If you inflate a spherical rubber balloon, why is the inflation difficult at first and much easier when the balloon is partially inflated? If the balloon is cylindered, why does the inflation begin in one place instead of throughout the balloon? As you continue to blow into the balloon, why does the bulging travel along the length of the balloon?
If you inflate a spherical soap bubble, you must provide air pressure that is larger than the existing air pressure inside the bubble. The internal air pressure depends on the curvature of the bubble’s surface. To see this point, consider a patch on the surface. The pull at the left side and the pull at the right are partially towards the middle of the bubble. The centered portion determines the air pressure. When the bubble is very curved, the center pulls on the patch of surface is large and therefore the internal pressure is as well. Now the bubble is hard to inflate. When the bubble is larger and the curvature is smaller, the center pull is also smaller and therefore the internal pressure is too. The bubble becomes easier to blow into at this point.
A rubber balloon is different compared to the soap bubble because the stretching membrane increases the pressure during inflation. During the initial stages of inflation, the resistance to stretching pushes the pressure up and requires a large pressure from you, if you are to inflate the balloon further. However, once the balloon has reached a certain size and pressure capacity, the decreasing curvature that follows begins to lower the internal pressure, and then the inflation becomes much easier than before. The ease is further aided by a lowered pressure to stretching of the rubber at about the same size of the balloon.
A peculiarity of some rubber balloons is that even if they are ideally spherical in nature, they may be noticeably non-spherical when their inflation is in a certain range. Soon, after the ease of the inflation sets in and before in the inflation again becomes difficult when the rubber is highly stretched, the balloon may develop an appreciable bulge on one side. When you inflate a cylindrical balloon, it first bulges at the weakest point, usually the part nearest to the opening. The section that joins the bulge to the not inflated is concave along the length. When the bubbles are connected by an opened tube, higher pressure in the smaller bubble pushes air through the tube to the large bubble, which has smaller pressure.
Another factor is also important. When you blow into the balloon, you insert a certain volume of air from your lungs, the “puff volume”. When the balloon is small, the added volume requires that the surface area increases significantly, which appreciably increases the resistance to further stretching. When the balloon is large, the added volume is small compared to the existing volume and does nor require as large an increase in the surface area, not as much increase in the stretching of the rubber.
http://scienceworld.wolfram.com/physics/Bubble.html>
This website provides the physics equation for the pressure of a soap bubble, which may help someone understand why it is difficult to blow up a bubble at the beginning.
http://www.britannica.com/EBchecked/topic/575080/surface-tension
This website provides information on the surface tension of a soap bubble. This relates to a balloon as well because surface tension is aspect that creates resistance for the inflation of the soap bubble or balloon.
Shamsipour, G. (2006). Simple Experiments for Teaching Air Pressure. Physics Teacher, 44(9), 576-577.
This article is a great extension to a unit on air pressure especially for elementary teachers.
Walker, J (1977). The Flying Circus of Physics. Wiley
This is original source of my question and answer for this article.