Matter is any substance that has mass and occupies space. Energy is the capacity to do work, existing in various forms like chemical, kinetic, and thermal energy. Adding energy to matter means increasing its internal energy, which is the microscopic motion of its constituent atoms and molecules. Greater internal energy translates directly to faster, more vigorous particle movement, which we often perceive as a temperature increase.
The Simplest Method: Thermal Energy Transfer
The most accessible and widely used method for increasing the internal energy of matter is through thermal energy transfer, commonly known as heating. This process involves the movement of energy from a region of higher temperature to one of lower temperature. At the atomic level, a hotter substance has particles moving with a greater average kinetic energy than a cooler substance.
When a cold object is placed in contact with a hot object, the faster-moving, high-energy particles of the hot object physically collide with the slower-moving particles of the cold object. This is a process called conduction, which is the direct transfer of kinetic energy through contact. Each collision transfers momentum and energy, causing the slower particles to speed up. The net effect is an increase in the random, chaotic movement of the particles in the cooler matter, which continues until both objects reach thermal equilibrium.
Converting Motion into Internal Energy (Mechanical Work)
Another common approach to energizing matter involves applying a physical force over a distance, also called mechanical work. The simplest, most relatable example of this is friction, such as vigorously rubbing one’s hands together. In this case, the ordered, large-scale kinetic energy of the moving hands is converted directly into the disorganized, microscopic motion of the skin cells and underlying molecules.
Friction is a force that opposes relative motion. The work done against this force translates macroscopic motion into intense, localized vibrations at the interface of the two surfaces. As the surface irregularities scrape against each other, the atoms within the material lattices are violently jostled. This results in the random excitation of particles, manifesting as an increase in the material’s internal thermal energy. The temperature rises because the directed, ordered energy of motion is converted into the random kinetic energy of the constituent particles.
Harnessing Light and Waves (Electromagnetic Absorption)
Matter can also absorb energy through non-contact methods, specifically by absorbing electromagnetic radiation like light, infrared, and microwaves. This process involves matter taking up a photon’s energy and transforming it into internal thermal energy. The specific mechanism of absorption depends on the energy of the wave, or the frequency of the photon.
Infrared light has an energy level that matches the natural vibrational frequencies of chemical bonds within molecules. When a molecule absorbs an infrared photon, the energy causes the bonds to stretch, bend, or vibrate more vigorously, directly increasing the molecule’s internal energy. Visible and ultraviolet light carry higher energy, which is typically absorbed by exciting electrons to a higher orbital energy level. These unstable, excited electrons quickly drop back down, releasing their excess energy through collisions with neighboring atoms, which then translate into increased molecular vibration and heat.
Microwaves, used in ovens, work differently by causing polar molecules, such as water, to rapidly rotate in alignment with the oscillating electric field. This rapid, forced rotation generates internal molecular friction, which in turn converts the rotational energy into the random kinetic energy that heats food.