Naked Science Forum
General Science => General Science => Topic started by: chris on 17/12/2017 10:02:04
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Georgia has a seasonal question:
Why are balloons most hard to blow up at the beginning?
There's a lot of confusing and unclear information around the web on this, so shall we try to produce a clear and concise answer for her?
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The molecules of a balloon are naturally squiggly and attract each other. You need to apply a force to straighten and separate them - i.e. to make the surface area of the balloon bigger.
When the balloon is new the fresh rubber molecules are tightly squiggled and tangled around each other, so it takes a lot of force to begin to separate them. As the balloon expands, the molecules straighten out and begin to slide over each other - it suddenly gets easier to stretch them once they are aligned, and if you allow the balloon to deflate from this condition, it ends up "thin and wrinkly" instead of the original shape. At some point the molecules can't slide any further without losing their grip on each other, and the balloon bursts.
If Georgia has long hair, she will have seen the mechanism at work every day! When you start brushing tangled hair you need to apply a lot of force (or patience) to open up the tightest tangles, but gradually the hairs align and what began as a stiff bunch ends up as a slippery aligned surface. You can see the opposite effect if you make felt from loose wool.
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Another factor is how much the surface of the balloon stretches as you add equal volumes of air. Let's say that you start with a balloon with a radius of 1cm and a volume of ~4 cm^3. If you add an additional 4 cm^3 of air, doubling its volume, you increase its radius and thus its circumference by ~25% or 1.5 cm.
The work needed for this is depends on the elasticity of the balloon material and the linear increase in the circumference.
If you add another 4 cm^3, the circumference increases by another 1.14 cm. Thus, even if the elasticity of the balloon material does not change, it takes less work to add the second 4 cm^3 of air to the balloon then it did the first 4 cm^3.
Another 4 cm^3 added, adds an even smaller increase to the radius, and takes even less work.
So, your lungs would have to work harder to add the same volume of air to the balloon when it is small than when it is larger, even if you don't assume that the material becomes easier to stretch.