Tell me more ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.
up vote 5 down vote favorite

I am currently working with pressure differences in the milibar range. However I do not have a good feeling for pressure differences. In every day life:

  • Where does a pressure of say 100 mbar over atmospheric pressure occur?
  • When I blow up my mouth, what is the pressure I get?
  • When I blow up an air balloon, what is the pressure inside?
share|improve this question

3 Answers

up vote 4 down vote accepted

Your mouth or balloon blowing are very bad examples, the reason is because there is no obvious reference.

A McDonalds straw is a better example. Say that it's 7 inches long. You know the density, you know gravity.

$$P=\rho g h$$

In this case, the full 7 inches will get you about 17 mbar. From my everyday experience and intuition, I believe that I can maintain a pressure in my mouth or blowing up a balloon several times what is required to suck up a (watery) drink the full length of a straw.

100 mbar is likely to overwhelm your mouth, but it might just be "athletic". In order to test it, you'll only need a few pipes from an aquarium store, a tub, and maybe some clamps (unless you have more specific equipment). Looking at some anecdotal information online, I see some people reporting in the neighborhood of 2 psi (137 mbar), corresponding with reports that "it felt like my eyes were about to pop out of my head".

Remember, safety first!

share|improve this answer
up vote 4 down vote

One millibar ($\text{mbar}$) is aproximately the pressure you feel on your hand when you hold a penny.

A penny has mass $ m = 0.0025\;\text{kg} $ and diameter $ D = 0.019\;\text{m} $ making the pressure

$$ P = \frac{m\;g}{\pi D^2/4} = 86\;\text{Pa} $$

and $ 100\;\text{Pa} = 1\;\text{mbar} $.

So if you stack 116 pennies you will get $100\;\text{mbar}$.

You could try with other coins, or household items, as long as you know their weight, or mass, and the contact area. A marble, for example, is not a good idea because the contact area depends on the load applied.

share|improve this answer
That's a great example for general physics instruction. I lol at the prospect of 116 stacked pennies, but still very helpful. An instructor could even have students feel some pressure on their hand w/ the pennies and experience the same pressure in their mouth using a straw. I feel like that kind of learning would be helpful. – AlanSE Jul 14 '11 at 3:04
up vote 2 down vote

A different way to get a feeling for pressure differences is a vacuum cleaner. Sometimes the possible vacuum is printed in the manual. A good one should provide a vacuum of 100-200 mbar below atmospheric pressure if you put your hand in front of the tube. With a piece of cardboard closing the tube you can get a feeling for the force that this small pressure difference can create.

To get 100 mbar over atmospheric pressure you can submerge a ballon or other container in water. In a depth of 1 m you have a hydrostatic pressure of 100 mbar.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.