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lecture10sf

# lecture10sf - Energy The capacity to do work or produce...

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Energy: The capacity to do work or produce heat. There are many different types of energy, and energy can be transformed from one type into another. Consider your textbook. What types of energy does it have? Gravitational Potential Energy It can drop, compress a spring, smash a finger. This is stored energy related to the position (height) of the book. Kinetic energy. If it’s moving it has kinetic energy. Even when apparently stopped, it has kinetic energy as part of the earth’s motion Heat The book can melt ice, or transfer its heat to another object at a colder temperature. Chemical energy It can burn. Chemical energy is a sort of potential energy related to the chemical nature of the book. It has the capability of reacting with oxygen and producing heat. Nuclear energy The book contains hydrogen nuclei which conceivably could be made to undergo nuclear fusion. Our textbook uses the symbol U for the total internal energy of a system. This internal energy means energy from all sources. It would be impossible to give an absolute value for the total energy of the book, and such a quantity is never asked for. Instead, we examine U, or changes of energy for well- defined initial and final states. U = U final - U initial where both initial and final states are clearly defined.

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Law of Conservation of Energy The energy of the universe is constant. Energy cannot be created or destroyed. In thermodynamics, we look at a system (such as the textbook, the chemicals in a beaker, or any other defined set), and U refers to the changes of energy of that system. Everything else in the universe is called the surroundings . A mathematical way of expressing the Law of Conservation of Energy is: U sys + U surr = 0 This equation states that any increase in the system’s energy must come from a decrease in the surrounding’s energy. Conversely, any decrease in the system’s energy appear as an increase in the surrounding’s energy. U sys and U surr must have equal and opposite signs. Textbook example: A person lifts the book which increases its gravitational potential energy. The system (textbook) increases in energy. The surroundings (person doing the lifting) decreases in energy--indirectly, some food was used to provide the energy to lift the book. If the person does enough heavy lifting, he will have to eat more. Textbook example: The book drops, smashing the table and making a loud noise. The system (textbook) decreases in energy. That energy appears in the surroundings largely as heat and sound when the book hit the table. An alternate mathematical expression of the Law of Conservation of Energy is: U = q + w U is the change of energy of the system. All (changes) refer to the system unless otherwise noted.
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