Teacher Demo - PDF document

Teacher Demo 2 5 – Keeping Dry | Page 1 200 8 Ruben Meerman | ABC Science Online Teacher Demonstration 2 5 Keeping Dry Materials: Tall, c lear plastic tub * Water Towel Tissue Coffee mug Clear glass * Cereal storage containers work well Instructions This demonstration is a golden oldie that truly astonishes ver y young children. E ven older kids (and adults) raise an eyebrow if they’ve never seen it – and surprisingly very few have! I t’s quick , easy , very effective and requires only the most basic equipment. 1 Gather all the materials and set them up on a desk with the students gathered around. 2 Push the tissue deep into the coffee mug. Ask your students to predict what will happen when you turn the mug upside down and submerge it in the water. Young students will get quite excited so expect some excitem ent and a few loud “uh - oh’s” ! 3. Submerge the coffee mug and hold it under water for a few moments. Young students will comment that the mug looks bigger, different or weird while it’s underwater. If you’re happy to let your hand soak and not in a ru sh, let them discuss their observations for a while. 4. Remove the mug and pull the tissue out. Allow a volunteer (or two, or three) to feel the tissue. They’ll notice that it’s as dry as a bone! Ask the class if they can explain why the tissue didn’ t get wet? Teacher Demo 2 5 – Keeping Dry | Page 2 200 8 Ruben Meerman | ABC Science Online 5. Repeat the demonstration with a clear drinking glass. Ask the students what they expect to see. 6 . Push the inverted glass deep under the surface of the water and let the students observe closely. Young children marvel at the fact no wat er enters the glass. They will want to see it several times. 7 . Remove the tissue and let the students observe that it still completely dry. To young children this is simply amazing! Teacher notes Atmospheric pressure is a considerable f orce we rarely notice. That’s because air exerts its pressure on us equally in all directions. At sea level, one cubic metre of air weighs around 1.2kg which is roughly the same as a litre of milk. Gravity keeps us and our atmosphere firmly stuck to the Ea rth but air is a fluid that flows readily , allowing us to move through it with ease. At low speeds it provid es few clues of its presence to our senses . Only when we pace through it at speed , or when huge bodies of air get moving as wind do we really appre ciate air’s very significant bulk. T he air inside the inverted glass is at atmospheric pressure. When you submerge the glass , the air inside exerts enough pressure on the water to prevents it from entering and wetting the tissue . Diving bells used in dee p sea diving rel y on exactly the same principle to transport divers beneath the sea. Modern diving chambers are more sophisticated because they usually double as decompression chambers, but the same principle allows divers to enter and exit the open chambe r while submerged . More info: Diving Bell - Wikipedia Article http://en.wikipedia.org/wiki/Diving_bell