Scale of Thermodynamic Temperature
- Temperature is a number used to indicate the level of hotness of an object on some scale
- To measure temperature a scale is needed involving two fixed points at known temperatures
- The temperature of other objects can then be defined as a position on this scale
- 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
- The fixed points on the absolute temperature scale are:
- Triple point of pure water
- The point where pure ice, pure water and pure water vapour all exist at the same temperature and pressure
- Absolute zero
- The lowest possible temperature
- Triple point of pure water
Units of Temperature
- 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
- 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
Worked Example
In many ideal gas problems, room temperature is considered to be 300 K.What is this temperature in Celsius?
Step 1: State the conversion between Kelvin and Celcius
θ / oC = T / K − 273.15
Step 2: Substitute in value of 300 K and calculate the value in Celcius
300 K − 273.15 = 26.85 oC
Exam Tip
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.
