5.1.2 Measurement of Temperature

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

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 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.

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