The Laws of Thermodynamics: Essential Concepts of Heat

What is Heat?

Heat is a form of energy that is transferred between objects or systems due to a temperature difference. It flows naturally from regions of higher temperature to regions of lower temperature. Heat is denoted by the symbol Q or q and its SI unit is Joules(J). Also measure in calories.

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Key Concepts in Heat

1. Temperature vs. Heat:
   • Temperature measures the hotness or coldness of an object, related to the average kinetic energy of particles.
   • Heat is the energy transfer due to temperature differences.
2. Thermal Energy:
   The total internal energy(ΔU) of a system, including the kinetic and potential energies of its particles.
3. Heat Transfer Mechanisms:
   • Conduction: Transfer of heat through direct contact between particles (e.g., metal rod heated at one end).
   • Convection: Transfer of heat in fluids (liquids and gases) through bulk movement (e.g., boiling water).
   • Radiation: Transfer of heat via electromagnetic waves, without needing a medium (e.g., sunlight). It is fastest method of heat transfer.

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Measurement of Heat

• Specific Heat Capacity (c):
  The amount of heat required to change the temperature of 1 kg of a substance by 1°C.
  Formula:Q=mcΔT, where Q= heat energy, m= mass, ΔT= temperature change.
• Latent Heat:
  Heat absorbed or released during a phase change without a temperature change (e.g., melting or boiling).
  Formula:Q=mL, where L= latent heat, m= mass.

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Laws of Thermodynamics

1. Zeroth Law:
   If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other.
2. First Law (Law of Energy Conservation):
   The change in internal energy of a system is equal to the heat added to the system minus the work done by the system:
   • ΔU=Q−W
3. Second Law:
   Heat flows spontaneously from hot to cold bodies, and entropy (disorder) tends to increase.
4. Third Law:
   As the temperature approaches absolute zero, the entropy of a system approaches a minimum value.

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Applications of Heat

• Everyday Life: Cooking, heating homes, refrigeration.
• Industrial: Metallurgy, power plants, chemical processes.
• Scientific: Thermodynamics, astrophysics, climate science.
• Transportation: Internal combustion engines, jet propulsion.
• Medicine: Sterilization, thermotherapy.

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Practical Examples

1. Boiling Water: Convection currents transfer heat throughout the water.
2. Metal Spoon in Hot Soup: Heat is conducted through the metal spoon.
3. Sunlight: Radiative heat warms the Earth.

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Important Heat-Related Phenomena

1. Thermal Expansion:
   Most materials expand when heated. This principle is used in bimetallic strips and thermal sensors.
2. Phase Changes:
   • Melting, freezing, boiling, condensation, sublimation.
   • Energy is exchanged during these changes, but temperature remains constant.
3. Heat Engines:
   Devices like steam engines, car engines, and turbines that convert heat into work.

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Units and Conversions

• 1 calorie (cal)=4.184 Joules (J)
• 1kWh (kilowatt-hour)= 3.6×10⁶J