Pressure:
General Pressure:
Description:
- The demonstrator sticks a cup onto their face by sucking the air out of it.
- A glass is filled to the brim with water, a piece of cardboard placed across the top, and the glass turned over. Instead of the water falling out of the glass, the cardboard supports the water.
- A few sheets of newspaper are placed over a wooden ruler on the edge of a desk. A sharp blow breaks the ruler in half.
Science:
Air consists of molecules, and these are in rapid motion. They collide with objects, exerting a pressure on them. The cup sticks to the demonstrators face because there are more air molecules colliding from the outside than from the inside. Similarly, the water cannot escape the glass because a small amount of air at the bottom of the glass is 'stretched' so that there are fewer molecules pushing the water out of the glass (helping gravity) than there are pushing up on the cardboard.
The ruler breaks because air cannot get under the newspaper rapidly enough, and a vacuum is created underneath. Hence there are many air particles pushing down on the ruler, and none pushing up. Another way to visualize this is to imagine the huge column of air above the newspaper. A small amount of air doesn't weigh much, but the entire column weighs a great deal.
Special Requirements:
None.
Safety Procedures:
Magdeburg Hemispheres:
Description:
The Magdeburg hemispheres are reinforced hemispheres that fit together with an airtight seal, leaving a cavity inside. A vacuum pump is used to evacuate the hemispheres. Once evacuated, they will not come apart, even when the entire audience pulls on either side (yes, we really do get the whole audience pulling!).
Science:
There are fewer molecules colliding with the inside walls, pushing them apart, than there are colliding with the outside walls, pushing them together. The force required to overcome this difference is about equivalent to hanging a 1 tonne car from the spheres! Each molecule doesn't have much momentum, but there are a great many colliding.
Special Requirements:
Electricity.
Sample Tutorial Questions:
- Estimate the dimensions of the hemispheres and calculate the amount of force necessary to pull them apart. What mass would you have to hang from the bottom?
- Estimate the mass of an average air molecule, and the density of air. Use this to calculate the average velocity of an air molecule at atmospheric pressure.
Safety Procedures:
Magdeburg Hemispheres.pdf - 11kb
Liquid Nitrogen Milo Tins:
Description:
A small amount of liquid nitrogen is poured into an empty milo tin. The lid is sealed. As the liquid boils, pressure builds in the tin until the lid pops off (often traveling several metres into the air).
Science:
At room temperature, the liquid nitrogen rapidly boils. A substance in gas form typically occupies about 700 times the volume that it occupies in a liquid form. Hence a very small amount of liquid can rapidly build enough pressure to pop the lid off the tin. To make the lid go higher, we can push the lid down more tightly, and allow more pressure to build.
Special Requirements:
None
Sample Tutorial Questions:
Students can estimate the height that the lid reaches, and calculate the amount of pressure that had built inside the can.
Safety Procedures:
Liquid Nitrogen Milo Tins.pdf - 11kb
Liquid Nitrogen B.O.N.G.:
Description:
BONG = Balloon Orientated Noise Gadget. Our BONG is a juice bottle, with a tube attached on the front. A balloon is attached on the tube, and a small amount of liquid nitrogen poured into the bottle. As the liquid boils, the balloon fills with nitrogen gas, and pops. The bottle can be placed in a pool of water to make the liquid nitrogen boil more rapidly.
Science:
At room temperature, the liquid nitrogen rapidly boils. A substance in gas form typically occupies about 700 times the volume that it occupies in a liquid form. Hence a very small amount of liquid can rapidly build enough pressure to pop the balloon. The water speeds the reaction, since it can conduct heat into the liquid nitrogen more easily than air.
Special Requirements:
None
Safety Procedures:
Liquid Nitrogen 'Bomb':
Description:
A small amount of liquid nitrogen is poured into a juice bottle, and the lid sealed. The bottle is placed into a specially reinforced wheelie bin. As the liquid nitrogen boils, the pressure inside the bottle builds until it pops. The bin is half filled with water to speed the boiling, and to dampen the blow when the bottle pops. The bottle has a weight attached to ensure that it sits below the water.
The procedure is repeated with a soft drink bottle, and the size of the 'explosion' compared.
