Temperature tells us how hot or cold matter is by describing the average kinetic energy of its particles. In physics, temperature scales are more than labels on a thermometer because they connect everyday measurements to molecular motion and energy. Celsius and Fahrenheit are common in daily life, while Kelvin is the standard scale for science.
Absolute zero is the lowest possible temperature, where a system has minimum thermal energy.
Key Facts
- Absolute zero is 0 K = -273.15°C = -459.67°F.
- Kelvin to Celsius: °C = K - 273.15.
- Celsius to Kelvin: K = °C + 273.15.
- Celsius to Fahrenheit: °F = (9/5)°C + 32.
- Fahrenheit to Celsius: °C = (5/9)(°F - 32).
- Average translational kinetic energy of one gas particle is KEavg = (3/2)kBT, where T is in kelvin.
Vocabulary
- Absolute zero
- Absolute zero is the lowest possible temperature, defined as 0 K, where a system has minimum thermal energy.
- Kelvin
- Kelvin is the SI temperature scale that starts at absolute zero and uses the same size degree interval as Celsius.
- Celsius
- Celsius is a temperature scale where water freezes at 0°C and boils at 100°C at standard atmospheric pressure.
- Fahrenheit
- Fahrenheit is a temperature scale where water freezes at 32°F and boils at 212°F at standard atmospheric pressure.
- Thermal energy
- Thermal energy is the internal energy associated with the random motion and interactions of particles in matter.
Common Mistakes to Avoid
- Using Celsius in gas law equations is wrong because temperature in equations like PV = nRT must be measured in kelvin.
- Writing 0°C as absolute zero is wrong because 0°C is the freezing point of water, while absolute zero is -273.15°C.
- Saying particles completely stop moving at absolute zero is too simple because quantum mechanics still allows zero-point energy in real systems.
- Adding 32 when converting Fahrenheit to Celsius is wrong because the correct order is subtract 32 first, then multiply by 5/9.
Practice Questions
- 1 Convert 25.0°C to kelvin and Fahrenheit.
- 2 A gas sample is cooled from 300 K to 150 K at constant volume. If its initial pressure is 200 kPa, what is its final pressure?
- 3 Explain why the Kelvin scale is used in formulas involving molecular kinetic energy instead of the Celsius or Fahrenheit scales.