Conservation of Energy

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Conservation of Energy (The First Law of Thermodynamics) The principal of conservation of energy, or the first law of thermodynamics, implies that the energy of a system does not change as it goes from one state to another; only its form changes. For example, a glass resting on the edge of a table has a certain potential energy. If the glass is knocked off the edge, its potential energy is converted to kinetic energy as it accelerates towards the ground. When the glass hits the ground the kinetic energy is converted to light energy (sparks), sound energy (a bang), thermal energy (heat), and chemical energy (the glass breaks). The first law of thermodynamics is the basis of all energy conversions from one form to another. Many of our biological activities are geared to perform these energy conversions. For example, during digestion, food molecules are broken down into progressively simpler molecules (chemical-chemical conversion); in the process some of the chemical energy stored in those molecules is converted to the thermal energy necessary to maintain our body temperature. Numerous practical devices have been designed that accomplish useful tasks through the conversion of energy from one form to another. Table 5-3 gives examples of different kinds of energy conversions. Question: If energy can be neither created nor destroyed, how can people claim that there is an “energy shortage”? Answer: Terms such as “energy shortage” and “energy waste” are misnomers. While total energy must remain constant, useful energy — that which can be used as fuel or perform work— may be in short supply. According to the first law, energy can never be wasted; it may only be converted to a form not readily usable to us. This is explained more when we talk about the second law of thermodynamics. Question: If energy can be transformed, what is it transformed into? Table 5-3. Examples of Different Kinds of Energy Conversions FROM/TO MECHANICAL THERMAL CHEMICAL ELECTRICAL LIGHT MECHANICAL Bicycle, Gearbox Friction Cigarette lighter Wind generator, Microphone Sparks THERMAL Gas turbines Heat exchanger Pyrometer Thermocouple Luminescence CHEMICAL Rockets, thermal engines Food, Fires Metabolism Battery, Fuel cell Candle ELECTRICAL Electric motor, Loudspeaker Resistor heater Electrolysis Transformer Light bulb LIGHT Galvanometer Solar collector Photosynthesis Solar cell Fluorescence The First Law of Thermodynamics: Energy can be neither created out of nothing nor destroyed into nothing, but it can be changed from one form to another. 95 Chapter 5 - Thermal Energy Answer: Although we often talk about transformation of energy, we should note that the nature of energy has not changed. We are only talking about its manifestation from one form to another.


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