18.4
Gas Thermometers and the Kelvin Scale
The definition of temperature in terms of molecular motion suggests that there should be a lowest possible temperature, where the average kinetic ener…
A gas thermometer evaluates temperature based on changes in pressure or volume of the gas-filled.
There are two types of gas thermometers: constant-volume, and constant-pressure, and the constant-volume gas thermometer is the most widely used.
The principle of this thermometer is that, at constant volume, the pressure of a gas increases as temperature increases.
To calibrate the gas thermometer, the pressure at 0 and 100 degrees Celsius is measured.
These values are plotted in a straight line, which is extrapolated to find the zero-pressure temperature.
This temperature is the zero point of the Kelvin scale. This is the standard scale for temperature, as the SI unit of temperature is Kelvin.
Absolute zero is the lowest possible temperature that ideally reflects the utter absence of thermal energy.
On the Kelvin scale, the freezing point of water is 273.15, while the boiling point is 373.15. The difference is the same as on the Celsius scale; only the zero point is different.
Thus, the relationship between Kelvin and Celsius scales can easily be established.
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Q1: How does a constant-volume gas thermometer measure temperature?
A constant-volume gas thermometer measures temperature by monitoring pressure changes in a sealed gas-filled vessel. As temperature increases, the gas pressure rises proportionally while volume remains fixed. This direct relationship between pressure and temperature allows precise temperature measurement and calibration across known reference points.
Q2: What is absolute zero and why is it important to the Kelvin scale?
Absolute zero is the lowest possible temperature where molecular kinetic energy is theoretically zero, representing the complete absence of thermal energy. The Kelvin scale is an absolute temperature scale with its zero point defined at absolute zero. This makes the Kelvin scale ideal for science because physical quantities like ideal gas volume are directly proportional to absolute temperature.
Q3: How is the Kelvin scale calibrated using a gas thermometer?
Gas thermometer calibration involves measuring pressure at two known temperatures: 0 and 100 degrees Celsius. These pressure values are plotted on a graph and extrapolated as a straight line to find where pressure reaches zero. This zero-pressure point defines absolute zero and establishes the zero point of the Kelvin scale.
Q4: What is the relationship between Celsius and Kelvin temperature scales?
The Celsius and Kelvin scales have identical temperature intervals; the difference between freezing and boiling points of water is 100 degrees on both scales. However, their zero points differ: water freezes at 273.15 K and boils at 373.15 K. Converting between scales requires only adding or subtracting this constant offset.
Q5: Why is the constant-volume gas thermometer chosen as the standard thermometer?
The constant-volume gas thermometer is the standard because its pressure readings correlate directly and linearly with temperature. When results are extrapolated to zero gas density, different gases produce consistent measurements that agree with absolute zero. This reliability and reproducibility make it ideal for calibrating other thermometers, despite being large and slow to reach thermal equilibrium.
Q6: What is the triple point and how does it define the Kelvin scale?
The triple point is the unique temperature and pressure condition where ice, liquid water, and water vapor coexist stably. Unlike freezing and boiling points, which vary with pressure, the triple point is constant at 273.16 K. This fixed reference point provides a more precise definition for the Kelvin scale than water's phase transitions.
Q7: Why is Kelvin the standard temperature unit in science?
Kelvin is the SI unit of temperature because it is an absolute scale with zero at absolute zero, making it ideal for scientific calculations. Many physical relationships, such as ideal gas behavior and thermal radiation, depend directly on absolute temperature. This fundamental connection between Kelvin and physical phenomena makes it essential for accurate scientific work.