# Vacuum energy

1. Oct 20, 2015

### Docak

If I induce vacuum in a glass tube and then I open the lid of the glass tube, air rushes in to fill in the vacuum. Can vacuum be regarded as a sort of potential energy, able to exert a force? Can a vacuum tube be considered as a potential storage device for energy, just like a container filled with compressed gas can be used as a storage device for energy. Is it possible that vacuum itself is responsible for expansion of universe?

2. Oct 20, 2015

### Bandersnatch

While there is something called vacuum energy, and it is indeed associated with accelerated expansion of the universe, what you describe is not that.
The air rushes to the vacuum tube not because of some inherent property of the vacuum, but because of gravity of Earth acting on the column of air.

3. Oct 20, 2015

### davenn

Hi There
welcome to the Physics Forums

Answer this .... how can something non-existent exert a force ? or do anything for that matter ?

but that is totally the opposite ... the gas IS something, it CAN exert a force

With air rushing into a vacuum, you need to reconsider where the force is coming from

cheers
Dave

4. Oct 20, 2015

### Staff: Mentor

I'm pretty sure that if you were to try this experiment on the International Space Station, you would get the same result without any help from gravity. Air molecules are moving in all directions randomly. When you open the vacuum tube, you simply allow some of them to enter the tube. The number of molecules in the tube increases until the rate at which they randomly leave equals the rate at which they randomly enter.

5. Oct 20, 2015

### Buckleymanor

I am pretty sure you are right .You will get the same result without any help from gravity though the air inside the space station will be pressurised to resemble the pressure gravity gives to the column of air on Earth.

6. Oct 21, 2015

### Bandersnatch

Of course jtbell is right. My train of thought went in the direction of why we have pressure on Earth in the first place.
But since it's remarkably easy to come up with examples of pressure maintained by means other than gravity, that answer turns out rather unsatisfactory.

7. Oct 21, 2015

### Docak

No, no did not mean energy in the sense of term energy used in physics. I mean inducing vacuum in a tube produces a capacity to do work, i.e., air rushing in to fill the tube when this vacuum tube is opened, hence causing displacement of air. I meant to ask if a vacuum tube can be considered a potential energy storage device?

8. Oct 21, 2015

### jbriggs444

It is a device. It requires energy to evacuate the tube in the first place. It can store... something. It can release energy then the surrounding air is allowed to re-fill the tube. The energy made available when the tube can be nearly identical to the energy required to empty it. So yes, it can be considered an energy storage device. However, there are other ways of looking at it as well...

Evacuating the tube on Earth displaces the rest of the Earth's atmosphere upward. The potential energy is that of the entire Earth's atmosphere lifted by a fraction of a micron.

Evacuating the tube in a space station compresses the air in the rest of the space station slightly. The potential energy is that of the increased pressure (and temperature) of the air in the rest of the space station.

Evacuating the tube while standing in a cylinder with a spring-loaded piston that maintains a constant pressure inside the cylinder has the effect of doing work against the spring. The potential energy is that stored in the spring.

What I am getting at is that the potential energy stored in a vacuum tube is not simply a property of the tube. It is a property of the tube in the context of its environment. If you remove the tube from its environment then that potential energy will have disappeared.

9. Oct 21, 2015

### ZapperZ

Staff Emeritus
I can't just leave this without pointing out the error. The expansion of the universe is nothing like the expansion of gas into a vacuum. The gas is expanding into a pre-existing space. The universe expansion is the expansion of our physical universe, i.e. our space and time. It is NOT expanding into a pre-existing space.

You may want to view this:

BTW, when you use the word "energy" in a "physics forum", you simply can't make a disclaimer that you are using the term not in the way it is used in physics. Doing so makes it not only ambiguous, but also undefined! You have no freedom to simply redefined physical terms.

Zz.

10. Oct 21, 2015

### Staff: Mentor

I'll set aside the self-contradiction of the first and last sentences (not to mention making that statement in the Physics section of Physics Forums!), but otherwise you are essentially correct.

This idea is a bit of a twist on how people usually think about energy storage via pressure, but it still counts. If you use such a device to spin a turbine/generator, the electrical device you power from it doesn't know or care about the details of how it works or what direction the turbine spins: it generates power, so it must have stored energy.

So the question is: where is the energy? (how is it stored?)

If you want to identify exactly how the energy is stored, the examples of using the device in a space station and on Earth are not interchangeable*. To illustrate more clearly, consider the device submerged under water in an open vessel (a pool, the ocean) as a third example. When you submerge the device under water, you displace water, which raises the water level of whatever vessel you are using. So the system includes both the device and the vessel of water and the energy that you stored is gravitational potential energy from raising the level of water in the vessel.

So, how does the device work in the atmosphere? When you pump the air out of the device, it displaces air in the atmosphere, raising the gravitational potential energy of the atmosphere.

So, how does the device work on a space station? The space station is itself a pressure vessel. So when you pump air out of the device, you increase the pressure in the space station, storing energy via that increase in pressure (often called "pressure energy").

Now, the critiques you got on the OP were based on the specific wording you used: while your device as part of a larger system stores energy, an isolated vacuum does not. And the conclusion; that such could power the expansion of the universe therefore doesn't follow either.

