What is the primary factor that causes objects to expand when heated?

The primary factor that causes objects to expand when heated is increased kinetic energy. When heat energy is applied to an object, the molecules within that object begin to move more rapidly. This increased movement leads to greater spacing between the molecules, resulting in the overall expansion of the material.

As temperature rises, the kinetic energy of the particles also rises, causing them to vibrate and move more vigorously. This phenomenon is observed across various materials, whether solids, liquids, or gases, as they all tend to expand in response to elevated temperatures.

For instance, in metals, the increased kinetic energy allows atoms to overcome some of the forces holding them in fixed positions, leading to a greater separation between adjacent atoms. This results in the metal expanding physically. It's a fundamental principle of thermodynamics that applies to most materials. Other factors like pressure and molecular structure can influence expansion but are secondary to the effect of kinetic energy on molecular movement.