Heat Transfer

1. Heat energy can be transferred between two containers by putting them in direct physical contact, and this heat energy will always flow from the hotter container to the cooler container.
2. The temperature of a substance is dependent on the kinetic energy of its atoms or molecules. Because atoms or molecules can transfer some of their kinetic energy by colliding with other atoms, heat energy can be transferred through atomic collisions.

3. If there are two substances each inside their own container, then heat energy can be transferred between the two substances through the following process:
   

   • The atoms of each substance collide with the atoms of their respective containers.
   • If the containers are touching each other, the atoms of one container can collide with the atoms of the other container which can then collide with the atoms of the substance within the container.
   • Eventually, the two substances and their containers come to thermal equilibrium [a state in which all atoms in the system have the same average kinetic energy].

    

4. During the process of reaching thermal equilibrium you might wonder how atoms know which way to transfer their kinetic energy. In fact, heat is flowing from the hot container to the cold AND from the cold to the hot. However, because the hotter container has a greater portion of atoms with higher kinetic energies, the rate of kinetic energy transfer from the hotter container to the cooler container is faster than the the rate of energy transfer back from the cooler container to the hotter container. The result is a decrease in the temperature of the hotter container and an increase in the temperature of the cooler container.
5. Eventually, when the temperatures become equal, the rate of energy exchange is equal and it appears that nothing is happening. However, energy is still being exchanged from one container to another. It’s just that the rate of exchange is equal. This state is known as thermal equilibrium.

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