| Interactive
Primary Newsletter 31 Under pressure |
 It
doesn’t work - it must be physics"Results! Why, I have gotten a lot of results. I know several thousand things that won’t work". "Just because something doesn’t do what you planned it to do doesn’t mean it’s useless." -Thomas Edison 1847 - 1931 We should all take heart from the above quotes by Edison, one of the most prolific inventors and experimenters of the last two centuries. Science is about questions. It’s about investigating ideas or hypotheses. It’s not about ‘right answers’. Nor is it simply about teacher demonstrations to prove some ‘scientific fact’. "Children share with geniuses an open, inquiring, uninhibited quality of mind." Chauncey Guy Suits 1905-1991. One time Director of the General Electric Research Laboratory. Look around the classroom on a cold Monday morning in October and we may not recognise the truth in the third of our quotes. Trite as it may sound, it is, and should be, memorable. Children, as do we all, have misconceptions about the world around them and their ‘alternative’ views of the world are based on their own experience. Such ideas are hard to shake. That this is so is often evident in their learning in science. Therefore we should ensure that pupils are encouraged to investigate their own conceptions, challenge them and seek alternatives. With a little luck, their newfound understanding will be based on guided observation and investigation. This brings us eventually, if not neatly, to the main topic of this Newsletter. We have a concern that teachers are too often being mis-directed by those who really should know better. Publications that offer ideas for investigations, activities to challenge misconceptions, or to demonstrate a key scientific idea, are commonplace in print and on the web. If so published, they should be tried and tested. Unfortunately a few seem to be based on theory and have never been exposed to a practical test. How often has the cry been heard "It doesn’t work. It must be me. There’s something I’m not doing properly!" It ain’t necessarily so. Remember Edison and consider the following examples.
"Go, measure earth, weigh air, and state the tides!" (Alexander Pope) Most young children believe that a balloon full of air is lighter than an empty one – probably as a result of the tearful loss / joyous sight of the helium-filled balloon inadvertently / deliberately released from grubby little hands. Well, air is light stuff, isn’t it? How do we challenge such an idea? We could try a theoretical approach. The pressure of air at sea level is approximately 10 N (Newtons) for every square centimetre (10 Ncm-2) – kind of like the force with which a 1 kg bag of sugar would press down on each and every key of a standard computer keyboard. Try working out how many square centimetres (apologies for the mis-spelling in the paper version of this in the last printed Newsletter - centimetres not centimeters!) you have pointing upwards (mine is approx. 42 x 18 = 756 cm-2 - funnily enough just about the area of the aforementioned keyboard) and that is quite a weight on your shoulders (7560 N to be precise). This is the force with which all that air presses down on your back. Imagine we had a very large beam balance and we could put a column of air extending to the top of the Earth’s atmosphere on one balance pan and a column of say, mercury (a very dense liquid metal) on the other. How much mercury would we need to balance the weight of air? Well, . . . you get the point, hopefully, but if not here we go. The density (mass per unit volume) of air is about 0.0012 gcm-3 (grammes per cubic centimetre) at sea level. Mercury has a density of 13.6 gcm-3, roughly ten thousand times that of air. This means we would need one thousand times less mercury to balance our scales. Air becomes less dense the higher you climb or fly. The force exerted by the Earth’s atmosphere comes mainly from that body of air extending up to about 10 kilometres above sea level. A wee sum will show that (very roughly) a one metre column of mercury has a weight that would be needed to balance the force of a ten kilometre column of air of the same diameter. If the 1 metre column of mercury has a radius of 1 cm it will contain 3,141 cm3 (volume = radius squared times the height times p [Pi]). This has a mass of about 42.716 kg (3,141 x 13.6 g [density of mercury]). Since the columns balance, the 10 km of air must have the same mass as a one-metre column of mercury. This is the theory behind the mercury barometer. Is this too much for most seven to ten-year olds? You betcha! |
