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Isobar (pressure): Difference between revisions

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[[Category:Done 2020-01-31]]  
[[Category:Done 2021-10-29]]  
[[File:isobar-proces.png|thumb|700px|right|The [[PV diagram]] of an isobaric process<ref>Wikimedia Commons [Online], Available: http://wiki.mitsted.dk/?page=Billede:Isobar-proces.png</ref>]]  
[[File:isobar-proces.png|thumb|700px|right|The [[PV diagram]] of an isobaric process<ref>Wikimedia Commons [Online], Available: http://wiki.mitsted.dk/?page=Billede:Isobar-proces.png</ref>]]  


<onlyinclude>An '''isobar''' in the context of [[thermodynamics]] refers to any process in which the [[system]] remains at a constant (unchanging) [[pressure]] throughout. </onlyinclude>
<onlyinclude>An '''isobar''' in the context of [[thermodynamics]] refers to any process in which the [[system]] remains at a constant (unchanging) [[pressure]]. </onlyinclude>


When a system expands at a constant pressure, such as a [[piston]] in an [[internal combustion engine]], its [[volume]] is increasing and this corresponds to an output of [[work]]. However, the system cannot do this work without an input (or output) of [[heat]]. By analyzing the [[first law of thermodynamics]], the amount of heat necessary to do a given amount of work without the pressure changing can be calculated by using the system's [[specific heat capacity]].  
When a system expands at constant pressure, such as a [[piston]] in an [[internal combustion engine]], it's [[volume]] increases which corresponds to an output of [[work]]. However, the system cannot do this work without an input (or output) of [[heat]]. By analyzing the [[first law of thermodynamics]], the amount of heat necessary to do a given amount of work without the pressure changing can be calculated by using the system's [[specific heat capacity]].  


To calculate the work done in an isobaric process, simply calculate the negative area under the curve. This is calculated using the [[ideal gas law]] the following equation:<ref name=Knight>R. Knight, Physics for scientists and engineers. Boston, Mass.: Addison-Wesley, 2012, p. 474.</ref>
To calculate the work done on a g as in an isobaric process, calculate the negative area under the curve. As seen in figure 1, this area will be a rectangle, so the area will be the base of the rectagle(<math>\Delta V</math>) times its height (<math>p</math>):<ref name=Knight>R. Knight, Physics for scientists and engineers. Boston, Mass.: Addison-Wesley, 2012, p. 474.</ref>


<center><math>W = -p\Delta V</math></center>
<center><math>W = -p\Delta V</math></center>


Therefore, a [[system and surrounding|closed system]] that is being compressed (decreasing volume), will have a positive <math>W</math> value—contrarily an expansion (increasing volume) will result in a negative <math>W</math> value. For a system to compress, it requires a cooling source (or output of heat from the system) to keep the pressure constant. Otherwise, the compressed volume ''without'' the removal of heat will increase the pressure. Contrarily, for expansion the system requires an input of heat, otherwise ''without'' the addition of heat, the pressure in the container would decrease.  
Therefore, a [[system and surrounding|closed system]] that is being compressed (decreasing volume), will have a positive <math>W</math> value—contrarily an expansion (increasing volume) will result in a negative <math>W</math> value.  


A real life example where an isobaric process is taking place is when cooking food. Since the pot is an [[system and surrounding|open system]], the molecules can escape (as vapour) as the contents in the pot get hotter. Referring to the ideal gas law: <math>pV = nRT</math>, if heat is added to this system temperature (<math>T</math>) increases, however, since it's an open system, molecules can escape (<math>n</math> decreases). When molecules escape this will decrease the volume (<math>V</math>) as well.  
<math>W > 0</math> when the gas is compressed. Energy is transferred from the environment to the gas.
 
<math>W < 0</math> when the gas expands. Energy is transferred from the gas to the environment.
 
 
For a system to compress, it requires a cooling source (or output of heat from the system) to keep the pressure constant. Otherwise, the compressed volume ''without'' the removal of heat will increase the pressure. Contrarily, for expansion the system requires an input of heat, otherwise ''without'' the addition of heat, the pressure in the container would decrease.
 
A real-life example of an isobaric process is cooking food in an open pot. Since the pot is an [[system and surrounding|open system]], the molecules can escape (as vapour) as the contents in the pot get hotter. Referring to the ideal gas law: <math>pV = nRT</math>, if heat is added to this system temperature (<math>T</math>) increases, however, since it's an open system, molecules can escape (<math>n</math> decreases). When molecules escape this will decrease the volume (<math>V</math>). Throughout this process, the pressure stays constant.  


To think of it through values imagine that pressure (<math>p</math>) is 6, <math>V</math> is 10, <math>n</math> is 3, and <math>T</math> is 20 (R is just a constant). Currently, both equations are equal to one another with the value of 60. The cooking will cause <math>T</math> to be 25 and <math>n</math> to be 2. Therefore, volume needs to decrease a bit (8.3), so then pressure can stay constant.


For more information, visit [http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/cppro.html#c1 Hyperphysics].[[Category:Uploaded]]
For more information, visit [http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/cppro.html#c1 Hyperphysics].[[Category:Uploaded]]
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*[[PV diagram]]
*[[PV diagram]]
*[[Ideal gas law]]
*[[Ideal gas law]]
*[[Isochore]]
*[[Isochore (volume)]]
*[[Isothermal]]
*[[Isothermal]]
*[[Adiabatic]]
*[[Adiabatic]]

Latest revision as of 19:47, 20 December 2021

The PV diagram of an isobaric process[1]

An isobar in the context of thermodynamics refers to any process in which the system remains at a constant (unchanging) pressure.

When a system expands at constant pressure, such as a piston in an internal combustion engine, it's volume increases which corresponds to an output of work. However, the system cannot do this work without an input (or output) of heat. By analyzing the first law of thermodynamics, the amount of heat necessary to do a given amount of work without the pressure changing can be calculated by using the system's specific heat capacity.

To calculate the work done on a g as in an isobaric process, calculate the negative area under the curve. As seen in figure 1, this area will be a rectangle, so the area will be the base of the rectagle([math]\Delta V[/math]) times its height ([math]p[/math]):[2]

[math]W = -p\Delta V[/math]

Therefore, a closed system that is being compressed (decreasing volume), will have a positive [math]W[/math] value—contrarily an expansion (increasing volume) will result in a negative [math]W[/math] value.

[math]W \gt 0[/math] when the gas is compressed. Energy is transferred from the environment to the gas.

[math]W \lt 0[/math] when the gas expands. Energy is transferred from the gas to the environment.


For a system to compress, it requires a cooling source (or output of heat from the system) to keep the pressure constant. Otherwise, the compressed volume without the removal of heat will increase the pressure. Contrarily, for expansion the system requires an input of heat, otherwise without the addition of heat, the pressure in the container would decrease.

A real-life example of an isobaric process is cooking food in an open pot. Since the pot is an open system, the molecules can escape (as vapour) as the contents in the pot get hotter. Referring to the ideal gas law: [math]pV = nRT[/math], if heat is added to this system temperature ([math]T[/math]) increases, however, since it's an open system, molecules can escape ([math]n[/math] decreases). When molecules escape this will decrease the volume ([math]V[/math]). Throughout this process, the pressure stays constant.


For more information, visit Hyperphysics.

For Further Reading


References

  1. ↑ Wikimedia Commons [Online], Available: http://wiki.mitsted.dk/?page=Billede:Isobar-proces.png
  2. ↑ R. Knight, Physics for scientists and engineers. Boston, Mass.: Addison-Wesley, 2012, p. 474.
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