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Would a syringe in space suck?

  1. May 29, 2012 #1
    Depends on how desperate you are? ;) This is a question of the nature of vacuums.
    Part 1- In the relatively pure vacuum of space, (for this case let's consider it pure please). Can volume of a closed chamber be increased or decreased without force? (other than the friction of materials, but for this purpose consider theoretical no friction) Pulling the plunger would create no pressure difference, so no suction, right, or in a close cylinder, no resistance to volume change right?
    Part 2 - I am given to understand that you cannot pull a liquid. Displacement pumps i understand to work by using the pressure difference of atmospheric pressure (the weight of air), to push a liquid into an area of lesser pressure, (created by increasing the volume per unit of material) in this case pulling a plunger! Say i have a 100 foot syringe. this syringe has the needle end sealed. let's say there is a cubic inch of air at the end, (the plunger is perfect and has no friction and an impermeable seal, the syringe cylinder is infinitely strong, as is the plunger). When i pull back on the plunger, i require greater and greater force to move the plunger. Will that force plateu and then become effectively constant? - Do i have this right: at equilibrium the pressure = the force with which the molecules are colliding against the plunger/piston and is equal on inside and out. When the plunger is pulled back, the inside molecules are not providing equivalent pressure/collisions and i must provide the extra force. This force is = to lifting the weight of a column of air the thickness of the area of the plunger surface minus the pressure exerted by whatever is in the volume inside the cylinder. Since this will become increasingly negligible, will the force necessary to expand the volume apprach a constant?
    If i start with nothing at all in the cylinder, will i be able to continue to increase the volume of the chamber with a Constant force of 14.7 pounds per square inch(i think thats standard atmospheric pressure) untill i reach the end of my syringe tube, or will i need to continually increase the force, if so, why. Thanks!
    Last edited: May 30, 2012
  2. jcsd
  3. May 29, 2012 #2


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    Staff: Mentor

    In a perfect vacuum you can move the plunger back and forth as much as you like, and the only force opposing you will be the friction of the plunger against the barrel of the syringe.

    The resistance you feel when you're in the earth's atmosphere and you pull the plunger out is caused by the air pressure outside the syringe wanting to push the plunger back in. In a perfect vacuum, there's no air, no pressure, hence no force to push the plunger in. In fact, if there were any air inside the syringe, its pressure would force the plunger out - this doesn't happen on earth because the atmospheric pressure outside balances the pressure from the air inside.
  4. May 30, 2012 #3


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    Staff Emeritus
    Science Advisor

    As you pulled the plunger back on your sealed 100ft syringe you would have to apply a greater and greater amount of force to keep going. If you were in space then it would not, as there would be no air pushing the plunger back in.
  5. May 30, 2012 #4


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    2017 Award

    Staff: Mentor

    Correct. The same happens if you try to pump water (which is at atmospheric pressure) to a height of >10m. During the first 10m of raised water level in the pump, the required force increases, afterwards you can try to "suck your vacuum away" as much as you want, nothing changes.
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