*If the experiment were set up with precise enough instruments/a large enough vessel, the pressure inside the space station would vary noticeably during the experiment, which would impact the results. Of course, if you had a large enough vessel on Earth you would also notice a pressure variation, but in the opposite direction.
[Edit: oops, pretty much all of that covered in intervening posts]

11. Oct 21, 2015

### OmCheeto

(though, you'll notice I've deleted your last sentence. See the note at the bottom of my post)

You can actually buy a "vacuum energy storage device" at most large food stores. They are called "flavor Injectors".
Just repurposed hypodermic syringes really.
But anyways, if you removed the needle, fully insert the plunger, put your finger over the open end, then withdraw the plunger, you will have stored some energy, as the plunger will travel all the way back to the fully inserted position, once you release the plunger.

You can calculate how much energy is stored with a simple equation: W=fd
W being work (or energy)
f being the the force, easily calculated by knowing atmospheric pressure, and the area of the cylinder
d being the distance the plunger travels

Of course, the piston will have some friction when traveling along the cylinder, so the system loses energy.
You can calculate the friction by reversing the process.
Start with the plunger, in the linked example, at the 1 ounce mark.
Place your finger over the open end, and compress the air.
Release the force on the plunger, and allow it to travel back towards the 1 ounce mark.
You will notice that the plunger doesn't go all the way back.
This tells you, via the combined gas law, PV/T = k, that the pressure inside the cylinder is higher than atmospheric.
Knowing that the temperature (T) and k (constant), don't change in this experiment, you should be able to determine the pressure (P) inside the relaxed system from the change in volume (V).
From that, you can determine the force differential.

ps. If you not willing to shell out the \$5.99 for the flavor injector, the dimensions are
diameter: 2.37 cm
plunger travel distance to 1 ounce: 6.68 cm
There's really no reason to buy this stuff, when you can just do the maths.

pps. As has been pointed out, the vacuum energy you can store in a flavor injector, is not the same as the "quantum fluctuation" vacuum energy.
I don't really know what the experts call it, but it should have a fancy name like that, as "vacuum energy" can be interpreted different ways; "My vacuum cleaner has way more energy than yours does. It has a 1 kwh battery, and yours only has only a 0.75 kwh battery. Ha!"

12. Oct 21, 2015

### CWatters

Just wanted to go back to the OP...

Interestingly Thomas Newcomen's steam engine didn't use high pressure steam to push on a piston. Instead they used the fact that condensing steam creates a partial vacuum in the cylinder and relied on external air pressure to push the piston down.

The problem with this approach is that no matter how good your vacuum is the maximum pressure difference between one side of the piston and the other is limited to atmospheric pressure. In a modern steam engine you can ramp up the pressure in the boiler to many times atmospheric pressure.

13. Oct 25, 2015

### Docak

Okay. Thanks for the discussion. What I get from this discussion is that although a vacuum does not store energy, but does have the potential to kind of absorb energy while the energy is applied by another mechanism. Anyway, let's consider another scenario. A man is holding a group of hostages inside a room. He has an explosive device attached to him. A SWAT team is getting ready to enter the room and rescue the hostages. A split second before entering the room, they throw in a large vacuum device, which has a sensor that detects and opens a valve to let energy inside the tube as soon as sensor detects explosion. If the hostage taker or terrorist detonates the explosive, will it be possible to minimize the damage by absorbing some or most of the energy released by the device. I mean in traditional methods, SWAT team is wearing heavy armor to protect themselves, but is it possible to use a vacuum device in such a scenario.

14. Oct 25, 2015

### davenn

you are still looking at a vacuum in an incorrect way
Its a vacuum in "that tube/vessel/device" nothing cannot absorb something. Rather, the surrounding air/explosive gasses etc "rush" into the vacuum aka void, till pressure equilibrium is reached between the inside and outside of the vacuum device

Dave

15. Oct 26, 2015

### Staff: Mentor

No. It should be clear by considering what the negative gauge pressure of a vacuum is (-14.7psi) vs the pressure that a pipe bomb can develop (thousands of psi). The energy density of your device is just far too low to absorb much from something else.

16. Oct 26, 2015

### Staff: Mentor

The story the OP told there doesn't imply to me a wrong way of looking at the situation but regardless, turning the scenario around and looking at it from the opposite direction does not answer the question...

17. Oct 27, 2015

### CWatters

When a bomb explodes it creates a ball of rapidly expanding gas. If that happens in a sealed vessel (like a room) the increase in pressure can blow the walls out etc. If the room contained a vacuum (rather than air at atmospheric pressure) when the bomb exploded then I suppose the peak pressure might be reduced very slightly.

18. Oct 27, 2015

### Loren

Not entirely. Air would displace the vacuum in the absence of gravity, assuming you had a concentration of it that represented a pressure greater than zero (the vacuum). For instance, a bottle of compressed air opened to a vacuum or into another chamber of lower pressure. In that instance gravity has no part of the physics.

As far as the original question goes, the vacuum itself does not represent a form of potential energy, but the higher pressure outside of it does.

This is sort of analogous to the movies where people get sucked out into space, but really they are pushed out into space by the higher pressure inside as it rushes out into vacuum.

19. Oct 27, 2015

### Loren

Worse yet, the valve acts as a constriction. Much like trying to get every car out of a huge parking lot after a big football game, but having only one 5 meter wide gate.