The Kelvin Scale (Cambridge International A-Level Physics)

Scale of Thermodynamic Temperature

Spec point — 14.2.2

• Temperature is a scalar quantity measured in degrees Celsius (°C) or kelvin (K)

  • Kelvin is the SI base unit for temperature

The Kelvin scale

• The scale of thermodynamic temperature does not depend on the property of any particular substance

  • The thermodynamic temperature scale is also known as the Kelvin scale

• The thermodynamic scale is an absolute scale because it is not defined in terms of the properties of any particular substances

  • This is unlike the degrees Celsius (^oC) temperature scale which is based on the properties of water

The degrees Celsius scale

• When the degrees Celsius (^oC) temperature scale was created

  • The freezing point of water was taken as 0 ^oC
  • The boiling point was taken as 100 ^oC

• The Celsius scale measures the temperature in a liquid-in-glass thermometer

  • The expansion of the liquid is not always linear

Using the Kelvin Scale

• Temperatures in the Celsius scale, θ, and temperatures in the Kelvin scale, T, can be converted using the equation:

θ/ ^oC = T / K − 273.15

T / K = θ /^oC + 273.15

Conversion chart for Kelvin and Celsius

Conversion chart relating the temperature on the Kelvin and Celsius scales.

• The divisions on both scales are equal which means that:

A change in a temperature of 1 K is equal to a change in temperature of 1^oC

Worked Example In many ideal gas problems, room temperature is considered to be 300 K.

What is this temperature in Celsius?

Answer

Step 1: Kelvin to Celsius equation

θ / ^oC = T / K − 273.15

Step 2: Substitute in value of 300 K

300 K − 273.15 = 26.85 ^oC

Exam Hint If you forget in the exam whether it’s +273.15 or −273.15, just remember that 0 ^oC = 273.15 K. This way, when you know that you need to +273.15 to a temperature in degrees to get a temperature in Kelvin. For example: 0 ^oC + 273.15 = 273.15 K.

Absolute Zero

Spec point — 14.2.4

• The lowest possible temperature on the thermodynamic temperature scale, known as absolute zero, is zero Kelvin

  • It is not possible to have a temperature lower than 0 K
  • So a temperature in Kelvin will never be a negative value

• Absolute zero corresponds to the temperature at which the average kinetic energy of the molecules is at its minimum

  • It is not possible to remove any more energy from a system with a temperature of 0 K
  • Even in space, the temperature is roughly 2.7 K, just above absolute zero

• The absolute temperature of a body is directly proportional to the average kinetic energy of the molecules within the body

Temperature-kinetic energy graph

At absolute zero, the temperature is zero Kelvin and the kinetic energy of the molecules is zero