Re: why the pressure inside a balloon is equal everywhere

Date: Sat Feb 20 04:00:23 1999
Posted By: Nigel Barker, Head of Science, International School of Lusaka
Area of science: Chemistry
ID: 919177498.Ch

Message:

I'm not familiar with the trick, but you seem to be asking two questions - why the pressure is equal everywhere inside a balloon, and how to measure the tension of a balloon.

Firstly, air particles are moving randomly and quickly all through the balloon. The pressure is caused by the collisions of the particles with each other and with the walls of the balloon. Because they are random moving in all directions, the pressure is the same in all directions.

Secondly, the best I can think of is this:

Part 1
Cut a narrow strip of balloon material. it has to be narrow to keep it linear as you stretch it. Measure its length, then suspend small weights from it one by one, measuring the new length each time. Plot a graph of extension vs. weight . The tension in the balloon material is equal to the weight (in Newtons).

Part 2
Next you need to project this linear tension to the two dimensional tension in the balloon's surface. To do this you need an estimate of how much the balloon expands when you blow it up.

Draw a 1cm square on a flat balloon (same brand and shape as the one you cut up), then inflate the balloon. When it gets to the size you want, find the new lengths of the sides of the square. If its too rounded for a ruler now, use a bit of cotton to measure the sides. Find the extension of each side, and use your graph to find out the tension which produces that extension. If your sides have stretched different amounts, you can find the average of the two (or four) tension values. That would be the tension in your balloon.

This is a very approximate value that you are getting, but its the best I can think of for your grade level. If you wanted to improve it, you could draw several squares on different parts of the balloon and find an average.
To eliminate inaccuracy from using two balloons, do the parts in reverse. Once you have taken all the measurements from the balloon in part 2, cut it up and do part 1.

Current Queue | Current Queue for Chemistry | Chemistry archives

Try the links in the MadSci Library for more information on Chemistry.