Heat, Temperature and Sound
Heat, Temperature and Sound
Heat questions ask scale conversions, modes of heat transfer and the meaning of specific and latent heat. Sound questions ask the speed of sound in different media, echo, the audible range and everyday uses of ultrasound. Temperature conversion has appeared in NTPC since the 2016 cycle.
1. Temperature and Heat
°F = (9/5) × °C + 32
K = °C + 273
−40 °C = −40 °F (the two scales meet here)
| Point | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Absolute zero | −273 (exactly −273.15) | −459.4 (exactly −459.67) | 0 |
| Ice melts | 0 | 32 | 273 |
| Normal body temperature | 37 | 98.6 | 310 |
| Water boils (sea level) | 100 | 212 | 373 |
| Idea | Fact |
|---|---|
| Conduction | Through solids, particle to particle; metals are good conductors |
| Convection | In liquids and gases by moving currents; sea breeze and land breeze |
| Radiation | Needs no medium; heat from the Sun reaches us this way |
| Specific heat | Heat to raise 1 kg by 1 °C; water's is high (about 4,186 J/kg °C), so coastal climates are mild |
| Latent heat | Heat absorbed or released during a change of state at constant temperature; steam at 100 °C burns more than water at 100 °C |
| Anomalous expansion of water | Water is densest at 4 °C, so lakes freeze from the top |
| Thermos flask | Cuts conduction, convection and radiation |
2. Sound
| Fact | Value or statement |
|---|---|
| Nature | Mechanical longitudinal wave; cannot travel in a vacuum |
| Speed in air | About 343 m/s at 20 °C |
| Order of speed | Solids > liquids > gases (steel about 5,960 m/s, water about 1,480 m/s) |
| Audible range | 20 Hz to 20,000 Hz |
| Infrasound | Below 20 Hz; heard by elephants and whales |
| Ultrasound | Above 20,000 Hz; used by bats, SONAR and medical scans |
| Echo | Needs the reflecting surface at least about 17.2 m away |
| Pitch | Depends on frequency |
| Loudness | Depends on amplitude; measured in decibels |
| Doppler effect | Apparent change in frequency due to relative motion |
✗ Sound travels fastest in air | ✓ Sound is fastest in solids and slowest in gases
✗ Pitch depends on amplitude | ✓ Pitch depends on frequency; loudness depends on amplitude
हिंदी नोट: °F = (9/5)°C + 32 और K = °C + 273; −40 पर सेल्सियस और फ़ारेनहाइट बराबर होते हैं। ध्वनि ठोस में सबसे तेज़ और गैस में सबसे धीमी चलती है तथा निर्वात में नहीं चलती। 20 Hz से कम आवृत्ति अपश्रव्य और 20,000 Hz से अधिक पराश्रव्य कहलाती है।
Exam Pointer: Verified NTPC pattern: Celsius to Fahrenheit conversion (April 2016 CBT-1). Heat transfer and sound questions are tagged syllabus-based.
Pariksha Pattern: Every Way NTPC Asks This Topic
Pattern 1: Temperature scale conversion
[PYQ: NTPC CBT-1 3-Apr-2016 Shift-1]
EXAM LEVEL
Q. Convert 25 °C to Fahrenheit.
°F = 9/5 × 25 + 32 = 45 + 32 = 77.
Answer: 77 °F
EXAMATLAS LEVEL
Q. At what temperature do the Celsius and Fahrenheit scales show the same number? Convert 300 K to Celsius.
Put F = C in F = 9C/5 + 32: C − 9C/5 = 32, so −4C/5 = 32 and C = −40. For 300 K, °C = 300 − 273 = 27.
Answer: −40; 27 °C
Pattern 2: Heat transfer, specific heat and latent heat
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. By which mode does heat from the Sun reach the Earth?
Radiation, which needs no medium.
Answer: Radiation
EXAMATLAS LEVEL
Q. Why does steam at 100 °C cause worse burns than boiling water at 100 °C? Why do lakes in cold countries freeze from the top?
Steam gives out its extra latent heat of vaporisation when it condenses on the skin. Water is densest at 4 °C, so colder water rises to the top and freezes there, while water at 4 °C stays below.
Answer: Latent heat; anomalous expansion of water
Pattern 3: Speed of sound and echo
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. In which medium does sound travel fastest: air, water, steel or vacuum?
Steel, a solid. Sound cannot travel through a vacuum.
Answer: Steel
EXAMATLAS LEVEL
Q. A person claps and hears the echo from a cliff after 2 seconds. If sound travels at 340 m/s, how far is the cliff?
Sound goes to the cliff and back, so distance = (340 × 2)/2 = 340 m.
Answer: 340 m
Pattern 4: Frequency ranges and characteristics
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. A sound of 15 Hz is called what?
Infrasound, since it is below 20 Hz.
Answer: Infrasound
EXAMATLAS LEVEL
Q. Which characteristic of sound depends on frequency and which on amplitude? Name two uses of ultrasound.
Pitch depends on frequency and loudness on amplitude. Ultrasound is used in SONAR to find sea depth and in medical scanning (sonography).
Answer: Pitch, loudness; SONAR and sonography
60-Second Revision
- °F = 9C/5 + 32; K = C + 273; −40 is common to °C and °F.
- Conduction in solids, convection in fluids, radiation without a medium.
- Sound: solids fastest, no sound in vacuum, about 343 m/s in air.
- Infrasound below 20 Hz, ultrasound above 20 kHz; echo distance = speed × time ÷ 2.