Interactive Primary Bulletin 38 |
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We start with a couple of tricks which rely on the effects of atmospheric
pressure. You’ve probably seen on the TV where the weather forecaster tells
us about lows and highs. Generally, High pressure gives
us good weather and Lows bad. This tells us that atmospheric pressure
can vary but is always all around us, pressing on the surface of everything
and everyone. Check out the first two tricks which let us know it’s there
and amaze us at the same time! The third looks at the effect of air
resistance - |
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There is a super little piece of apparatus made by PASCO called the Atmospheric Pressure Demonstrator. Consisting of a thick rubber mat (20 cm square with a central handle), it costs a hefty £35 and is available though Feedback Instruments in the UK. If the mat is placed on the top surface of a stool (needs to be pretty flat), see what happens when we try to lift it up with the centre handle? The mat ‘sticks’ and the stool can be lifted, as if by magic. Show the mat can be easily peeled away from the surface. Figure 1 - PASCO Atmospheric Pressure Demonstrator Why does the mat ‘stick’ to the surface? What keeps the mat ‘stuck’ to the stool. You’ve guessed it - atmospheric pressure and a good seal betwixt mat and stool. Why not try making your own version using some rubber from a cheap hot-waterbottle, wood handle, glue, card and some paper-clips. See Figs. 2 & 3 for a DIY design.
Figures 2, 3a & 3b - DIY Atmospheric Pressure Demonstrator in action and to different methods of construction.
We also successfully used a mousemat and a plastic cotton reel in place of the rubber and wooden handle (Figure 3b). |
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King of Spades
holds back the water!
Ask the children to suggest what might happen if you turn the glass upside down with the card in place. Invert it carefully, maintaining a little pressure on the card to make sure there is a good seal between the glass and the card. Take your hand away from it, the card should stay in position and the water in the glass. (Practise over a sink!). Figure 4 - Defying gravity? Explanation - Air is pushing upwards on the bottom of the card with a greater force than the force exerted downwards by the weight of the water. |
DIY
construction - Cut a rectangular piece from a hot water bottle, maximising
the area obtained. The dimensions are not critical but bigger does tend to
be better! Get a handle (Wood is good). Again, shape is not critical but you
need to have a flat bottom and something to get a hold of. Glue (try
superglue - care required) the handle to the centre of the rubber (original
external surface). You may need to experiment with different types of glues,
depending on the handle material and the rubber. Cut a piece of thick card,
slightly smaller than the rubber. Cut a hole as shown in Figure 3a. The card
braces the rubber and gives it some rigidity. Slide four large paperclips
over the edges as shown.
Take
a glass and fill it with water. Lay the King of Spades (or any other playing
card) on top of the glass.
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Magic carpet? Cut an A4 sheet of
card into 8 pieces. The challenge is to drop as many pieces of card as you
can (not crumpled) from arm’s length into an empty basin on the floor. Keep
a score and allow each child to choose how the paper is released. Don’t
suggest what method of release may be more successful. Allow a second round
and see if a preferred method of release emerges. You will find that if the
pieces are held vertically and dropped, the pieces will flip, swerve and
weave their way to the floor, frequently missing the target. If, however,
you hold the piece of paper horizontally, it tends to descend straight down
and you have a better chance of it reaching the basin. |
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