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Vacuum Chamber Investigation

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Planning: Apparatus List: When conducting the experiment I will need the following pieces of apparatus: * Rotary Vacuum Pump * Vacuum Chamber * Manometer * Valve * Short Pipe * Long, thin pipe * Stop Clock Apparatus Diagram: Safety Considerations: It is important that when setting up and carrying out the experiment no objects are poked into the belt drive mechanism of the rotary pump. The mains voltage in the mains powered equipment is also dangerous but is screened in normal use. Obeying these two safety points will help prevent physical injury and electric shock. Variables which could affect the Experiment: * The length and diameter of the pipe connecting the vacuum chamber to the vacuum pump. * The size of the vacuum chamber. * Changes in atmospheric pressure may affect the experiment. This is perhaps one of the more major variables because it can account for up to plus or minus 50 mbar. * The consistency of the Rotary Pump may be a major variable in this experiment. It is unlikely that its performance will remain constant when evacuating the vacuum chamber. This is because of heat build up in both electrical and mechanical components such as the mains transformer and seals when pumping takes place. ...read more.


Typically, meteorological variations of 'atmospheric pressure' are approximately plus or minus 50 mbar). A manometer is the perfect instrument to carry out this kind of measurement as it displays pressure in millibar units. It also has the ability to work in the low pressure scale (less than or equal to 200 mbar with a 0.1 mbar resolution. This particular feature will come in useful when determining the zero offset. However when measuring pressure for evacuation purposes, measuring to the nearest mbar is ample. Treatment of Results: Graphical plotting will be used to extract information. Results for the first part of the experiment will be plotted graphically. Pressure (mbar) against Time (s) will be plotted and also further logarithmic graphs. The results for the second part will be treated in a similar fashion but with more analysis due to the more complex nature of air flow through the long, narrow pipe. Pilot Experiment: As stated earlier in the project, it will be necessary to carry out an appropriate pilot experiment. This will put the investigation into a more concise context. It also aids planning. I plan to carry out a routine experiment that in some way simulates the actual pumping speed experiment. The pilot experiment will involve discharging a known capacitor through an unknown resistor. ...read more.


against Time (s) was plotted as well as a logarithmic graph to confirm that the capacitor discharged exponentially. Analysis of Part II of Pilot Experiment: Quite clearly the larger resistor made the overall discharge time much longer. This is not surprising because of the fact that the time constant equation gives us a measure of how fast the resistor-capacitor combination discharges. The capacitor used in this part of the experiment was the same as the one used in the first part (100 micro Farad). However the resistor was much larger. A 700K? resistor was used in place of the 150K? resistor. Therefore the time constant was much larger, and hence the capacitor took longer to discharge. By comparing the values of the time constants in the two parts of the experiment we can see that the 700K? resistor made the overall discharge time approximately 4.5 times longer than with the 150K? resistor: Time Constant of Pilot Experiment I: 15 seconds Time Constant of Pilot Experiment II: 75 seconds ==> 70/15 = 4.67 However it did discharge in the same fashion: -exponentially. This was again confirmed by the logarithmic graph. This factor is important, because it tells us that regardless of the resistor size, the capacitor will still discharge exponentially. From the pilot experiment I have improved my understanding of exponentiality. This will prove to be important in the actual experiment; however I do not feel that my plan needs any revisions at this stage. ...read more.

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