Heat and Thermodynamics
~11 min read
- Heat: Energy transferred due to temperature difference. SI: Joule. Q = mcΔT (sensible heat); Q = mL (latent heat).
- Three laws: 0th law: thermal equilibrium. 1st law: ΔU = Q - W (energy conservation). 2nd law: heat flows hot → cold.
- Heat engines: Convert heat to work. Carnot engine: max possible efficiency η = 1 - T_cold/T_hot.
Thermodynamics is the science of heat, work, and energy transformations. NDA tests temperature scales, specific heat, latent heat, three laws, and applications like refrigerators and engines.
Temperature and Scales
| Scale | Freezing | Boiling |
|---|---|---|
| Celsius (°C) | 0 | 100 |
| Fahrenheit (°F) | 32 | 212 |
| Kelvin (K) | 273.15 | 373.15 |
Conversions: F = (9/5)C + 32. K = C + 273.15. Absolute zero: 0 K = -273.15 °C.
Specific Heat and Latent Heat
- Specific heat (c): Heat needed to raise 1 kg of substance by 1°C. SI: J/(kg·K). Water: 4186 J/(kg·K) — very high.
- Q = mcΔT (sensible heat — temperature changes).
- Latent heat (L): Heat needed to change state without temperature change. SI: J/kg.
- Q = mL.
- L_fusion of ice = 334 kJ/kg. L_vaporisation of water = 2260 kJ/kg.
- Water's high specific heat moderates climate (large water bodies maintain steady temperatures).
Modes of Heat Transfer
| Mode | Medium | Mechanism |
|---|---|---|
| Conduction | Solid (mostly) | Direct atomic vibration; no bulk movement of matter |
| Convection | Liquid/Gas | Bulk movement of warmer fluid; rises, cooler descends |
| Radiation | No medium needed (works in vacuum) | Electromagnetic waves (mostly infrared) |
- Metals are best conductors (copper, silver, aluminium).
- Wood, plastic, glass are poor conductors (insulators).
- Sea breezes and land breezes are convection effects.
- Sun's heat reaches Earth by radiation (no medium between).
Three Laws of Thermodynamics
- Zeroth Law: If A is in thermal equilibrium with B, and B with C, then A with C. Basis of temperature.
- First Law: ΔU = Q - W (or Q = ΔU + W). Energy can be converted from one form to another but cannot be created/destroyed.
- Second Law: Heat flows spontaneously from hot to cold. Entropy of an isolated system never decreases. No engine can convert 100% heat to work.
- Third Law: Entropy approaches zero as temperature approaches absolute zero (0 K). Absolute zero is unreachable.
Heat Engines and Refrigerators
- Heat engine: Takes heat from hot reservoir (Q_h), converts part to work (W), rejects rest (Q_c) to cold reservoir.
- Efficiency: η = W/Q_h = 1 - Q_c/Q_h.
- Carnot engine: Theoretical maximum efficiency: η = 1 - T_c/T_h (temperatures in Kelvin).
- Refrigerator/Heat pump: Heat engine in reverse. Pumps heat from cold to hot using external work.
- COP (Coefficient of Performance): Q_c/W for refrigerator.
- No engine can have efficiency 100% (would violate 2nd law).
NDA PYQ Examples
Q: Absolute zero is equivalent to:
(a) 0°C (b) -100°C (c) -273.15°C (d) 100°C
Answer: (c) -273.15°C = 0 Kelvin.
Q: Heat reaches us from the Sun mainly by:
(a) Conduction (b) Convection (c) Radiation (d) Compression
Answer: (c) Radiation — no medium between Sun and Earth.
Q: Specific heat of water is highest among common substances. Result:
(a) Heats up quickly (b) Moderates climate near water bodies (c) Boils easily (d) Conducts electricity
Answer: (b) Moderates climate near water bodies.
Q: Which law states that entropy of an isolated system never decreases?
(a) Zeroth law (b) First law (c) Second law (d) Third law
Answer: (c) Second law of thermodynamics.
Drill Heat and Thermodynamics for NDA
NDA-pattern items on Heat and Thermodynamics with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why does sweating cool the body?
Sweat evaporates by taking latent heat (~2260 kJ/kg) from the body. This heat loss cools the body. Same principle for clay pots cooling water through porous walls.
Why are houses in hot regions painted white?
White reflects most sunlight and absorbs little. Dark colours absorb more radiation and heat up. White roofs keep interiors cooler — a cheap form of climate control.
Why are utensils made of metal but their handles of wood/plastic?
Metal is a good thermal conductor — distributes heat for cooking. Wood/plastic is a poor conductor — prevents the handle from getting hot. Same logic for kettles, pans, soldering irons.
What is the difference between heat and temperature?
Temperature measures the average kinetic energy of molecules (how hot something is). Heat is the total thermal energy transferred. A cup and a swimming pool at 30°C have same temperature, but the pool has far more heat.
Why does water expand when frozen?
Water molecules form a hexagonal crystal lattice when freezing, which has more space between molecules than liquid water. This anomalous expansion (4°C is densest) is why ice floats and why water pipes burst in winter.