Heat, Thermodynamics and Sound

4.1 Temperature and Heat Transfer

Temperature scales:

• Celsius (°C): 0°C (melting ice), 100°C (boiling water at 1 atm)
• Fahrenheit (°F): °F = (9/5)°C + 32
• Kelvin (K): K = °C + 273.15 (absolute zero = 0 K = −273.15°C — lowest possible temperature; molecules cease all motion)

Heat Transfer Mechanisms:

Mechanism	Medium Required	How it works	Examples
Conduction	Required (solids best)	Molecule-to-molecule energy transfer	Metals cooling, cooking
Convection	Required (fluids)	Bulk movement of heated fluid	Sea breezes, room heaters, ocean currents
Radiation	Not required	EM wave (infrared) emission	Sun's heat reaching Earth, thermos flask

Specific Heat Capacity: Amount of heat needed to raise 1 kg by 1°C.

Water has exceptionally high SHC (4,186 J/kg·K). This is why oceans moderate coastal climates and why water is used as coolant in engines.

4.2 Laws of Thermodynamics

Law	Statement	Implications
Zeroth	If A is in thermal equilibrium with C, and B is in equilibrium with C, then A and B are in equilibrium	Defines temperature as a measurable property
First	Energy cannot be created or destroyed; Q = ΔU + W	Refrigerators, heat engines obey energy conservation
Second	Heat does not spontaneously flow from cold to hot; entropy of universe always increases	No heat engine is 100% efficient; refrigerator needs external work
Third	As T → 0 K, entropy → constant (minimum)	Absolute zero is unattainable

Carnot Efficiency: Maximum theoretical efficiency of a heat engine = 1 − (T_cold/T_hot). The larger the temperature difference, the more efficient the engine.

4.3 Sound Waves

Sound is a longitudinal mechanical wave — particles oscillate parallel to wave propagation direction. It requires a material medium and cannot travel in vacuum.

Speed of sound:

• In air at 0°C = 332 m/s; at 25°C ≈ 346 m/s
• In water ≈ 1,500 m/s; in steel ≈ 5,100 m/s
• Sound travels faster in denser media because particles are closer together

Frequency ranges:

• Infrasound: < 20 Hz — earthquakes, elephant communication, weather
• Audible range: 20 Hz – 20,000 Hz (human hearing)
• Ultrasound: > 20,000 Hz

Applications of ultrasound:

• Medical USG: 2–15 MHz sound pulses reflect from organ boundaries — images of foetus, kidney stones, cardiac function.
• Sonar (Sound Navigation and Ranging): Used by submarines and bats (echolocation) — measures depth/distance by time of echo.
• Industrial NDT: Detects internal cracks in metal castings without cutting.
• Lithotripsy: High-intensity ultrasound pulses break kidney stones non-surgically.

Doppler Effect: When source and observer move relative to each other, observed frequency changes.

• Apparent frequency increases when approaching, decreases when receding.
• Applications: radar speed guns, weather Doppler radar, redshift in astronomy (universe expansion evidence), Doppler ultrasound in cardiology.

---