What is Pressure Difference?

Answer 1

Is is simply an area or region where there are two separate pressures at the same time.

In a bottle od Pepsi there is a higher pressure than atmospheric pressure, so you can say that there is a pressure difference across the plastic of the bottle from the inside to the outside.

Answer 2

Think about a whole bunch of marshmallows...

In the air, we have very very light marshmallows that are floating around. Now, let's say that that you take a bunch of those marshmallows and cram them into a box. You sit in the box, and tape it shut. Then you take another box, and just open it up. Some marshmallows will just naturally float into the box. Then you close that box, taping it up. The box with all of the marshmallows will really want to explode out of that box, right? Also, that box will be lots heavier than the box with only a few marshmallows, right?

Think of a space ship...again, let's use the marshmallow example. Inside of the ship, we need to keep a fairly constant amount of marshmallows around us...too many and we are squished between them. Too few and the marshmallows inside of us want to get out to even everything out. In space, there are very very very few marshmallows. So, since we have a normal amount of marshmallows around us inside of the ship for being on earth, what do you think the marshmallows inside of the ship would want to do? Get out to even it all out!

Air molecules (remember, the air that we breath isn't "nothingness", it is actually a whole bunch of molecules that you can't see, and in most cases, can't smell.) Jam a whole bunch of those molecules together in a closed container, and the "pressure" in the container is higher than the pressure out of the container. If you have too many molecules in the container, or the container isn't strong enough...guess what happens? (Coke all over the fridge.)

The same works in the reverse. A vacuum (not to be confused with the thing you clean your floors with) has less molecules than the air around it. If the vacuum is broken (puncture a hole in it) guess what happens then??

In short, space (like a vacuum) has very little pressure. Being on the top of Mt. Everest has a little more pressure. Being on the ocean has a little more pressure still. Being underwater has lots and lots of pressure. It is these pressure differences that cause your ears to "pop" when you are in a plane or driving up and down very high mountains.

Makes sense?