radiation D. Air trapped between the fibres is a poor conductor of heat
23. Consider the following statements:
Statement I: Absolute zero corresponds to about −273°C.
Statement II: Temperature on the Kelvin scale is written with a degree symbol, as in 300°K.
Which of the above statement(s) is/are correct?
A. Only I B. Only II C. Both I and II D. Neither I nor II
24. Consider these statements for ordinary heat transfer.
Statement I: Heat flows spontaneously from a colder body to a hotter body when the two are merely put in thermal contact with no external work or other change.
Statement II: A refrigerator uses external work to transfer heat from its cold interior to warmer surroundings.
Which statement(s) is/are correct?
A. Only I B. Only II C. Both I and II D. Neither I nor II
25. Which of the following does NOT undergo sublimation on heating?
A. Camphor B. Candle wax C. Naphthalene balls D. Dry ice (solid CO₂)
26. Which of the following is NOT a wind instrument (aerophone), in which a column of air vibrates?
A. Veena B. Flute C. Shehnai D. Trumpet
27. Which of the following is NOT an example of heat transfer by convection?
A. Sea breeze blowing during the day B. A room heater warming the whole room C. Circulation of water being heated in a pot D. A steel spoon becoming hot in a cup of tea
28. Match the musical instrument in List I with the part that vibrates to produce sound in List II.
List I: A. Sitar; B. Tabla; C. Flute; D. Cymbals
List II: 1. Air column; 2. Stretched string; 3. Body of the instrument; 4. Stretched membrane
A. A-2, B-1, C-4, D-3 B. A-2, B-4, C-1, D-3 C. A-4, B-2, C-1, D-3 D. A-3, B-4, C-2, D-1
29. The persistence of sound in a large hall due to repeated reflections from the walls and ceiling is called:
A. Echo B. Resonance C. Reverberation D. Refraction
30. For the human ear to hear an echo distinctly from the original sound, the minimum time gap between the two should be about:
A. 0.01 second B. 0.5 second C. 1 second D. 0.1 second
31. A SONAR pulse sent from a ship returns after 4 s. If sound travels at 1,480 m/s in sea water, what is the depth of the sea?
A. 2,960 m B. 5,920 m C. 1,480 m D. 370 m
32. If the speed of sound in air is 340 m/s and a distinct echo needs a gap of at least 0.1 s, what is the minimum distance of the reflecting surface from the listener?
A. 34 m B. 8.5 m C. 170 m D. 17 m
33. A trained singer can shatter a wine glass by holding a note. This is an example of:
A. Reverberation B. Resonance C. Echo D. Doppler effect
34. On a night when air near the ground is cooler than the air above, why can distant sound sometimes be heard more clearly than under a warmer-ground daytime profile?
A. Sound travels faster in the cooler air near the ground B. The frequency of sound increases at night C. Cooler air near the ground refracts sound back down towards the surface D. Clouds reflect more sound at night
35. At an ordinary reference temperature, approximately how fast does sound travel in liquid water?
A. About 332 m/s B. About 5,000 m/s C. About 3,40,000 m/s D. About 1,480 m/s
36. SONAR stands for:
A. SOund Detection And Reflection B. SOund Navigation And Radiation C. Sea Object Navigation And Ranging D. SOund Navigation And Ranging
Answers and explanations
1. C. On the Fahrenheit scale water freezes at 32°F and boils at 212°F, a gap of 180 degrees, compared with 0°C and 100°C on the Celsius scale. The figure 373 is the boiling point in kelvin.
2. B. K = °C + 273 = 27 + 273 = 300 K. Kelvin is the SI unit of temperature and, unlike °C and °F, is written without a degree symbol. Subtracting 27 instead of adding would wrongly give 246.
3. C. Absolute zero is 0 K = −273.15°C = −459.67°F, the lowest theoretically possible temperature. Water freezes at 0°C (273 K), far above absolute zero, so that statement is the incorrect one.
4. A. An echo is sound reflected from a hard surface such as a wall or cliff that reaches the ear at least 0.1 s after the original sound, so the two are heard separately. Refraction is bending of sound, resonance is forced vibration at the natural frequency and interference is the overlapping of waves.
5. A. In the introductory comparison, pitch mainly tracks frequency, loudness tracks amplitude or intensity, and timbre depends on the mixture of frequency components. 'Loudness – frequency' is the incorrect primary pairing. Real perceived loudness also depends on frequency and hearing sensitivity, so this is a school-level comparison, not a complete psychoacoustic law.
6. C. Under the stated school approximation, 212°F = 100°C and 273 K ≈ 0°C; the other values are −40°C and 37°C. Thus C < B < D < A. With the exact offset 273.15, 273 K is −0.15°C, which does not change the order.
7. D. Wool does not generate heat; it keeps us warm because air trapped between its fibres is a poor conductor and stops body heat from conducting away. So A is false and R is true. Several thin layers keep us warmer than one thick layer because each layer traps its own air.
8. D. A pyrometer measures very high temperatures (furnaces, molten metal) where an ordinary thermometer would melt, usually from the radiation emitted. A hygrometer measures humidity, a manometer gas pressure, and a lactometer the purity of milk.
9. C. Shiny, silvered surfaces reflect thermal radiation and reduce radiative heat transfer. The vacuum between the walls greatly reduces conduction and convection through the gap; supports and other pathways can still transfer some heat. Thus radiation is the best answer for the silvered walls.
10. A. This is the zeroth law, which defines temperature and makes thermometers meaningful. The first law is energy conservation, the second concerns direction of heat flow and entropy, and the third says absolute zero cannot be reached.
