Pressure in fluids
These resources explore pressure through investigation. The list provides a range of activities, lesson ideas, background information, practical tips and suggested teaching strategies. and links to the following areas of the 2014 National Curriculum:
• atmospheric pressure, decreases with increase of height as weight of air above decreases with height
• pressure in liquids, increasing with depth; upthrust effects, floating and sinking
• pressure measured by ratio of force over area – acting normal to any surface
Visit the secondary science webpage to access all lists: www.nationalstemcentre.org.uk/secondaryscience
Machines 11-14
This whole suite of materials is good, but of particular relevance to this topic are the first three activities in the second unit; What’s in Pressure? You’ll find lesson plans in document Teaching Appraoches. Background information and teacher guidance are available in Physics Narrative and Teaching and Learning sections.
Having “played” with a tray of marbles in the first activity, students should develop quite a sophisticated understanding of pressure at a particulate level. They then go on to apply their ideas, thinking about the relationship between pressure, force and area before feeling the forces for themselves with a pair of syringes. Teachers will be able to assess students' understanding of this topic by asking them to draw diagrams showing the moving particles inside the syringes.
Vacuum Bazooka
The bazooka demonstration is great fun but its real value is in helping students to appreciate the forces and pressures that cause the movement. So long as your school has a vacuum cleaner, your technician should be able to put the equipment together fairly easily – just make sure the tube is the right diameter.
Demonstrating Physics: Forces
There are a couple of demonstrations included on this film which fit snugly into this topic.
Expanding Marshmallows starts about 3½ min into the film and shows how the air trapped inside marshmallows will expand significantly if the air pressure outside is reduced, it’s a great demonstration.
It is also an experiment that can be carried out quickly and easily by students using small marshmallows (the type used on top of hot drinks) and 10 or 20cm3 plastic syringes.
1. After removing the plunger, put the marshmallow into the syringe.
2. Replace the plunger and gently push it up until it is just below the marshmallow.
3. Putting the finger over the open end of the syringe, pull down on the plunger whilst observing the marshmallow expand.
4. The marshmallow shrinks rapidly when the finger is removed.
5. Similarly, using the plunger to compress a syringe full of air will shrink the marshmallow and it will now expand when the finger is removed.
The Cartesian Diver starts about 8½ min into the film and it is excellent for showing the effect of pressure in liquids. It’s good to demonstrate it with a test tube inside a two litre bottle so that students can see the air bubble changing size. Once they've seen this they could carry out the experiment for themselves, perhaps using sauce sachets which are easier to handle than the tubes.
Physics for Non-Physicists: Forces
Significant numbers of students will find the two demonstrations in this film counter-intuitive and that’s exactly why you should consider using them with your classes. The film shows a specialist from the Institute of Physics running some training for science teachers who are then seen trying the ideas out in their own classrooms.
The air bubbles demonstration (about 7 min into the film) takes a phenomenon that students will have seen many, many times; bubbles moving floating upwards in a liquid, and slows it down by using glycerol (propan-1,2,3-triol) in place of water. Challenging students to predict whether the smaller or larger bubbles will reach the top first is straightforward but asking them to give a give a reason will really put their ideas to the test. Encourage the use of physics terms and notice how the syringe has been set up to provide a good supply of bubbles.
Demonstrating Physics: Electrostatics
Ignore the fact that this film is entitled Electrostatics and fast forward to 12 min into the film. This is a fantastic, and really quick, demonstration that brings home what determines whether something will float or sink. Just make sure the students don’t really think that the metal ball turns into a polystyrene one!
Air Pressure
This is a linked pair of demonstrations which show how large forces can arise when atmospheric pressure is unbalanced. What's particularly useful is the explanation of why the crushed can demonstration may be confusing for students and how the second demonstration can be used to more clearly illustrate the effect of air pressure.
