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Temperature and thermal energy are fundamental concepts in physics. They describe how hot or cold objects are and the energy associated with their particles' motion. Understanding these ideas is crucial for grasping heat transfer and thermodynamics.

Temperature scales like Celsius, Fahrenheit, and Kelvin help us measure and compare temperatures. Converting between these scales is essential for practical applications and scientific research. The concept of absolute zero, the lowest possible temperature, is a key principle in thermodynamics.

Temperature and Thermal Energy

Temperature and thermal energy

  • Temperature measures the average kinetic energy of particles in a substance
    • Higher temperature indicates particles have higher average kinetic energy (boiling water)
    • Lower temperature means particles have lower average kinetic energy (ice cube)
  • Thermal energy represents the total kinetic energy of all particles in a substance
    • Depends on both temperature and the number of particles or mass (cup of coffee vs. pot of coffee)
    • Objects with the same temperature can have different thermal energies if they have different masses (ice cube vs. iceberg)
  • Heat capacity is the amount of heat required to raise the temperature of an object by one degree
    • Specific heat is the heat capacity per unit mass of a substance

Conversion of temperature scales

  • Celsius (°C) and Fahrenheit (°F) are relative temperature scales, Kelvin (K) is an absolute scale
  • Convert between Celsius and Fahrenheit:
    • $°F = (°C × \frac{9}{5}) + 32$
    • $°C = (°F - 32) × \frac{5}{9}$
  • Convert between Celsius and Kelvin:
    • $K = °C + 273.15$
    • $°C = K - 273.15$
  • Convert between Fahrenheit and Kelvin:
    • $K = (°F - 32) × \frac{5}{9} + 273.15$
    • $°F = (K - 273.15) × \frac{9}{5} + 32$
  • Absolute zero is the lowest possible temperature, corresponding to 0 K or -273.15°C

Thermal Equilibrium and the Zeroth Law of Thermodynamics

Thermal equilibrium in heat transfer

  • Thermal equilibrium occurs when two or more systems in thermal contact have the same temperature
    • No net heat transfer happens between systems in thermal equilibrium (room temperature coffee and air)
  • Heat always flows from higher to lower temperature system
    • Temperature difference determines the direction of heat transfer (hot coffee to cold mug)
  • When systems with different temperatures are in thermal contact, heat flows until thermal equilibrium is reached (ice melting in warm water)

Zeroth law of thermodynamics

  • States if two systems are in thermal equilibrium with a third system, they are also in equilibrium with each other
    • Allows defining temperature and establishing temperature scales (Celsius, Fahrenheit, Kelvin)
  • Practical applications:
    1. Thermometers: Zeroth law enables calibrating thermometers using fixed points (freezing and boiling points of water)
    2. Thermostats: Zeroth law allows thermostats to control temperature in devices (ovens, refrigerators, HVAC)
    3. Thermal insulation: Zeroth law helps understand the effectiveness of insulation materials in preventing heat transfer (fiberglass, foam)

Thermal Effects

  • Thermal expansion occurs when materials increase in size as temperature increases
  • Latent heat is the energy absorbed or released during a phase change without changing temperature