Edexcel A Level (IAL) Physics-5.5 Temperature & Absolute Zero- Study Notes- New Syllabus

Absolute Zero and the Relationship Between Average Kinetic Energy and Absolute Temperature

Absolute zero is the lowest possible temperature that a substance can have. At this temperature, the average kinetic energy of molecules is at its minimum value.

What Is Absolute Zero?

• Absolute zero is the temperature at which molecular motion is minimal.
• It is equal to \( 0\,\mathrm{K} \) on the Kelvin scale.
• This corresponds to \( -273\,^\circ\mathrm{C} \).
• At absolute zero, molecules still possess minimal quantum motion, but no thermal motion.

Key idea: Absolute zero represents the point where the average kinetic energy of molecules is minimum, not necessarily zero.

 Absolute Temperature (Kelvin Scale)

The Kelvin scale is an absolute temperature scale:

• Zero on the Kelvin scale corresponds to absolute zero.
• There are no negative temperatures on the Kelvin scale.
• Temperature intervals are the same size as on the Celsius scale.

The conversion between Celsius and Kelvin is:

\( T = \theta + 273 \)

• \( T \) = temperature in kelvin (K)
• \( \theta \) = temperature in degrees Celsius (°C)

 Average Kinetic Energy of Molecules

The average kinetic energy of molecules in a substance is directly proportional to its absolute temperature:

\( \text{average kinetic energy} \propto T \)

This means:

• Higher absolute temperature → molecules move faster on average
• Lower absolute temperature → molecules move more slowly
• At \( 0\,\mathrm{K} \), average kinetic energy is minimum

Important distinction:

• Temperature depends only on average kinetic energy.
• Internal energy depends on both kinetic and potential energy.

Physical Meaning of Absolute Zero

• Molecules are not completely stationary, but thermal motion is minimised.
• No further thermal energy can be removed.
• Substances approach perfect order.

Absolute zero cannot be reached in practice, only approached.

Importance of the Kelvin Scale

• Used in gas laws and thermodynamics
• Directly proportional to molecular kinetic energy
• Essential for equations involving energy

Example: If temperature doubles in kelvin, the average kinetic energy of molecules doubles.