Introduction A great excuse threatening to pour water over your audience – but with a surprise Twist, thanks to physics. B. Presentation Proper Materials needed: ;large glass ;ashtray or similar ;water ;handkerchief Procedures: 1 . Push the centre of the handkerchief into the glass, so that the edges are hanging over the outside of the rim of the glass. 2. Pour water into the glass, through the loose handkerchief. Make sure that your audience can see the water easily passing through the handkerchief into the glass.
Keep pouring the water until the glass is roughly half full. 3. Pull the corners of the handkerchief so that the material is taut over the top of the glass. Hold the glass and handkerchief so that the material stays tightly stretched over the opening. For younger audiences you may like to say some ‘magic words’ that make the hanky water proof. 4. Place the ashtray on the top of the glass and tip it all upside down, being careful to keep the handkerchief pulled tight. 5.
Choose a likely suspect from your audience to threaten with a drenching! Hold the upside-down glass and ashtray above their head, making sure that the glass is vertical and the handkerchief is tight. Remove the ashtray and voila! – Nothing happens! The water stays inside the glass. How to Operate: When the handkerchief is loose, the water can pour through the gaps in the fabric. But when it is pulled tight, the water molecules are able to form a single membrane across the material due to surface tension.
Science Concept: It’s the same with this trick – due to surface tension, the water forms a membrane cross the hanky, but if you poke the hanky with your finger then you break this surface tension and the water comes through the cloth. The force of the surrounding air acting upon this membrane is sufficient to overcome gravity, and so the water stays in the glass. This best explains La Place’s Law, The larger the vessel radius, the larger the wall tension required to withstand a given internal fluid pressure. This demonstration is based on surface tension and air pressure.
Application: Imagine blood flowing through a blood vessel which has a certain radius and a retain wall thickness. The blood vessel wall is stretched as a result of the difference between the blood pressure inside the vessel and the surrounding pressure outside the vessel. La Place’s law describes the relationship between the transmutable pressure difference and the tension, radius, and thickness of the vessel wall. Obviously, the higher the pressure difference the more tension there will be. On the other hand, the thicker the wall the less tension there is. Also, the larger the radius the more tension there is.
