The Kelvin Scale (CIE A Level Physics)

• 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 taken as 0 ^oC
  • The boiling point as 100 ^oC
• The Celsius scale measures the temperature in a liquid-in-glass thermometer
  • The expansion of the liquid is not always linear

• 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

• 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