# 6.5.1 The Kelvin Scale & Absolute Zero

### The Kelvin Scale & Absolute Zero

• 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 s as:

The temperature at which the molecules in a substance have zero kinetic 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

#### Using the Kelvin Scale

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

θ / °C = T / K − 273.15

T / K = θ / °C + 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 °C

• This is why when using the specific heat capacity equation

Q = mcΔθ

• Δθ does not require the temperature to be in either unit
• This is because the difference in temperature between two values whether in Kelvin or Celsius will be exactly the same

#### Worked Example

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

What is this temperature in Celsius?

Step 1: Kelvin to Celsius equation

θ / °C = T / K − 273.15

Step 2: Substitute in value of 300 K

300 K − 273.15 = 26.85 °C

#### Exam Tip

If you forget in the exam whether it’s +273.15 or −273.15, just remember that 0 °C = 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 °C + 273.15 = 273.15 K.

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