describe an effect of atmospheric pressure
(a) Heat an opened drinks can and then submerse in water. The can should collapse. Ask the students to explain, paying particular attention to answers such as: 'the can collapsed because the vacuum sucked the walls in'. Students need to appreciate that the can collapsed because there is an imbalance between the force exerted by particles outside the can and the force exerted by particles inside the can. (b) Show the students of flat fish that live on the bottom of the sea eg skate, and tell them that we are at the bottom of a sea as well - a sea of air! Discuss what the effects of living at the bottom of the sea would be for the fish. 'It doesn't notice it because its used to it' and explain that we dont notice the effects of 'air pressure' because we are used to it, but on an average table there is a force equivalent to the weight of 5 cars on it! Students could draw a poster to display this interesting fact. (c) Demonstrate the use of sink plunger or dent removers that can lift smooth boards. Draw a cartoon strip showing the forces on the plunger due to air before and after its use.
describe some effects (a) Offer the students syringes connected by rubber tubing. Fill one of the syringes with water and and uses of liquids under and the push the plunger to move the water into the other chamber. Consider what use this could be pressure, and ask the students to invent something that could use this effect. (b) Ask a volunteer to lightly hold a balloon between flat hands. Fill the balloon with water from a tap and ask if the balloon is trying to push the hands apart. Show pictures of hydraulic machines like big yellow bulldozers and state these work on a similar principle. Discuss how bulldozers can lift heavy loads (c) Discuss how remarkable brake systems are on a car - how they can stop a car travelling fast. Explain that they work by moving brake fluid around and challenge the students, working in groups to explain how brakes might work. Discuss the properties of the brake fluid - it is runny, non-compressible and doesnt freeze at normal temperatures - why are these properties important?
describe an effect of underwater pressure
(a) The pressure in a liquid or gas is the same in all directions. Distinguish between force which acts in a particluar direction. Remind students of the last time they had their head underwater. They were not pushed to any direction - the force due to water was the same from every direction. (b) Elicit sudents' ideas as to why special submarines have to be used to explore the very deep oceans. What would be the effect of going too deep? (c) Refer to a diver who said: 'The pressure of the water pushes directly on my ear drums' and ask if this statement is scientifically correct. It is important to distinguish between the words 'force' and 'pressure'. The pressure causes a force, pressure has not direction associated on it and a force is exerted by one thing acting on another. Ask the students to rewrite the statement correctly, by substituting the word 'pressure' for 'force'.
apply the concept of transmission of pressure to predict the resulting force
(a) Some students develop a picture of forces being carried, by the fluid, from cylinder to cylinder. Care is needed not to suggest the force 'slides around the bends' from one piston to another. Instead make the connection by pressure values. Consider that the pressure throughout the fluid increases by the same amount when a force is applied to one cylinder and then the system equilibrates back to its original level and a force can be felt at the other cylinder. (b) Show students a set up were two syringes of different cross-sectional area are connected via a rubber tube. Fill one of the chambers with water and push in the plunger so the water is transferred to the other chamber - this is a simple hydraulic system. Hold both syringes vertically downwards and add a weight to the small plunger so that the larger plunger goes up. Vote on how much weight has to be put on the large plunger to push the small plunger back up. Is it less, more or equal to the weight on the other side. It is counterintuitive to many but the large piston will need considerably more weight to push the small piston back up.
apply the model of the particle theory of matter to explain the behaviour of gases under pressure
(a) Agitate a tray of marbles and ask the students to listen for the collisions the marbles have with each other and wall. Distinguish that there are two distinct sounds and try to count the number of collisions with the walls of the tray only. Discuss how this helps to explain gas pressure. Repeat with a smaller tray or move one of the slides in, this time there should be move collisions with the walls in a certain time. Conclude that pressure is increased as volume is decreased. (b) Use the Gas Properties applet at http://phet.colorado.edu/web-pages/simulations-base.html. to demonstrate the effect on gas pressure as volume is varied (c) Role play gas particles in an enclosed room. The particles must obey the rules of travelling in a straight line, not speeding up and bouncing off each other at a sensible angle. Discuss the effect of giving energy to the system - the individual particles all speed up. Try to observe that there are now more collisions in a certain time i.e. the pressure has gone up!