Heat and Thermodynamics
Subject: General Science — Physics | Topic: Heat and Thermodynamics Exam: AP Group 2 (APPSC) | Sources: NCERT Class IX-XI, Lucent's GS
Introduction
Heat and thermodynamics covers temperature measurement, heat transfer, phase changes, and the laws governing thermal energy. APPSC typically asks 1-2 questions — favourites include specific heat of water, modes of heat transfer, and practical applications like pressure cookers.
Temperature and Measurement
| Scale | Freezing Point of Water | Boiling Point of Water | Conversion |
|---|---|---|---|
| Celsius (°C) | 0°C | 100°C | — |
| Fahrenheit (°F) | 32°F | 212°F | F = (9/5)C + 32 |
| Kelvin (K) — SI unit | 273.15 K | 373.15 K | K = C + 273.15 |
- Temperature measures the average kinetic energy of particles.
- Heat is energy transferred due to temperature difference. SI unit: Joule (J).
- Absolute zero = 0 K = −273.15°C (no molecular motion).
- Clinical thermometer: 35°C to 42°C. Laboratory: −10°C to 110°C.
Heat Transfer
| Mode | Mechanism | Medium Required | Best In |
|---|---|---|---|
| Conduction | Direct molecular contact | Yes (solids best) | Metals (silver best, copper second) |
| Convection | Actual movement of fluid | Yes (liquids/gases) | Liquids and gases |
| Radiation | Electromagnetic waves | No medium needed | Vacuum/any medium |
- Dark, rough surfaces absorb and emit radiation better than light, smooth surfaces.
- Thermos flask prevents all three modes: vacuum (conduction/convection), silvered walls (radiation).
Specific Heat and Calorimetry
Specific Heat Capacity (c): Heat needed to raise 1 kg of a substance by 1°C.
Q = mc(ΔT)
- Water has the highest specific heat among common substances: 4,186 J/kg/°C. This is why water moderates climate.
- Principle of Calorimetry: Heat lost by hot body = Heat gained by cold body (insulated system).
Change of State
| Change | Latent Heat | Value for Water |
|---|---|---|
| Melting (solid→liquid) | Latent Heat of Fusion (Lf) | 3.34 × 10⁵ J/kg |
| Boiling (liquid→gas) | Latent Heat of Vaporization (Lv) | 22.6 × 10⁵ J/kg |
- Melting point of ice: 0°C. Boiling point of water: 100°C (at 1 atm).
- Boiling point increases with pressure (pressure cooker principle) and decreases with altitude.
- Evaporation causes cooling (sweat cools the body). Rate increases with temperature, surface area, wind speed; decreases with humidity.
- Sublimation: Solid→Gas directly. Examples: camphor, dry ice (solid CO₂), naphthalene.
Thermal Expansion
- Solids expand on heating — linear, area, and volume expansion.
- Railway tracks have gaps to allow for thermal expansion.
- Bimetallic strips (two metals with different expansion rates) used in thermostats.
- Anomalous expansion of water: Water is densest at 4°C, expands both above and below. This is why ice floats and aquatic life survives in frozen ponds.
Laws of Thermodynamics
| Law | Statement |
|---|---|
| Zeroth | If A is in thermal equilibrium with C, and B with C, then A is in equilibrium with B (basis of temperature measurement) |
| First | Q = ΔU + W (conservation of energy: heat = internal energy change + work done) |
| Second | Heat cannot spontaneously flow from colder to hotter body; entropy of an isolated system never decreases |
| Third | Absolute zero cannot be reached in a finite number of steps |
Entropy: Measure of disorder. In any natural (irreversible) process, total entropy of the universe increases.
Practical Applications
| Application | Principle |
|---|---|
| Pressure cooker | Increased pressure raises boiling point → faster cooking |
| Desert cooler | Evaporation causes cooling |
| Refrigerator | Evaporation of refrigerant absorbs heat |
| Greenhouse effect | Glass/atmosphere traps radiation, raising temperature |
- Good conductors: metals (silver, copper, aluminium).
- Bad conductors (insulators): wood, plastic, air, wool.
Units and Conversions
- 1 calorie = 4.186 Joules (Joule's mechanical equivalent of heat)
- 1 kilocalorie (food Calorie) = 4,186 J
Likely Exam Questions
-
Water has the highest specific heat capacity among common substances at: Ans: 4,186 J/kg/°C
-
The mode of heat transfer that does not require a medium is: Ans: Radiation
-
At what temperature is water the densest? Ans: 4°C
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The First Law of Thermodynamics is essentially the law of: Ans: Conservation of Energy
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A pressure cooker cooks food faster because: Ans: Increased pressure raises the boiling point of water
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The SI unit of temperature is: Ans: Kelvin (K)
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Sublimation is the direct change from: Ans: Solid to gas (e.g., camphor, dry ice)
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The principle behind a thermos flask is prevention of heat transfer by: Ans: All three modes — conduction, convection, and radiation
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1 calorie equals how many Joules? Ans: 4.186 Joules
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Entropy is a measure of: Ans: Disorder or randomness in a system