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HoneyBadgerDC
08-12-2014, 08:05 AM
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?

Colophon
08-12-2014, 08:08 AM
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.

HoneyBadgerDC
08-12-2014, 08:15 AM
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?

leahcim
08-12-2014, 08:15 AM
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).

Isilder
08-12-2014, 08:18 AM
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?

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 ..

HoneyBadgerDC
08-12-2014, 08:23 AM
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.

am77494
08-12-2014, 08:41 AM
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.

Gary T
08-12-2014, 11:21 AM
The syringe is the whole aparatus. What you're pulling back is the plunger.

HoneyBadgerDC
08-12-2014, 11:30 AM
The syringe is the whole aparatus. What you're pulling back is the plunger.

I was brain dead this morning and looking for that word.

md2000
08-12-2014, 02:37 PM
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.

Chronos
08-12-2014, 02:47 PM
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.

Thin Ice
08-12-2014, 02:59 PM
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?

HoneyBadgerDC
08-12-2014, 03:11 PM
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.

Quercus
08-12-2014, 03:21 PM
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?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).

RitterSport
08-12-2014, 03:31 PM
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.

Machine Elf
08-12-2014, 03:31 PM
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?

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.

Chronos
08-12-2014, 03:39 PM
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.

HoneyBadgerDC
08-12-2014, 04:34 PM
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.

scr4
08-12-2014, 05:12 PM
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 (http://www.tested.com/science/physics/453458-ping-pong-ball-launcher-blows-past-sound-barrier-through-paddle/), aka vacuum bazooka, works. Although Mythbusters tested it and found that there is a limit to it, due to air leakage around the projectile.

md2000
08-12-2014, 05:37 PM
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?

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.

Chronos
08-12-2014, 06:42 PM
It takes no energy to hold anything steady.

Thin Ice
08-12-2014, 08:33 PM
I misunderstood. Thanks so much.

md2000
08-12-2014, 08:48 PM
It takes no energy to hold anything steady.

Here, hold this 100-pound sack of flour for a few hours.


If the question is "work = force x distance" when dragging a weight along the floor against friction resistance, then why is it not work when holding a weight steady against force of gravity or a plunger held steady against the force of air pressure?

Energy is a measure of ability to do work - your muscles have (chemical) energy. They expend it holding something in equilibrium against an opposing force. The plunger or bowling ball, of course, does not gain energy.

Colophon
08-13-2014, 06:19 AM
Here, hold this 100-pound sack of flour for a few hours.


If the question is "work = force x distance" when dragging a weight along the floor against friction resistance, then why is it not work when holding a weight steady against force of gravity or a plunger held steady against the force of air pressure?

Energy is a measure of ability to do work - your muscles have (chemical) energy. They expend it holding something in equilibrium against an opposing force. The plunger or bowling ball, of course, does not gain energy.
Because your muscles are moving when doing work. They are contracting, so they will tire out.

If you lie down and I place a bowling ball on your chest, you can hold it steady without doing any work.

Chronos
08-13-2014, 09:09 AM
If the question is "work = force x distance" when dragging a weight along the floor against friction resistance, then why is it not work when holding a weight steady against force of gravity or a plunger held steady against the force of air pressure?
Because the distance is zero.

Machine Elf
08-13-2014, 10:45 AM
If the question is "work = force x distance" when dragging a weight along the floor against friction resistance, then why is it not work when holding a weight steady against force of gravity or a plunger held steady against the force of air pressure?

As Chronos notes, the distance traveled by the item is zero; you have not done any work, i.e. you have not transferred any mechanical energy, to the object.

Energy is a measure of ability to do work - your muscles have (chemical) energy. They expend it holding something in equilibrium against an opposing force. The plunger or bowling ball, of course, does not gain energy.

This is unique to the physiological mechanism of your muscles. Note that a cable suspending a bowling ball from the ceiling is not doing any work on the bowling ball, and it's also not expending any energy to create that force. Likewise, your muscles don't do any work on a bowling ball when they hold that ball at constant altitude - but your muscles, because of how they operate, are pissing away energy internally in order to exert that steady force.

A hovering helicopter is a related example. No mechanical work is being done on the chassis of the helicopter - but the engine and rotor are pissing away energy in order to exert the force required to offset gravity. You could take away the engine and rotor and set the chassis on top of a flagpole; from the chassis' point of view it's the same thing, i.e. no work is being done on it.

scr4
08-13-2014, 10:54 AM
If the question is "work = force x distance" when dragging a weight along the floor against friction resistance, then why is it not work when holding a weight steady against force of gravity or a plunger held steady against the force of air pressure?

Because muscles are inefficient. Just because you used up energy doesn't mean you actually did work. A helicopter hovering in one spot hasn't done any work, but it still uses fuel because a helicopter is an inefficient method of keeping something stationary.

scabpicker
08-13-2014, 12:28 PM
A helicopter hovering in one spot hasn't done any work, but it still uses fuel because a helicopter is an inefficient method of keeping something stationary.

I'm probably just being nitpicky because physics can claim I didn't do any work after I busted ass holding up 100lbs, but I think that helicopter is doing lots of work. I think the air it's hurling at the ground would agree if it could.

And I predict someone will correctly nitpick my nitpick. Don't disappoint me, folks.

ETA: And Machine Elf didn't get nitpicked because I can't find a nitpick with his statement.

md2000
08-13-2014, 05:06 PM
The physics concept of work is an exercise in theoretical physics that applies to spherical chickens in a vacuum.

The concept does not really take into account the exertion of effort (to avoid the technical terms "work" or "energy"). Obviously performing an essentially isometric exercise like a person holding up a weight against gravity, or a helicopter or rocket or Harrier jet hovering, is using energy in the strict, measurable sense.

A bowling ball resting on a table (or on you lying down) converts a small amount of potential energy into deformation and compression of the supporting surface as it sinks however much to reach equilibrium. That work is "recovered" in that t is slightly easier to lift it the first fraction on a centimeter as the table rebounds (or the ball drops slightly less of a distance as it rolls off the table, and then the table top rebounds, releasing spring-like potential energy).

Similarly, your muscles convert biological energy (glucose?) into heat while maintaining a pose; the engine in the helicopter burns fuel and creates circular motion of the blades against air resistance, creates heat and motion of the air, vibrations (noise) in the air, radiant heat, etc.

Once you get into second and third level detail, the physics is a lot less theoretical...

scr4
08-14-2014, 11:27 AM
I'm probably just being nitpicky because physics can claim I didn't do any work after I busted ass holding up 100lbs, but I think that helicopter is doing lots of work. I think the air it's hurling at the ground would agree if it could.

I'll be more precise: A helicopter carrying a 100-lb weight and hovering (i.e. stationary in the air) is not doing any work on the 100-lb weight. It's doing work on air, any loose object being blown around by the air flow, etc.

Similarly, a person holding a 100-lb weight is not doing work on the weight, but the muscles may be generating heat, twitching, etc.

scabpicker
08-14-2014, 12:11 PM
I'll be more precise: A helicopter carrying a 100-lb weight and hovering (i.e. stationary in the air) is not doing any work on the 100-lb weight. It's doing work on air, any loose object being blown around by the air flow, etc.

Similarly, a person holding a 100-lb weight is not doing work on the weight, but the muscles may be generating heat, twitching, etc.

Nitpick resolved. Plus, I'm even more sorry about being butthurt about physics denigrating my "work". :)