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AP_Physics_B_-_Thermodynamics

# AP_Physics_B_-_Thermodynamics - Thermodynamics AP Physics B...

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Thermodynamics AP Physics B

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Work done by a gas Suppose you had a piston filled with a specific amount of gas. As you add heat, the temperature rises and thus the volume of the gas expands. The gas then applies a force on the piston wall pushing it a specific displacement. Thus it can be said that a gas can do WORK.
Work is the AREA of a P vs. V  graph positive W negative W V V V P W on by = = = - = = + = - = gas the ON done is Work gas the BY done is Work The “negative” sign in the equation for WORK is often misunderstood. Since work done BY a gas has a positive volume change we must understand that the gas itself is USING UP ENERGY or in other words, it is losing energy, thus the negative sign. When work is done ON a gas the change in volume is negative. This cancels out the negative sign in the equation. This makes sense as some EXTERNAL agent is ADDING energy to the gas.

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Internal Energy ( U) and Heat Energy (Q) All of the energy inside a system is called INTERNAL ENERGY, U. When you add HEAT(Q), you are adding energy and the internal energy INCREASES. Both are measured in joules. But when you add heat, there is usually an increase in temperature associated with the change. 0 , 0 , = = U T if U T if T U α
First Law of Thermodynamics “The internal energy of a system tend to increase when HEAT is added and work is done ON the system.” by Add on Add W Q U or W Q U W Q U - = + = + = Suggests a CHANGE or subtraction You are really adding a negative here!

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