11. B. Sublimation is solid to gas (camphor), deposition is gas to solid (frost), condensation is gas to liquid (dew), and vaporisation is liquid to gas (boiling). Code: A-2, B-1, C-4, D-3.
12. B. Frost forms when water vapour turns directly into a solid (deposition); camphor goes directly from solid to gas (sublimation); steam changing into droplets is condensation; and ice turning into water is melting (fusion). Hence A-4, B-3, C-1, D-2.
13. A. A traditional glass clinical thermometer commonly spans about 35°C to 42°C to focus on human body temperatures. Digital devices may use different ranges and displays, so the question identifies the traditional instrument.
14. B. One calorie is the heat needed to raise 1 g of water by 1°C and equals about 4.18 J (the mechanical equivalent of heat). Conversely 1 J ≈ 0.24 cal, which is why 0.24 appears as a trap.
15. D. One calorie warms 1 g of water by 1°C, so the heat needed = 200 × 15 = 3,000 cal. In joules this is 3,000 × 4.18 = 12,540 J. The value 3,000 is the answer in calories, not in joules.
16. C. °F = (9/5 × °C) + 32 = (9/5 × 50) + 32 = 90 + 32 = 122°F. Stopping at 90 (forgetting to add 32) is the most common slip.
17. B. Put °F = °C = x in °F = (9/5)°C + 32: x = 1.8x + 32 → −0.8x = 32 → x = −40. So −40°C = −40°F. At 0°C the Fahrenheit reading is 32°F, not 0.
18. A. Water expands on freezing, so ice is less dense and floats. Lakes therefore freeze from the top, and the ice sheet, a poor conductor, insulates the liquid water beneath where fish and plants survive.
19. D. Boiling point falls when pressure falls. In the mountains air pressure is lower, so water boils below 100°C and food cooks slowly. A pressure cooker does the reverse: higher pressure raises the boiling point above 100°C.
20. C. Both are at 100°C, but when steam condenses on skin it first gives up its large latent heat of vaporisation (about 2260 J per gram) and only then cools like hot water. That extra hidden heat causes the worse burn.
21. A. Water's unusually high specific heat capacity lets it absorb a lot of engine heat with only a small rise in temperature. The same property keeps coastal climates mild. A low boiling point would actually be a disadvantage for a coolant.
22. D. Wool does not generate heat; it traps air between its fibres, and still air is a poor conductor, so body heat does not escape. This is also why several thin layers keep us warmer than one thick layer.
23. A. Statement I is correct: absolute zero, 0 K, is −273.15°C (≈ −273°C). Statement II is wrong: kelvin is an absolute SI unit and is written without the degree symbol, e.g. 300 K.
24. B. With the stated no-work condition, net heat transfer between bodies is spontaneously from hotter to colder, so I is false. A refrigerator uses external work to move heat from its cold interior to warmer surroundings, so II is true. The qualifier matters because a cold-to-hot transfer can be driven by work.
25. B. Camphor, naphthalene balls and dry ice change directly from solid to gas without passing through the liquid state, which is sublimation. Candle wax first melts into a liquid, so it does not sublime.
26. A. The veena is a string instrument (chordophone): its sound comes from vibrating stretched strings. Flute, shehnai and trumpet are aerophones, where blowing sets a column of trapped air vibrating.
27. D. The first three examples involve bulk movement of a fluid, so convection contributes. Heat travels along the solid steel spoon by conduction; energy transfer in a metal involves mobile electrons and lattice interactions, not bulk flow of the metal. Hence the spoon is the exception.
28. B. Sitar is a chordophone (string, 2); tabla is a membranophone (stretched skin, 4); flute is an aerophone (air column, 1); cymbals are idiophones, where the metal body itself vibrates (3). So the code is A-2, B-4, C-1, D-3.
29. C. Reverberation is the lingering of sound caused by multiple reflections in an enclosed space. An echo is a single distinct reflected sound, resonance is forced vibration at the natural frequency, and refraction is bending of sound.
30. D. The sensation of sound persists in the ear for about 0.1 second, so a reflected sound must arrive at least 0.1 s later to be heard as a separate echo. This is why echoes are not heard in small rooms.
31. A. The pulse travels down and back, so the one-way depth is half of the total path: depth = (1,480 × 4) ÷ 2 = 2,960 m. Forgetting to halve the distance gives the wrong value 5,920 m.
32. D. In 0.1 s sound covers 340 × 0.1 = 34 m. This is the to-and-fro path, so the surface must be 34 ÷ 2 = 17 m away. Forgetting to halve the distance gives the trap answer 34 m.
33. B. When the singer's note matches the glass's natural frequency, the glass absorbs energy efficiently and its vibrations build up until it breaks; this is resonance. A radio tuner picks a station the same way, and soldiers break step on a bridge to avoid resonant build-up.
34. C. Under the stated temperature inversion, sound speed is lower in the cooler near-ground air. Refraction can bend sound paths back toward the ground, increasing audibility at some distant locations. This is conditional, not a rule for every night or every sound source.
35. D. A representative value is about 1,480 m/s in water, faster than sound in air but slower than in many metals. The precise value changes with temperature, pressure and composition. The contrast is explained by the medium's elastic properties and density, not simply by how close its particles appear.
36. D. SONAR expands to SOund Navigation And Ranging. Active sonar sends acoustic pulses and measures returning echoes to estimate range or depth; passive sonar listens without transmitting. RADAR instead uses radio waves.