The Kelvin Scale (CIE A Level Physics)

• As an everyday scale of temperature, Celsius (^oC) is the most familiar
• This scale is based on the properties of water - the freezing point of water was taken as taken as 0 ^oC and the boiling point as 100 ^oC
  • However, there is nothing special about these two temperatures
  • The freezing and boiling point of water will actually change as its pressure changes

• The Celsius scale is used to measure the temperature in a liquid-in-glass thermometer
  • However, the expansion of the liquid might be non-linear

• Other temperature scales include:
  • Fahrenheit, commonly used in the US
  • Kelvin, used in thermodynamics

• The Kelvin scale is known as the thermodynamic scale and was designed to overcome the problem with scales of temperature
• The thermodynamic scale is said to be an absolute scale that is not defined in terms of a property of any particular substance
• This is because thermodynamic temperatures do not depend on the property of any particular substance

• On the thermodynamic (Kelvin) temperature scale, absolute zero is defined as:

The lowest temperature possible. Equal to 0 K or -273.15 °C 

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

• Absolute zero is defined in kinetic terms as:

The temperature at which the atoms and molecules in all substances have zero kinetic and potential energy

• This means for a system at 0 K, it is not possible to remove any more energy from it
• Even in space, the temperature is roughly 2.7 K, just above absolute zero

• To convert between temperatures θ in the Celsius scale, and T in the Kelvin scale, use the following conversion:

θ / ^oC = T / K − 273.15

T / K = θ / ^oC + 273.15

Conversion chart relating the temperature on the Kelvin and Celsius scales

• The divisions on both scales are equal. This means:

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

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