Internal energy

Definition

Internal energy is the total energy contained within a system due to the kinetic and potential energies of its molecules. It encompasses all forms of energy at the molecular level, including translational, rotational, vibrational, and potential energies, which are crucial for understanding how heat and work interact in thermodynamic processes.

5 Must Know Facts For Your Next Test

1. Internal energy is denoted by the symbol U and is an extensive property, meaning it depends on the amount of substance in the system.
2. Changes in internal energy can occur due to heat transfer (q) into or out of the system and work (W) done on or by the system.
3. In an isolated system where no heat or work transfers occur, the internal energy remains constant according to the first law of thermodynamics.
4. The internal energy of an ideal gas can be calculated using only its temperature, since it depends solely on the kinetic energy of the molecules.
5. Phase changes, such as melting or boiling, involve changes in internal energy without temperature changes, as energy is absorbed or released during these transitions.

Review Questions

• How does internal energy relate to the first law of thermodynamics, and what implications does this have for thermodynamic processes?
  • Internal energy is directly linked to the first law of thermodynamics, which states that the change in internal energy (ΔU) of a closed system is equal to the heat added to the system (q) minus the work done by the system (W). This relationship implies that any heat transfer or work performed affects the internal energy, highlighting how different processes, such as heating or compression, can alter a system's energy state.
• Discuss how phase changes affect internal energy and provide examples of how this can be observed in real-world scenarios.
  • During phase changes like melting or boiling, internal energy increases even though temperature remains constant. For instance, when ice melts to water, heat is absorbed without a temperature rise, indicating that internal energy increases as intermolecular forces weaken. Similarly, when water boils to steam, more energy is required to break intermolecular bonds before temperature starts to rise again. These examples illustrate how heat input during phase changes alters internal energy despite no change in temperature.
• Evaluate the role of thermal energy in determining a system's internal energy and how this impacts the behavior of gases in different conditions.
  • Thermal energy plays a crucial role in determining a system's internal energy since it corresponds to the average kinetic energy of its molecules. In ideal gases, this means that higher temperatures lead to increased molecular motion and therefore higher internal energy. Conversely, at lower temperatures, molecular motion decreases, reducing thermal energy and internal energy. This relationship helps explain gas behavior under various conditions, such as expansion or compression: as gas expands, it cools and loses thermal energy, thus lowering its internal energy unless heat is added from an external source.

Related terms

enthalpy: A thermodynamic quantity that represents the total heat content of a system, used to calculate changes in internal energy when pressure-volume work is done.

thermal energy: The part of the internal energy associated with the temperature of a system, reflecting the average kinetic energy of its molecules.

first law of thermodynamics: A principle stating that energy cannot be created or destroyed, only transformed, which connects internal energy changes to heat added to or work done by the system.