Chapter 7: Conservation Laws

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A conserved quantity is one that does not change with . Physicists are interested in conserved quantities because they find it helpful to focus on things that are unchanging amid the more general confusion of change. Conserved quantities often provide a shortcut to understanding the essence of complex processes. Until the 20th century, mass was considered to be a strictly conserved quantity, i.e., it was taken to be impossible to either create or destroy mass. Mass is still considered to be a strictly conserved quantity at the level of chemical reactions and is treated as such by chemists. In a later chapter, we will see how the concept has to be refined. Electrical charge is a strictly conserved quantity. Although individual charges can be created, the total amount of positive charge less the total amount of negative charge does not change. "Energy" is a name given to a particular quantity observed to be conserved in nature. In fact, energy is broken down into several subcategories: "Kinetic" energy, "gravitational potential" energy, "internal" energy, "electrical potential" energy, "radiant" energy. These forms of energy are NOT individually conserved. It is the of them that is conserved. "Kinetic Energy" is the energy of . Kinetic energy = 1/2 (mass)x(speed)2. Gravitational potential energy is the energy associated with the of gravitationally attracting objects. Gravitational Potential Energy = ( ) x (height). Gravitational potential energy (decreases, increases?) with the increased separation of two gravitationally attracted objects. energy is energy "hidden" inside of materials. It is associated with the temperature of materials, the physical state of materials (solid, liquid, gas), the chemical state of materials (dynamite, natural gas), and the shape of objects (spring). Friction (of one surface dragging on another) converts energy into internal energy. Electrical potential energy is the energy associated with the position of a charged object relative to another. The electrical potential energy of unlike charges (increases, decreases?) with increased separation (like gravity), BUT the electrical potential energy of like charges (increases, decreases?) with increased separation. Radiant energy is the energy of . Sunlight carries radiant energy. Mechanisms by which energy is transferred from one object to another or from one form to another: Work, heat conduction, radiation, convection, chemical reaction. Work is the transfer of energy from one object to another that occurs when the first exerts a force on the second and the object moves in the direction of the force. Work = (Force)x(Distance moved in the direction of the force). Heat conduction is the flow of energy from a (hot, cold?) object to a (hot, cold?) when the objects are in contact. Radiation is the transfer of energy through space in the form of . Convection is the transfer of energy from one place to another because the energy is stored in matter and the matter is moved from one place to another. A chemical reaction transforms energy from the form of internal energy to some other form (such as kinetic energy).

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