Thermal Equilibrium (HL IB Physics)

• Thermal energy is always transferred from a hotter region to a lower region
  • Eventually, once the regions reach the same temperature, no more thermal energy is transferred

• Thermal equilibrium is defined as:

When two substances in contact with each other no longer exchange any heat energy and both reach an equal temperature

• There is no longer thermal energy transfer between the regions

Thermal equilibrium and the direction of energy flow

Two regions of different temperatures will eventually reach thermal equilibrium

• The two regions need to be in thermal contact for this to occur
  • The hotter region will cool down and the cooler region will heat up until they reach the same temperature

Graph showing water and ice reaching thermal equilibrium

The final temperature of the water depends on the initial temperature difference between the water and ice

• An example of this is ice in room temperature water
  • The ice cubes heat up as thermal energy is transferred from the water
  • Therefore, the water cools down as thermal energy is transferred away from the water to the ice