Suppose I had a perfect syringe about 10 feet long. When the syringe was pushed all the way in it would be void of an gasses. Now I start to pull back on the syringe. Would the effort required to pull back level off at a point or at least almost level off?
The differential between the pressure inside and outside the syringe cannot be greater than atmospheric pressure. So, that should be the limit, once the pressure inside drops to zero.
Thanks for the reply, for some reason I am having trouble grasping that. Does that mean I could not pull the syringe back past a certain point or that once I reached a certain point the effort to pull it back would remain constant?
It depends on the air pressure outside the syringe. If you do your experiment in a vacuum chamber, there is no additional effort to pull back the syringe (beyond the friction from the perfect seals).
Level off ? Certianly not to zero. The force to pull the syringe back is constant or almost …
There is static friction that is known to be larger than when its moving, and there is the force to squash the gasket down, and so on, that means its harder to get it started than keep it moving… So there is that starting force, and then it gets easier… but it doesn’t go down to zero …
Thats clear now, if I would have thought about pulling back the syringe in a vacuum chamber it would have cleared it right up. For some reason I was having trouble wrapping my brain around this.
As a note - while the force on the syringe arm has been properly addressed, I’d like to note that you will not get a “good” vacuum after a certain point. This is because molecules will permeate from the syringe, or small gases will remain in the space, etc. etc.
You can get a better vacuum, if you machine the end of the syringe body and the piston to have minimal gap, cryogenic ally Liquify gases in the gap to take them out before pulling, etc etc
Sounds a little extreme but this is the level of vacuum needed for making semiconductors.
[nitpick]The syringe is the whole aparatus. What you’re pulling back is the plunger.[/nitpick]
I was brain dead this morning and looking for that word.
There will always be atmospheric pressure pushing on the plunger. So, like holding a bowling ball (always pulled by gravity) there is a specific fixed amount of energy and work to moving the plunger further and further up the barrel (assuming the perfect seal, perfect vacuum inside the syringe, no friction, etc. “Assume a spherical chicken”) Let go and the atmospheric pressure will push the plunger back all the way in, just as when you let go of the bowling ball, it falls until it hits ground that stops it.
It is not a fixed amount of work; it is a fixed amount of work per distance (or in other words, a fixed amount of force). If you couldn’t see the body of the plunger, you couldn’t tell by feel how far it had already been pulled out, and pulling it an additional centimeter would take the same amount of energy.
Could someone explain further. Why doesn’t it take more effort to create a larger volume of vacuum than a smaller volume of vacuum in the syringe?
The way I understand it so far is that it would take more effort but not more peak force.
It does take more overall effort to create a larger volume. What people are saying is that every cubic inch of vacuum created takes the same effort, regardless of how much you’ve already created. So it doesn’t get harder as you create more (but you still have to keep working).
This may seem obvious, but it’s not the way most springs work, for instance: it’s easier to stretch a spring the first inch than the ninth inch (assuming you haven’t stretched it so far it’s broken, of course).
So, the idea is that the syringe’s open end is sealed after all the air is pushed out, right? So, then the only force you have to overcome is the force of the air against the plunger. If the plunger had a 1 square inch cross-section, then the force required to pull the plunger would be about 14.7 pounds. As long as you maintain that force, you can pull the plunger out.
I understand it, sort of, but it sure is counter-intuitive.
Suppose you are using the syringe to squeeze a volume of air. The farther you compress it, the greater the pressure there is, therefore the greater the force resisting your push. You’re probably used to this idea, especially if you’ve ever tried to inflate a road bike tire with a hand pump.
Now consider instead a syringe containing a vacuum, i.e. the pressure inside is exactly zero. No matter how far back you pull the plunger, the pressure inside will always be exactly zero, which means there will be a constant force resisting your pull.
On the other hand, if you start with a small but nonzero amount of air in the syringe (which is probably a more likely situation to encounter), then it will in fact require more force the further you pull it out, to an asymptote of the vacuum value.
I was actually thinking of different ways that I may be able to use Vacuum to power a pumpkin chucker. there may be some advantages to that constant force. A 3ft cyl would give you a constant force of about 15,000# over as much distance as you decided to apply it.
That’s how the vacuum ping-pong cannon, aka vacuum bazooka, works. Although Mythbusters tested it and found that there is a limit to it, due to air leakage around the projectile.
If you were in a sealed room, creating more vacuum means compressing the air in the room into a smaller volume. In the open or a non-sealed room, the air displaced by creating more vacuum is just going into the general atmosphere. You are in fact making the rest of the atmosphere deeper (and increasing pressure) in the same way that adding a cup of water raises the sea level around the world… sufficiently small to be effectively zero.
Like I said, consider it analogous to raising a bowling ball. There’s a constant pressure on the plunger, equal to the difference in atmospheric pressure (times area of plunger) on each side - 0 on the inside, 14.7 psi (depending on the weather and altitude) on the outside. Similarly (ignoring gravity getting less as you get higher by the foot) it takes X amount of energy to raise a bowling ball, the same each foot it goes up. It takes a lesser but significant amount of energy to hold steady the plunger/ bowling ball against the force being exerted by air pressure/gravity.