Question

The internal energy of a substance does not depend upon (a) translational energy (b) vibrational energy (c) energy due to gravitational pull (d) rotational energy

Short answer

(c) energy due to gravitational pull

Step-by-step solution

Understand the Concept of Internal Energy

Internal energy is the total energy contained within a substance due to the movement and interactions of its molecules. It includes translational, vibrational, and rotational energies, which are associated with different types of molecular motion.

Identify Components of Internal Energy

Internal energy includes various types of kinetic and potential energies at the microscopic level: translational energy (straight-line movement of molecules), vibrational energy (oscillation of atoms within molecules), and rotational energy (spinning of molecules around an axis).

Evaluate the Role of Gravitational Energy

Energy due to gravitational pull is a macroscopic energy and does not contribute to the internal energy of a substance. Internal energy focuses on molecular-level energies, and gravitational potential energy affects the system as a whole when considering position relative to a gravitational field.

Determine the Correct Answer

Given the nature of internal energy, it is influenced by molecular motion and interactions (translational, vibrational, and rotational energies), but not by external forces such as gravity. Therefore, energy due to gravitational pull is not a component of internal energy.

Key concepts

Molecular Motion

Molecular motion is a fundamental concept in understanding internal energy. It refers to how molecules move within a substance and contributes significantly to its internal energy.
In any material, molecules are constantly in motion, which can include various types of movements such as translational, vibrational, and rotational motions. Each type of motion plays a role in the total energy possessed by a substance.

• **Translational motion:** This is the linear movement of molecules. It occurs when molecules travel in straight lines, moving from one location to another within the substance.


• **Vibrational motion:** This involves the oscillation of atoms within a molecule. Atoms in a molecule can vibrate back and forth or stretch within bonds, adding to the internal energy.


• **Rotational motion:** Molecules can also spin around an axis, contributing further to the internal energy. This spinning motion is more pronounced in gaseous substances.

Understanding these molecular motions helps in analyzing and calculating the internal energy of a substance.

Kinetic Energy

Kinetic energy is directly related to the movement of molecules, making it a vital component of a substance's internal energy. It acts as a measure of how the energy of a particle, such as a molecule, changes with motion.
When molecules move, they carry kinetic energy due to their motion. This type of energy is crucial for explaining the temperature of a substance because the faster the molecules move, the higher the temperature.

• **Translational kinetic energy:** This part comes from the linear motion of molecules. The faster they translate through space, the more kinetic energy they acquire.


• **Vibrational kinetic energy:** As atoms vibrate within a molecule, their movement generates kinetic energy. Even at room temperature, molecules possess vibrational energy due to the constant oscillation.


• **Rotational kinetic energy:** This form is due to the spinning of molecules around an axis. Larger molecules tend to have more rotational energy because they have more axes around which they can spin.

Thus, kinetic energy at the molecular level is an essential part of what we consider as internal energy.

Potential Energy

Potential energy in the context of molecular interactions is an important aspect of a substance's internal energy. While kinetic energy relates to motion, potential energy is linked to the position and interactions of molecules.
This energy arises from the forces acting between atoms and molecules, particularly the attractive and repulsive forces that govern their interactions.

• **Chemical potential energy:** Derives from the interactions between atoms within a molecule, such as bonds that can store and release energy during chemical reactions.


• **Intermolecular potential energy:** This accounts for the forces between different molecules. Weak forces like Van der Waals forces and stronger forces such as hydrogen bonds contribute to the potential energy, affecting the physical properties like boiling and melting points.

Potential energy doesn't directly cause observable motion but instead affects the stability and structure of the material, playing a crucial role in its overall internal energy.