๐ Introduction: Heat vs Temperature
Temperature is a measure of the average kinetic energy of molecules in a substance. It is a scalar quantity measured in Kelvin (K), Celsius (°C), or Fahrenheit (°F). Heat, on the other hand, is the total energy transferred between two bodies due to temperature difference. Heat is a form of energy in transit, measured in Joules (J) or calories (cal).
The key difference: Temperature tells how hot or cold a body is; Heat tells how much thermal energy is being transferred. Two objects at the same temperature can have vastly different amounts of heat energy depending on their mass and specific heat capacity.
๐ก️ Temperature Scales
| Scale | Melting Point of Ice | Boiling Point of Water | Absolute Zero |
|---|---|---|---|
| Celsius (°C) | 0°C | 100°C | -273.15°C |
| Fahrenheit (°F) | 32°F | 212°F | -459.67°F |
| Kelvin (K) | 273.15 K | 373.15 K | 0 K |
Conversion formulas:
- °C = K - 273.15
- °F = (9/5)°C + 32
- K = °C + 273.15
๐ Thermometers and Their Types
A thermometer measures temperature based on a thermometric property that changes linearly with temperature. Common thermometers include:
- Mercury Thermometer: Range -39°C to 357°C. Mercury is shiny, good conductor, and expands uniformly.
- Alcohol Thermometer: Range -115°C to 78°C. Used for very low temperatures. Colored for visibility.
- Clinical Thermometer: Range 35°C to 42°C. Has a constriction (kink) to hold the reading.
- Platinum Resistance Thermometer: Measures temperature by change in electrical resistance. Very precise.
- Thermocouple: Uses Seebeck effect. Range: -200°C to 1600°C.
๐ Thermal Expansion
Most substances expand on heating and contract on cooling. Water is an exception — it expands when cooled below 4°C (anomalous expansion).
1. Linear Expansion
ฮL = L₀ ฮฑ ฮT, where ฮฑ is the coefficient of linear expansion.
2. Superficial (Area) Expansion
ฮA = A₀ ฮฒ ฮT, where ฮฒ = 2ฮฑ (approximately).
3. Volumetric (Cubical) Expansion
ฮV = V₀ ฮณ ฮT, where ฮณ = 3ฮฑ (approximately). For liquids, we talk about apparent expansion and real expansion because the container also expands.
๐ฅ Calorimetry
Calorimetry is the measurement of heat transferred. The fundamental principle is the Principle of Heat Exchange: Heat lost by hot body = Heat gained by cold body (assuming no heat loss to surroundings).
Specific Heat Capacity (c)
Amount of heat required to raise temperature of 1 kg of substance by 1°C (or 1 K). Unit: J/kg·K. Q = mcฮฮธ
Water has the highest specific heat capacity (4200 J/kg·K) among common substances, which is why it is used as a coolant and why coastal areas have moderate climate.
Heat Capacity (C)
Amount of heat required to raise temperature of whole body by 1°C. C = mc. Unit: J/K.
Latent Heat (L)
Heat required to change state of 1 kg of substance without change in temperature. Q = mL
- Latent heat of fusion (ice): 336,000 J/kg (or 80 cal/g)
- Latent heat of vaporization (water): 2,260,000 J/kg (or 540 cal/g)
๐จ Modes of Heat Transfer
- Conduction: Transfer in solids without particle movement. Metals are good conductors. Q/t = kA(ฮธ₁-ฮธ₂)/d (Fourier's Law).
- Convection: Transfer in fluids by actual movement of particles. Causes winds, ocean currents.
- Radiation: Transfer without medium via electromagnetic waves. Hot bodies emit infrared radiation. Stefan-Boltzmann Law: E = ฯT⁴
๐ก Key Constants & Values
- Specific heat of water: 4200 J/kg·K
- Latent heat of ice: 336,000 J/kg
- Latent heat of steam: 2,260,000 J/kg
- Absolute zero: -273.15°C = 0 K
- Stefan-Boltzmann constant (ฯ): 5.67 × 10⁻⁸ W/m²·K⁴
- Water's maximum density: at 4°C (1000 kg/m³)
- Relation: ฮณ = 3ฮฑ, ฮฒ = 2ฮฑ
๐ก️ Heat and Temperature
NEB / CEE Physics • 80 MCQs • 4 Rounds