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.
Understanding Physics: Absolute Zero and Temperature Scales
The Kelvin scale is especially useful because its zero point has a physical meaning. Equal steps on the Kelvin and Celsius scales are the same size, so a rise of one kelvin is equal to a rise of one degree Celsius. The difference is where counting begins.
This matters whenever temperature appears in a formula. For example, the pressure of a fixed amount of gas rises in direct proportion to its temperature only when temperature is measured in kelvin. Using Celsius in that relationship can give impossible results near the freezing point of water.
Gas laws show why the lowest limit matters. Imagine a gas sealed in a container with flexible walls. As it is cooled, its particles move more slowly and strike the walls less often and less strongly.
The pressure falls. If the pressure stays constant instead, the container can shrink as the gas cools.
A graph of gas volume against Celsius temperature can be extended toward a volume of zero near minus 273.15 degrees Celsius. Real gases turn into liquids or solids before this simple graph reaches that point, but the pattern helped scientists identify the Kelvin scale.
Absolute zero does not mean that every particle becomes completely still. Classical physics suggests that motion would stop, but quantum physics gives particles a small unavoidable minimum motion called zero point motion. Scientists cannot cool an object exactly to zero kelvin.
Removing the last tiny amount of thermal energy becomes harder as a sample gets colder. Laboratories can reach temperatures extremely close to zero kelvin by using methods such as laser cooling and evaporation cooling. At these temperatures, unusual effects can appear, including superconductivity, where some materials carry electric current with no electrical resistance.
Students meet temperature scales in weather reports, ovens, medical thermometers, refrigerators, engines, and science experiments. Celsius is convenient for daily conditions because water freezes near zero degrees Celsius and boils near one hundred degrees Celsius at normal air pressure. Kelvin is needed for calculations involving gases, radiation, entropy, and thermal energy.
Pay close attention to whether a problem asks for a temperature change or an actual temperature. A change of ten kelvin equals a change of ten degrees Celsius.
However, a temperature of ten kelvin is not the same physical state as ten degrees Celsius. Convert to kelvin before substituting a temperature into most physics equations.
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.