4.1 Thermal Equilibrium
Thermal Energy, Heat and Temperature
- Heat is the flow of thermal energy.
- Temperature is a measure of the average kinetic energy which each molecule of an object possesses.
Thermal Energy and Heat
- Thermal energy is a measure of the sum of kinetic and potential energy in all the molecules or atoms in an object.
- The SI unit of thermal energy is Joule, J.
- Heat is the flow of thermal energy, from a hotter body to a colder one.
Temperature
- Temperature is a physical quantity which measures the degree of hotness of an object.
- Temperature is a measure of the average kinetic energy which each molecule of an object possesses.
- One object is at a higher temperature than another if the average kinetic energy of each of its molecules is greater.
- The SI unit of temperature is Kelvin, K.
Differences between Thermal Energy and Temperature
Thermal Energy | Temperature |
| A form of Energy | Degree of hotness of an object. |
| Unit: Joule (J) | Unit: Kelvin (K)/ Degree Celsius (oC) |
| Sum of the kinetic energy and potential energy of the particles. | Average kinetic energy of the particles. |
| Derived quantity | Base quantity |
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Thermal Equilibrium
Thermal Equilibrium
- Two objects are in thermal contact when heat energy can be transferred between them.
- Two objects are in thermal equilibrium when there is no net flow of heat between two objects that are in thermal equilibrium.
- Two objects in thermal equilibrium have the same temperature.
Example:
Figure below shows 2 blocks in thermal contact with each other. Initially, the temperature of the 2 blocks are different, and there is a net flow of thermal energy from higher temperature to lower temperature.
After some time, thermal equilibrium achieved, where the temperature of the 2 blocks become the same, and there is no net flow of thermal energy between the 2 blocks.
Before
- Initially, the temperature of block A is higher than block B.
- The rate of thermal energy transfer is higher from block A to the block B (1000J/s).
- There is also thermal energy transfer from the block B to block A, but with lower rate (only at 200J/s).
- Therefore, there is a net heat flow of thermal energy from the block A to block B.
- As a result, the temperature of block A decreases whereas the temperature of B increases.
After
- Thermal Equilibrium Achieved.
- The temperature of the 2 blocks become the same.
- Heat flow is still goes on between the blocks.
- However, the rate of flow of heat are equal in both direction. As a result, the net heat flow is equal to 0.