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Internal energy is the total energy of all particles in a substance, including kinetic and potential energy.
In more detail, internal energy is a key concept in understanding how particles behave in different states of matter. It is the sum of two types of energy: kinetic energy, which is the energy of motion, and potential energy, which is the energy stored due to the positions of the particles relative to each other.
When we talk about kinetic energy in the context of internal energy, we are referring to the movement of particles. In a gas, for example, particles move rapidly and freely, colliding with each other and the walls of their container. This high-speed motion means that gases have a high kinetic energy component to their internal energy. In liquids, particles are still moving but are closer together and slide past each other, resulting in moderate kinetic energy. In solids, particles vibrate in fixed positions, so their kinetic energy is lower compared to liquids and gases.
Potential energy, on the other hand, is related to the forces between particles. In a solid, particles are tightly packed in a regular arrangement, and the potential energy is high because of the strong forces holding them together. In a liquid, particles are less tightly packed, so the potential energy is lower. In a gas, particles are far apart, and the potential energy is very low because the forces between them are minimal.
Changes in internal energy can occur through heating or cooling. When a substance is heated, its particles gain kinetic energy and move faster, increasing the internal energy. Conversely, when a substance is cooled, its particles lose kinetic energy and move more slowly, decreasing the internal energy. Understanding internal energy helps explain why substances change state, such as melting, boiling, or freezing, as these processes involve changes in the energy of the particles.
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