Paper I · 2 General Science & Technology · 2.1 Physics in everyday life
2.1.3 Heat: temperature scales, expansion and modes of heat transfer
Heat is energy that flows because of a temperature difference, by conduction, convection or radiation, and substances differ in how much heat they need to warm up, expand or change state.
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Core facts
- Heat is energy in transit (unit joule; also calorie, 1 cal = 4.18 J). Temperature measures hotness (SI unit kelvin).
- Reference points:
| Point | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Ice melts | 0 | 32 | 273.15 |
| Water boils | 100 | 212 | 373.15 |
| Absolute zero | -273.15 | -459.67 | 0 |
- Conversion: F = 1.8C + 32; K = C + 273.15. -40 is the same on Celsius and Fahrenheit.
- Clinical thermometer (mercury, with a kink to hold the reading) reads 35 to 42 °C (94 to 108 °F). Normal body temperature: 37 °C (98.6 °F).
- Three modes of heat transfer:
- Conduction: through solids by contact (metals are good conductors).
- Convection: by movement of fluids (liquids and gases).
- Radiation: by electromagnetic waves; needs no medium (Sun's heat reaches Earth).
- Water has maximum density at 4 °C. This is why ponds freeze from the top and aquatic life survives below.
- Water has a high specific heat capacity (about 4200 J/kg per K).
Explained
Specific heat and latent heat
- Specific heat capacity: heat needed to raise 1 kg by 1 K. Water's high value makes it a good coolant and moderates coastal climates.
- Land and sea breeze: by day land heats faster than sea, so cooler air flows from sea to land (sea breeze); at night the direction reverses (land breeze).
- Latent heat: heat absorbed or released during change of state without change in temperature. For water: fusion about 336 kJ/kg, vaporisation about 2260 kJ/kg. Steam burns are worse than boiling water burns because steam releases this latent heat on condensing.
- Evaporation causes cooling: sweating, and earthen pots (matka) keeping water cool.
Thermal expansion
- Solids, liquids and gases generally expand on heating. Railway tracks and bridges have gaps for expansion.
- A bimetallic strip (two metals, different expansion) bends on heating; used in thermostats and fire alarms.
- Gases expand most, then liquids, then solids.
- Anomalous expansion of water between 0 and 4 °C: it contracts on heating.
Mode of transfer: examples
- Conduction: a metal spoon in hot tea; woollen clothes trap air (poor conductor) and keep us warm.
- Convection: boiling water circulates; sea breeze; hot air rising from a heater; cooling coil of a refrigerator at the top of the freezer.
- Radiation: dark, rough surfaces absorb and radiate best; white clothes reflect heat, so are worn in summer.
- Thermos (Dewar) flask: double-walled glass with vacuum between (stops conduction and convection) and silvered walls (reduce radiation).
Mercury and alcohol thermometers: mercury freezes at about -39 °C, so alcohol is used for very low temperatures.
Common traps
- ✗ "Heat and temperature are the same." ✓ Heat is energy; temperature is its measure of hotness.
- ✗ "Water is densest at 0 °C." ✓ Water is densest at 4 °C.
- ✗ "Radiation needs a medium." ✓ It travels through vacuum.
- ✗ "The clinical thermometer reads -10 to 110 °C." ✓ It reads only about 35 to 42 °C.
- ✗ "Sea breeze blows from land to sea by day." ✓ By day it blows from sea to land.
- ✗ "Water boils at 373 K at all heights." ✓ 100 °C (373.15 K) is at sea-level pressure; it falls with altitude.
Quick revision
- Heat in joule; temperature in kelvin; 1 cal = 4.18 J.
- F = 1.8C + 32; K = C + 273.15; -40 is common.
- Body temperature 37 °C = 98.6 °F.
- Conduction, convection, radiation; radiation needs no medium.
- Water densest at 4 °C.
- Water's specific heat is high: coastal moderation, sea breeze by day.
- Thermos flask: vacuum plus silvered walls.
- Absolute zero = 0 K = -273.15 °C.
Go deeper: Deeper, Mains writing (beyond what APPSC Paper I usually asks)
Deeper Advanced
- Laws of thermodynamics: Zeroth (thermal equilibrium defines temperature), First (energy conservation: change in internal energy = heat added minus work done by the system), Second (heat does not flow by itself from cold to hot; entropy tends to increase), Third (absolute zero cannot be reached).
- Radiation laws: Stefan-Boltzmann law (radiated power proportional to T⁴); Wien's displacement law (peak wavelength inversely proportional to temperature). A black body is a perfect absorber and emitter.
- Greenhouse effect is a radiation phenomenon: gases such as carbon dioxide and methane absorb outgoing infrared radiation from Earth.
- Heat engines convert heat to work and cannot be fully efficient; refrigerators and air conditioners move heat from cold to hot with external work.
- Triple point of water is 273.16 K; dry ice is solid carbon dioxide, which sublimes at -78.5 °C at normal pressure.
- Kinetic theory: temperature is a measure of the average kinetic energy of molecules.
Mains writing Mains writing
- Frame: heat physics links directly to climate, cooling demand and energy efficiency.
- Analysis:
- Rising cooling demand from air conditioning adds to emissions and peak electricity load.
- The urban heat island effect results from dark surfaces absorbing radiation and lower convective cooling.
- Passive design (white roofs, ventilation, thermal mass) uses conduction, convection and radiation principles.
- Way forward: cool-roof standards, building energy codes and efficient cooling technology.
- Example: reflective "cool roofs" and ventilated courtyards in traditional Indian architecture reduce indoor temperatures without power.
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