Asia-Pacific Forum on Science Learning and Teaching, Volume 3, Issue2, Article 3(Dec., 2002)
Russell TYTLER
Using toys and surprise events to teach about air and flight in the primary school
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It's the atmosphere!

Activities
The magic finger
Small plastic soft drink container with screw top. Holes can be put in the bottom, and top, using a small hot nail held by pliers

A container full of water has three holes in the bottom.

What can be done to make the water come out of the holes?

Is the finger really magic?

Can you explain what is happening?

(There is a hole in the top that can be opened or closed with the finger)

Watch video 4: ordinary (817 kB) or high (8.3 MB) resolution (in Real Player format)


The upturned glass
Glass, container to catch mistakes. Card or plastic sheet. The card should be only marginally larger than the glass.

Fill the glass with water, to the top, and put the piece of white card on top.

Hold the card while you turn the glass upside-down.

Make sure you do this over the tray, so it won't matter if it spills.

What do you think will happen if you take your finger off the card?

Does it make any difference to what happens if the glass is only half full of water?

What holds the card on?

Text books give the standard answer: "Atmospheric pressure"

But does air inside the glass play a role?

Will it work for a very tall glass?

Does it work with different liquids? Sparkling mineral water? Oil?

Is the card really needed? Lift water with a straw by putting your finger on the top of the straw. Try it with different size straws.

Does surface tension play a role? Try it with a some detergent in the water.

Watch video 5: ordinary (870 kB) or high (8.8 MB) resolution (in Real Player format)

Commentary
The idea of atmospheric pressure is counter intuitive for two reasons - students do not have a confident idea of the gas state because they do not associate matter with an insubstantial, invisible presence. 'If you can't see it or feel it, then forget about it!' The other reason is that the effects of atmospheric pressure are mainly masked by the fact that air is everywhere. We do not collapse under the weight of the atmosphere because every part of our bodies is composed of air or water or other substances which are at atmospheric pressure and resist the effect of the atmosphere.

Another, more technical difficulty is sometimes encountered in that if students accept that the weight of the atmosphere above us is bearing down on us, they imagine the force due to this must be downwards also, and not applied in all directions, as is evident from the upturned glass or magic finger.

For all these reasons, I'd recommend saving a serious discussion of these concepts until the upper primary or lower secondary school. Nevertheless, some of these activities work well as challenges with lower year levels. Don't be disappointed, though, if they revert to simpler ideas even after you've discussed the principles thoroughly.

You will also find that students will use a variety of explanations for these activities, and will hold onto naive ideas for some despite considerable discussion. Learning is a slow process, and it's important to monitor what students are thinking about each activity.


UPTURNED GLASS
Most of these activities, involving water being supported counter intuitively, are related to the same principle. The upturned glass is a case in point, but I have never seen an explanation of this that is satisfactory. I've now run this activity with many students and adults, and the following questions come up:

The questions and extra challenges in the activity are meant to address some of these.

The reason the card stays on is because of the outside air pressure acting upwards. The complication of the air inside can be explained thus: when the glass is upturned, the water level and card drop very slightly, increasing the volume of the enclosed air. This drops the pressure, and the card settles when the upwards pressure exactly matches the downward weight of the water and card, and the downwards but reduced pressure from the air inside.

If the card is relatively rigid, you will be able to see it drops just a bit so there's a slight gap between the card and glass rim, filled with water. The surface tension of the water allows this to happen by maintaining the surface and even providing a small adhesive force. The trick works even if the water is taken out, provided the card is wet! I've found a piece of thin plastic works better than paper since it doesn't soak, but I've also done it with table napkins and glasses of wine, when pushed! You need to be careful the card or napkin is not too big so that it droops and breaks the water seal.

The trick doesn't work with lemonade because the bubbles increase the pressure inside the glass. It works without a card provided the surface is small enough to maintain the surface through cohesion, as with a straw used for transferring water by the action of the thumb at the top (identical in principle to the magic finger).


MAGIC FINGER
The magic finger is really a double trick. The original version had the magic finger on the hand not holding the container! Every time the finger points, the water comes out because, unknown to the audience, the finger on top of the (secret) hole rolls slightly to let air in. I've kept classes going on this by challenging others to see if their fingers are magic. They're always delighted to learn the trick and talk about it.

In fact even quite young students can get some sense of this activity. The easiest way to explain it is in terms of air needing to be let into the top hole to take up the space the escaping water will leave. Some students are attracted to a 'trapped/released' image and claim that the finger allows water to escaped at the bottom and air to escape out the top! The air pressure explanation involves the outside pressure pushing on the water at the holes (again, you can see the effect of surface tension as the water forms half drops) and keeping it up provided the air inside is not at atmospheric pressure also.

Copyright (C) 2002 HKIEd APFSLT. Volume 3, Issue 2, Article 3 (Dec., 2002). All Rights Reserved.