Lecture214Week9

Lecture214Week9 - Heat Heat is a form of energy and any...

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05/07/09 Physics 214 Fall 2008 1 Heat Heat is a form of energy and any object has internal energy in the form of kinetic energy of the atoms or molecules and in potential energy connected with the molecular structure and the electromagnetic forces among the constituents. If we add energy to an object the internal energy increases. Energy can be added in many ways. For example we can do work such as the frictional forces on a moving object. electromagnetic radiation from the sun. a source of heat like a hair dryer.
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05/07/09 Physics 214 Fall 2008 2 First Law of Thermodynamics The increase in the internal energy of a system is equal to the amount of heat added to the system minus the amount of work done by the system. U = internal energy Q = heat that is added or removed W system = is the work done by the system Looking at the pictures you can see that the force pushing on the piston does positive work and therefore is putting energy into the gas. The work done by the gas is negative because the force the piston exerts is to the right and the movement of the piston is to the left. ENERGY CONSERVATION ΔU = Q – W system or ΔU = Q + W external
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05/07/09 Physics 214 Fall 2008 3 Temperature Heat flows from a hot body to a cold body until they reach thermal equilibrium. Temperature is the quantity that measures whether one body is hotter than another and at thermal equilibrium both bodies have the same temperature. The simplest picture is a gas of free molecules where the energy is kinetic. A higher temperaturemeans higher velocities and more stored energy. Increase U and T increases http://jersey.uoregon.edu/vlab/Thermodynamics/index.html http://www2.biglobe.ne.jp/~norimari/science/JavaApp/Mole/e-gas.html
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05/07/09 Physics 214 Fall 2008 4 Measuring temperature Physical properties of an object change with temperature. For example mercury expands as the temperature increases and we can use that expansion to measure T. There are three temperature scales, Celsius, Fahrenheit and Kelvin c f f c K c
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05/07/09 Physics 214 Fall 2008 5 Absolute zero An ideal gas obeys the law PV = constant x T if we keep V constant the pressure is proportional to T. If we measure P versus T at constant volume we find a temperature where the pressure is zero. This is absolute zero where the stored internal energy has it’s lowest possible value. This is Zero degrees Kelvin
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05/07/09 Physics 214 Fall 2008 6 Heat capacity For a given object at a specific temperature the total internal energy depends on how big the object is. If we add a fixed amount of energy the temperature will rise but the rise will be smaller the larger the object is. So we define a quantity called specific heat c = is the quantity of heat required to raise unit mass of a substance by one degree. Q = mcΔT
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Lecture214Week9 - Heat Heat is a form of energy and any...

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