Interactive Primary Newsletter 31 Under pressure

Magic Card Tricks We will end on a more positive note with a collection of activities that may help to reinforce some ideas about air pressure. They also show that, maybe, you can teach a new dog old tricks. Those longer in the tooth may recall favourite uncles entertaining them with such nonsense. These days, we have to cry such tricks “opportunities to introduce cognitive conflict”. Eh? For the first of these old favourites, one that always works well (he said, tempting fate), you will need a piece of card and drinking glass. The open end of the glass has to be completely covered and a post card works well for this. Ask the class what will happen if you fill the glass with water put the card on top and turn the glass upside down? Fill the glass to overflowing, firmly press the card over the open end and invert the glass (Fig.4). Fig.4 - Don’t try this in the local! If you have tried this before demonstrating and are very confident you could invert the assembly over the head of a pupil (well maybe not, unless you’re nearing retirement). The card is held in place mainly by external air pressure with a little surface tension thrown in for luck. Next, crush a paper towel into a small ball and press it into the bottom of a tumbler or a half-pint glass (Fig.5). Ask the class what will happen when the glass is placed in a bowl of water. They will invariably say “the paper will get wet”. Fig.5 - Keeping your paper dry? Hold the tumbler by the base and place it in the water, open end first (Fig.6). Water rises up the glass but should stop before it reaches the paper towel. Try this out for yourself, before you demonstrate it. Fig.6 - Is it still dry?   Why does the paper stay dry? The air inside the tumbler is compressed and the pressure is enough to stop the water rising high enough to wet the paper. This last activity was a favourite with one of my more interesting and entertaining uncles. All that is needed is an old-fashioned cotton reel, one with a single hole, not the plastic variety with 4 or 5 holes, and a piece of card about 75 cm x 75 cm. Find the centre of the card by drawing diagonals. Push a drawing pin through the intersection of the diagonals. Place the card under the cotton reel with the pin in the hole in one end and blow hard down the hole in the other. How far can you blow the card? Fig.7 - Blown away Surprise! The card does not blow away but stays firmly attached to the bottom of the reel. How can this be? It is all to do with moving air and the differences in pressure it can create. To give it a posh name this is a manifestation of the Bernoulli Effect. Put simply, moving air is at a lower pressure than still air. Air escaping from the hole around the sides of the card lowers the air pressure on the inside of the card. The greater pressure in the static air on the other side keeps the card ‘stuck’ to the bottom of the cotton reel. My uncle assured us it was magic. We all believed him. It‘s in a book, it must be true!