Energy And Matter

Scientists define energy as the ability to do work. Energy can occur in various forms, and one form of energy can be converted to another form. In a light bulb's filament, electrical energy is converted to radiant energy (light) and thermal energy (heat). Some forms of energy important to biological systems include chemical energy, thermal energy, electrical energy, and mechanical energy. Inside any single organism, energy may be converted from one form to another. For example, after you eat a meal, your body changes the chemical energy found in food into thermal and mechanical energy, among other things.

States of Matter

Although it is not apparent when we observe matter, all the atoms and molecules in any substance are in constant motion. The motion of and spacing between atoms or molecules of a substance determine the substance's state: solid, liquid, or gas, as shown in Figure 2-5. In general, the atoms or molecules of a solid are more closely linked together than in a liquid or gas. Water is an exception to this, as will be described later. Solids move less rapidly than the particles that make up a liquid or a gas. A solid maintains a fixed volume and shape. A liquid maintains a fixed volume, but its particles move more freely than those of a solid, which gives a liquid its ability to flow and to conform to the shape of any container. Particles of a gas move the most rapidly. Gas particles have little or no attraction to each other, and they fill the volume of the container they occupy. Thermal energy must be added to the substance to cause a substance to change states.

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