AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Ideal gases (A-level only): Concept of absolute zero of temperature.
Absolute zero is the lowest possible temperature, at which a system has minimum internal energy.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 2 of 4
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Absolute zero is the lowest possible temperature, at which a system has minimum internal energy. For an ideal gas, absolute zero is the temperature at which pressure would be zero at constant volume, or volume would be zero at constant pressure, according to pV = nRT. It is defined as 0 K on the Kelvin scale, equivalent to −273.15 °C. The Kelvin scale is an absolute thermodynamic scale, so temperatures in gas law calculations must be in kelvin. For example, converting 27 °C to kelvin gives 300.15 K. In the classical ideal gas model, at absolute zero, particles have zero kinetic energy and zero internal energy. The concept is assessed by explaining why the Kelvin scale is used and converting between Celsius and kelvin.
What earns the marks
- Absolute zero is the temperature at which the pressure of an ideal gas would be zero at constant volume, or the volume would be zero at constant pressure.
- Absolute zero is defined as 0 K, which is equivalent to −273.15 °C.
- The Kelvin scale is an absolute thermodynamic temperature scale, and temperatures in kelvin are used in the ideal gas equation because T is directly proportional to the average kinetic energy of particles.
- In the classical ideal gas model, at absolute zero, particles have zero kinetic energy and the gas has zero internal energy.
- Converting between Celsius and kelvin: T(K) = θ(°C) + 273.15, often approximated as +273.
Common mistakes and how to put them right
Using degrees Celsius in the ideal gas equation instead of kelvin.
Put it right: Convert to kelvin by adding 273.15 (or 273 for approximate calculations).
Stating that real gases have zero volume at absolute zero.
Put it right: Real gases liquefy and solidify before reaching absolute zero; zero volume only applies to the theoretical ideal gas model.
Believing that absolute zero can be reached.
Put it right: It is a theoretical limit; it cannot be reached in practice, though temperatures very close to it have been achieved.
Exam tips
- Always convert temperatures to kelvin before using the ideal gas equation; show the conversion explicitly.
- When explaining absolute zero, refer to the extrapolation of a pressure–temperature graph to zero pressure at constant volume.
Try the lesson
The start of the Ideal gases (A-level only): Concept of absolute zero of temperature. lesson, made from checked teaching notes for this specification point.
Have a go first
What is the value of absolute zero on the Celsius temperature scale?
Listen
Absolute zero is the lowest possible temperature, at which a system has minimum internal energy. For an ideal gas, absolute zero is the temperature at which pressure would be zero at constant volume, or volume would be zero at constant pressure, according to pV = nRT. It is defined as 0 K on the Kelvin scale, equivalent to −273.15 °C. The Kelvin scale is an absolute thermodynamic scale, so temperatures in gas law calculations must be in kelvin. For example, converting 27 °C to kelvin gives 300.15 K. In the classical ideal gas model, at absolute zero, particles have zero kinetic energy and zero internal energy. The concept is assessed by explaining why the Kelvin scale is used and converting between Celsius and kelvin. A strong answer earns marks for these points. Absolute zero is the temperature at which the pressure of an ideal gas would be zero at constant volume, or the volume would be zero at constant pressure. Absolute zero is defined as 0 K, which is equivalent to −273.15 °C. The Kelvin scale is an absolute thermodynamic temperature scale, and temperatures in kelvin are used in the ideal gas equation because T is directly proportional to the average kinetic energy of particles. In the classical ideal gas model, at absolute zero, particles have zero kinetic energy and the gas has zero internal energy. A common slip: Using degrees Celsius in the ideal gas equation instead of kelvin. Instead, convert to kelvin by adding 273.15 (or 273 for approximate calculations). Exam tip: Always convert temperatures to kelvin before using the ideal gas equation; show the conversion explicitly.
See it
Key words
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Absolute zero
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Kelvin scale
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See every definition
- Absolute zero
- The lowest possible temperature, at which a system has minimum internal energy, defined as 0 K, equivalent to −273.15 °C.
- Kelvin scale
- An absolute thermodynamic temperature scale, and temperatures in kelvin are used in the ideal gas equation because T is directly proportional to the average kinetic energy of particles.
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