The Temperature Scale

This lesson covers: 

1. Understanding absolute zero and the thermodynamic temperature scale
2. Converting between Celsius and Kelvin temperature scales
3. The relationship between temperature and internal energy
4. Defining thermal equilibrium

Understanding absolute zero

The kelvin scale, also known as the absolute or thermodynamic temperature scale, is fundamental in thermal physics. Unlike the Celsius scale, which is based on the freezing and boiling points of water, the Kelvin scale is independent of the properties of any specific substance.

Key Features:

• Zero kelvin (0 K) represents the lowest conceivable temperature, known as absolute zero.
• At 0 K, particles possess the least possible internal energy and all motion theoretically stops.
• As temperature increases above 0 K, particles acquire more internal energy, which is linked to their motion.

Converting between celsius and kelvin temperature scales

To convert temperatures from degrees Celsius (°C) to kelvins (K):

T(K) = θ(°C) + 273

To convert temperatures from kelvins (K) to degrees Celsius (°C):

θ(°C) = T(K) − 273

Worked example: Converting from °C to K

Convert 40°C to Kelvin.

Step 1: Formula

T(K)=θ+273

Step 2: Substitution and correct evaluation

T(K)=40+273=313K

Understanding the relationship between temperature and internal energy

Temperature is directly linked to the internal energy of particles in a substance. Higher temperatures correspond to greater internal energy, indicating increased particle movement. Conversely, lower temperatures signify reduced internal energy and decreased particle activity.

Key points:

• Internal energy is the sum of the kinetic and potential energy of particles in a system.
• Increasing the temperature of a substance increases the mean kinetic energy of the particles in the substance
• When a substance changes state, the position of the particles changes and hence the potential energy changes.
• There is no change in kinetic energy during a change of state.

Defining thermal equilibrium

Thermal equilibrium is a key concept in thermodynamics.

When the rate of net energy transfer between two or more objects in thermal contact reduces to zero, they are said to be in thermal equilibrium.

When two or more objects in thermal contact eventually stop exchanging heat energy, they have reached thermal equilibrium. This occurs when:

• The objects attain the same final temperature.
• There is no further net energy transfer between them.

This principle is in line with the zeroth law of thermodynamics:

If objects A and B are each in equilibrium with a third object C, then A and B are in equilibrium with each other.