PRELIMS

Heat and Thermodynamics

Nature and Units of Heat

Heat is energy flowing between bodies due to temperature difference. Amount of heat depends on the mass of the body.

• Mechanical Equivalent of Heat (J): , where is heat produced and .
• Units of Heat:
  1. Calorie (cal): Heat to raise 1 g of water by .
  2. International Calorie: Heat to raise 1 g of water from to .
  3. B.Th.U (British Thermal Unit): Heat to raise 1 pound of water by ( ).
  4. Therm: B.Th.U.
  5. Pound Calorie: 453.6 cal.

Temperature and Measurement Scales

Temperature decides the direction of heat flow (always higher to lower).

• Fixed Points: Freezing point (LFP) and Boiling point (UFP) of water.

• Celsius (°C): LFP: 0°C. UFP: 100°C. Intervals: 100. Conversion Formula:
• Fahrenheit (°F): LFP: 32°F. UFP: 212°F. Intervals: 180. Conversion Formula:

09. Heat

1. Concept and Units of Heat

Heat is a form of energy that flows from a hot body to a cold body due to a temperature difference.

• Units:
  • SI Unit: Joule.
  • Calorie: Heat required to raise the temperature of 1 gram of water by .
  • Relationship: .
  • British Thermal Unit (BTU): .

2. Temperature Scales and Thermometers

• Fundamental Relation:
• Absolute Zero: The theoretical lowest temperature, where molecular motion ceases. ( ).
• Thermometer Types:
  • Mercury: Ranges from to .
  • Alcohol: Used for very low temperatures (Alcohol freezes at ).
  • Platinum Resistance: to .
  • Thermoelectric: to (based on Seebeck effect).
  • Total Radiation Pyrometer: Measures temperatures above using Stefan's Law ( ). Used for the Sun.

3. Specific Heat and Thermal Expansion

• Specific Heat Capacity: Heat required to raise the temperature of unit mass by .
  • Water: Has the highest specific heat ( ).
• Thermal Expansion:
  • Ratios: Linear ( ), Superficial ( ), and Volume ( ) expansion coefficients follow the ratio 1 : 2 : 3.
• Anomalous Expansion of Water:
  • Water has maximum density and minimum volume at .
  • Significance: Allows fish to survive in frozen lakes (bottom stays at ).

4. Latent Heat and Phase Change

• Latent Heat: Heat absorbed or released at constant temperature during a change of state.
  • Latent Heat of Fusion (Ice): .
  • Latent Heat of Vaporisation (Water): .
  • Fact: Steam causes more severe burns than boiling water at because of latent heat.

5. Evaporation and Humidity

• Evaporation: Conversion of liquid to vapour at any temperature below the boiling point. It causes cooling.
• Relative Humidity: Ratio of water vapour present to the maximum it can hold.
  • Measured by a Hygrometer.
  • Comfort: Humans feel comfortable at 60 to 65% relative humidity.

6. Heat Transfer Modes

1. Conduction: In solids; heat flows without actual movement of particles.
2. Convection: In liquids and gases; heat flows due to actual movement of particles.
   • Natural Examples: Sea breeze, land breeze, and the trade winds.
3. Radiation: Heat travels in straight lines at the speed of light without a medium.
   • Newton's Law of Cooling: Rate of cooling is directly proportional to the temperature difference between the body and the surroundings.

7. Thermodynamics and Misc Facts

• Triple Point of Water: Temperature and pressure where all three phases (solid, liquid, gas) coexist ( at ).
• Greenhouse Effect: Atmosphere traps solar radiation (mainly IR), warming the Earth.
• Cloudy Nights: Are warmer than clear nights because clouds reflect back the Earth's long-wave radiation.

8. Heat vs Temperature and Calorimetry

• Heat: Form of energy; total kinetic energy of all molecules; depends on mass; SI unit Joule.
• Temperature: Measure of the average kinetic energy of molecules (degree of hotness); independent of mass; SI unit Kelvin.
• Principle of Calorimetry: Heat lost by a hot body = Heat gained by a cold body, when mixed in an insulated system (no heat exchange with surroundings).

Confused Pairs: Specific Heat vs Latent Heat

• Specific Heat Capacity: Heat needed to raise temperature of unit mass by without change of state.
• Latent Heat: Heat needed to change the state of unit mass at constant temperature (no temperature rise).
• Common Trap: A temperature-vs-time graph is flat (plateau) during latent heat absorption (melting/boiling), but rises during specific-heat-driven warming.

UPSC Relevance

• Scale Conversions: Standard relationship formulas between Celsius and Fahrenheit.
• Everyday Science: Radiation Pyrometers for non-contact measurement and Kirchhoff's law applications.
• Common Trap: Heat vs Temperature, and Specific Heat vs Latent Heat, are recurring conceptual MCQ pairs.

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