understand that the lowest possible temperature is zero kelvin on the thermodynamic temperature scale and that this is known as absolute zero

Temperature Scales

Learning Objective

• Understand that the lowest possible temperature is zero kelvin on the thermodynamic temperature scale.
• Recognise that this temperature is called absolute zero.

Why Different Scales Exist

Temperature is a measure of the average kinetic energy of particles. Different historical and practical reasons have led to the development of several temperature scales. The three most commonly used scales in physics are:

1. Celsius (°C)
2. Fahrenheit (°F)
3. Kelvin (K)

Kelvin – The Thermodynamic Scale

The Kelvin scale is based on absolute thermodynamic principles. Its zero point, absolute zero, corresponds to the complete cessation of thermal motion of particles (in the classical sense). No temperature can be lower than 0 K.

Conversion Between Scales

The relationships between the three scales are linear, so they can be expressed with simple equations.

\$ T{\text{K}} = T{\text{°C}} + 273.15 \$

\$ T{\text{°C}} = T{\text{K}} - 273.15 \$

\$ T{\text{°F}} = \frac{9}{5}\,T{\text{°C}} + 32 \$

\$ T{\text{°C}} = \frac{5}{9}\,(T{\text{°F}} - 32) \$

\$ T{\text{K}} = \frac{5}{9}\,(T{\text{°F}} - 32) + 273.15 \$

Key Reference Points

Understanding Absolute Zero

At absolute zero (0 K), the thermal motion of particles reaches its minimum possible value. In quantum mechanics, particles still possess zero‑point energy, but no further removal of thermal energy is possible.

Experimental evidence for absolute zero includes:

• Extrapolation of the ideal gas law \$PV = nRT\$ to \$T = 0\$ K.
• Behaviour of superconductors and superfluids at temperatures approaching 0 K.
• Measurements of specific heat capacities that tend to zero as \$T \to 0\$ K.

Practical Implications

Absolute zero provides a fundamental reference for:

1. Defining the Kelvin scale.
2. Calculating thermodynamic efficiencies (e.g., Carnot efficiency \$ \eta = 1 - \frac{T{\text{cold}}}{T{\text{hot}}}\$).
3. Understanding low‑temperature phenomena such as Bose‑Einstein condensation.