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15 May 2020, 15:01

The SFK Project: #5 Try blowing it up!

What you need:

  • Balloon
  • Plastic bottle (best choice would be one with a plastic label all around which can be pulled up or down without destroying it)
  • Big strong needle
  • Sticky tape
  • Candle or flame to heat the needle

What you will get at the end:

A magic trick. Only you can inflate a balloon inside a bottle. Everyone else will fail to do so. But there is more magic: When you’ve blown up the balloon, you can leave it open, and the air magically stays inside!

How to build:

Get a balloon halfway into a plastic bottle with its nozzle (the part where you need to blow the air in) staying outside. This nozzle needs to be pulled over the rim of the bottleneck. Just look at the picture, and you’ll know what I mean.

Now try to blow up the balloon. You won’t be able to!

If you drill a small hole into the bottom of the bottle using a hot needle, things will change:

When you managed to blow up the balloon inside the bottle, hold your finger onto the hole in the bottle so that no air can get in or out the bottle. You may then open your mouth and leave the nozzle open, but the balloon still keeps inflated. No air will get out of the nozzle although it is wide open!

Now put a small piece of sticky tape onto the hole in the bottom and ask the best balloon artist in your family to inflate the balloon using his lungs only. He will be very embarrassed when realising that he is not able to do so while you can easily inflate the balloon. What he doesn’t know is that you’ve secretly removed the sticky tape before you start inflating the balloon. Relax and enjoy the adults marvelling!

You may make this even more mystic when using one of those bottles having a plastic label that fits tight all around the bottle like a bandage but still can be pushed a little bit up or down the bottle. Pull the label up and drill your hole. Then slide the label back down and over the hole. If you’re lucky, the label will block the air from streaming through the hole, and no one will be able to inflate the balloon. No one except you! Because you secretly pull up the label until the label no longer covers the hole. Once the air is inside the balloon, you slide the label back, and the air stays magically inside the balloon without touching it!

Keeping the air inside the balloon will only work with a bottle made from rigid thick plastic! You may have difficulties to find one in your country. Most bottles made for recycling can be easily compressed. They won't stay stable when the blown-up balloon tries to shrink.

In Germany, we do have "multi-trip-bottles" which do not get compressed and recycled when being returned but which get cleansed and used multiple times. This type of bottle is the kind that will not collapse. But I have not yet found such bottles with an all-around label. So one solution is to use the sticky tape with them. Another solution is to fake the label by using crystal clear cellular tape to fill the gap and making it all-around. Here is a step by step instruction:

  • Apply a hot water jet to the label to get it off the bottle (let the jet do the work - do not try tearing the label off).
  • Then cleanse the bottle from any glue (using dishwashing soap)
  • Dry the two parts of the label (put them on tissue paper)

  • Hold a piece of tape in place, sticky side up, and stick each part of the label on the edges of this tape leaving a gap between. Use a ruler to align the two parts!

  • Stick another piece of tape on top of the first one, this time sticky side down

  • Trim off the exceeding parts of the tape
  • Repeat the steps to get an extra length to both open ends of the label

  • This time trim all three sides and leave about 2/3 of the required gap width to form a complete ring around the bottle
  • If you have, apply silicon spray on the backside of the label. It needs to easily slide on the bottle's surface later.

  • Put the label around the bottle and close the ring by applying a piece of sticky tape. Make the ring as tight as possible. It needs to seal the hole airtight!

Here are pictures of the hole in open state and with the label pulled over it:

But how does this trick work, and why does the air stay inside the balloon? I know you want to know the secret and explain the thing to the adults. They will marvel even more, believe me!

How it works:

The air around us does press on everything with lots of pressure. This includes you and me! We just don’t realise this fact because we’ve got used to it since our birth. This “atmospheric pressure” is so intense that it may destroy things. It’s just because this force is normally pressing from all sides equally that things do not get damaged by it.

If you are on a high mountain or in an airplane, there is less air above you, and that's why the pressure gets less.

Inside the bottle, we have normal atmospheric pressure as it is all around us. But inside an inflated balloon we need a higher pressure to stretch the balloon’s skin. If you do not seal the nozzle by a knot, air will flow out of the balloon because air is always escaping from high pressure to lower pressure.

If you inflate a balloon the normal way (outside the bottle), the inside pressure increases at the same time. On the outside, there is enough room to push away the outside air. 

But when you try to inflate the balloon inside the bottle, things are different: The air inside the bottle and outside the balloon can’t be pushed out of the way. Because there is no way out of the bottle. So the balloon inside the bottle needs to compress surrounding air. This increases the pressure inside the bottle to extremely high values. It will be so high that no human can generate higher pressure with his lungs. If there is not higher pressure in your mouth or lungs, then the air can’t flow into the balloon. No way to inflate the balloon.

Drilling a hole into the bottle means we are giving the air a way to stream out of the bottle when the inside pressure gets higher. So the balloon gets the space it needs to be inflated. The pressure inside the bottle does not increase very much, and therefore our lungs may generate a higher pressure to inflate the balloon.

Then we block the air from flowing back inside the bottle by sealing the hole with our finger (or the plastic label). The bottle is filled with the inflated balloon. If air flowed out of the balloon, it would need to shrink. But if it would shrink, air would need to fill the place inside the bottle where the inflated balloon used to be. If no air can get inside shrinking the balloon would produce a “vacuum” which is nothing else but a pressure much lower than the normal atmospheric pressure around us. This vacuum kind of “sucks” the balloon into the bottle so that it keeps being inflated. Air stays inside because the pressure in the balloon is equal to the pressure outside the bottle. The same pressure means no air is flowing at all.

By the way:

Our lungs work with a vacuum too. When breathing in (“inhaling”) the “respiratory muscles” produce a vacuum because they enlarge our chest. Put a measuring tape around your chest, and you will see what I’m talking about: Breath in and keep the air inside while measuring the circumference of your chest (that’s the length of the tape around your chest). Then Breath out (“exhale”) all air and do the same. You will find the chest being several centimetres bigger when breathing in. This is producing a vacuum inside the lungs and the airways. Air is sucked in through mouth and nose. When breathing out, our respiratory muscles make the chest small again. They stop making the chest big. The chest and the lungs are elastic like a balloon. And they kind of squeeze the air out by producing pressure inside the lungs which is higher than the surrounding atmospheric pressure. This makes waste air flowing outside the body again.

Repeat measuring your chest while inhaling. But this time you hold close your mouth and your nose using your fingers. You will find no different size of the chest. The muscles did produce a vacuum again, but there was no possibility for air flowing inside the lungs. Our muscles are not strong enough to widen the chest against this vacuum. Be careful not to do this experiment longer than 10 seconds, or you might get dizzy or even faint. This is because our brain constantly needs fresh air to get the oxygen in and carbon dioxide out of the body. This is the reason why you can’t dive underwater as long as you want.

Where I’ve got the idea from:

“The Magic of Science“ by Tony Griffith, seminar booklet

What's coming up:

Next week in SFK #6, we will levitate a ping-pong-ball.

Volker de Haas started electronics and computing with a KIM1 and machine language in the 70s. Then FORTRAN, PASCAL, BASIC, C, MUMPS. Developed complex digital circuits and precise analogue electronics for neuroscience labs (and his MD grade). Later database engineering, C++, C#, hard- and software developer for industry (transport, automotive, automation). Ended up designing and constructing the open source PLC / IPC "Revolution Pi". Today engaged in advanced development as a service.

15 May 2020, 15:01