Difficulty: Easy
Correct Answer: Both atoms and molecules moving randomly
Explanation:
Introduction / Context:
Thermal energy is one of the most important ideas in basic physics because it connects temperature, particle motion, and heat transfer. When we say that a substance is hot or cold, we are really describing how fast its particles are moving on average. This question tests whether you understand that thermal energy is related to the random motion of microscopic particles such as atoms and molecules, rather than being something separate that floats around independently of matter.
Given Data / Assumptions:
Concept / Approach:
According to kinetic theory, all matter is made up of particles such as atoms and molecules. In solids, these particles vibrate around fixed positions. In liquids, they move more freely while remaining close together. In gases, they move rapidly and are widely spaced. Thermal energy is the total internal kinetic energy due to this random motion of particles. The faster the atoms and molecules move on average, the higher the temperature of the substance. While electrons also move and carry energy, the basic school level explanation of thermal energy focuses on the motion of atoms and molecules as whole particles forming the material.
Step-by-Step Solution:
1. Recognize that solids, liquids, and gases are all made up of very small particles, which can be individual atoms or molecules made of several atoms.2. Understand that thermal energy comes from the random motion of these particles, including vibrations, rotations, and translations.3. Know that when a substance is heated, its atoms and molecules move faster, and when it is cooled, they move more slowly.4. Realize that this idea applies whether the basic particles are atoms, as in noble gases, or molecules, as in water or oxygen gas.5. Therefore, thermal energy is associated with the motion of both atoms and molecules, not only one of these groups and not electrons alone.6. The most accurate option is the one that includes random motion of both atoms and molecules.
Verification / Alternative check:
Everyday examples support this explanation. When you heat a metal rod, the atoms vibrate more strongly. When water is heated, its molecules move faster and eventually escape into the air as steam. Gas in a cylinder exerts more pressure when heated because the gas molecules move faster and collide more often with the walls. All these cases involve the motion of atoms or molecules. Textbook diagrams of kinetic theory always show small particles moving randomly inside containers, which confirms that thermal energy is tied to their motion rather than to electrons only or to some mysterious separate fluid.
Why Other Options Are Wrong:
Option A, atoms only, is incomplete because many substances, especially gases and liquids, consist mainly of molecules, and their motion must also be included. Option B, molecules only, is also incomplete because in some elements such as noble gases, the basic particles are single atoms. Option D, electrons only, is incorrect at this level because thermal energy in ordinary matter is described by the motion of entire atoms and molecules, not just electrons inside atoms. Therefore, only the option that mentions both atoms and molecules gives a complete answer.
Common Pitfalls:
A common mistake is to think that only gases have moving particles, while solids are completely rigid. In reality, particles in solids vibrate constantly, and that vibration contributes to thermal energy. Another pitfall is to think of heat as something separate from matter, instead of understanding that it is the energy of moving particles. Remembering that all states of matter have atoms or molecules in motion and that temperature is a measure of their average kinetic energy helps avoid these errors.
Final Answer:
Thermal energy in a substance is mainly caused by the random motion of both atoms and molecules that make up the material.
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