Science:
At room temperature, the liquid nitrogen rapidly boils. A substance in gas form typically occupies about 700 times the volume that it occupies in a liquid form. Hence a very small amount of liquid can rapidly build enough pressure to pop the bottle. The water speeds the reaction, since it can conduct heat into the liquid nitrogen more easily than air.
Soft drink bottles are made to withstand very large pressures, so they usually take longer to pop, and will make a larger 'bang' when they do.
Special Requirements:
This demonstration must be performed outside, since water will go everywhere! Also, a perimeter of at least 2m in all directions must be cordoned off.
Safety Procedures:
Liquid Nitrogen Bomb.pdf - 13kb
Liquid Nitrogen Rocket:
Description:
The 'rocket' is a soft drink bottle with a metal tube attached along the length of one side. A guiding string is looped through this tube, across the front of the lecture theatre, and tied to a bar on a high elevated platform.
A small amount of liquid nitrogen is poured into the bottle, and a cork put into the top. The guiding string is held taught. As the liquid nitrogen boils, the pressure inside the bottle builds until it pops the cork, and the bottle shoots along the string.
Science:
At room temperature, the liquid nitrogen rapidly boils. A substance in gas form typically occupies about 700 times the volume that it occupies in a liquid form. Hence a very small amount of liquid can rapidly build enough pressure to pop the cork out of the bottle at a high speed.
Momentum is conserved during the demonstration. Initially, the bottle and cork are at rest, and the total momentum is zero. After the cork pops out, the cord and bottle travel in opposite directions, with velocities related to their masses so that their total momentum cancels to zero.
Special Requirements:
This demonstration must be performed in 7-222 on the UQ campus, since it requires an elevated platform to tie the guiding string to. It could possibly be modified for other venues, but will require reconnaissance.
Safety Procedures:
Liquid Nitrogen Rocket.pdf - 12kb
Shrinking Balloon Animals:
Description:
Balloon animals are pushed into a container of liquid nitrogen. The gas inside the balloons condenses, and the balloons collapse. When they are taken out they rapidly warm to room temperature again, and increase back to their previous size.
Science:
At room temperature, the liquid nitrogen rapidly boils. A substance in gas form typically occupies about 700 times the volume that it occupies in a liquid form. Hence the gas in the balloons occupies a much smaller volume when it condenses to a liquid.
The balloons will sometimes pop when removed from the liquid nitrogen. This is due to the change in properties of the balloon rubber when it is frozen (it becomes brittle). When the gas warms before the balloon, they pop.
Special Requirements:
None.
Can Crush:
Description:
A small amount of water is poured into an empty soft drink can. The water is boiled until the can is entirely filled with steam. The can is upended into a bucket of water. The steam in the can rapidly cools and condenses, and the can is crushed by atmospheric pressure.
Science:
A substance in gas form typically occupies about 700 times the volume that it occupies in a liquid form. Hence the steam in the can occupies a much smaller volume when it condenses back to a liquid. There are then many fewer air molecules colliding with the inside of the can, than with the outside of the can. Unlike the Magdeburg hemispheres, the can is not strong enough to withstand atmospheric pressure, so it collapses.
Special Requirements:
None.
Safety Procedures:
Bell Jar:
Description:
A balloon animal is placed inside a sealed bell jar. A vacuum pump is used to evacuate the air from inside the jar. The balloon expands and eventually pops. A mashmallow man is then put inside the jar and the air evacuated. The marshmallows expand. When the air is let back into the jar, the marshmallows shrink to smaller than their original size.
Science:
Without air pressure outside the balloon, the pressure of the air inside the balloon is no longer balanced. Hence the balloon with expand. It will pop when the external pressure is so low that the rubber of the balloon can no longer hold the air inside it.
The marshmallow expands for the same reason - it contains many bubbles of air. When it is expanded a long way, the bubbles near the surface of the marshmallow will pop. When the external air pressure is returned to normal these popped bubbles will collapse, meaning that the marshmallows return to smaller than their original size.
Special Requirements:
Electricity.
Safety Procedures:
Vacuum Tube:
Description:
The air is evacuated from a tall, transparent chamber using a vacuum pump. A ball bearing and a piece of foam are allowed to drop down the tube. Because there is no air resistance, they hit the bottom at the same time.
Science:
Without air resistance to slow down the foam, it accelerates under gravity at exactly the same rate as the ball bearing.
Special Requirements:
Electricity.
Safety Procedures:
Bed of Nails:
Description:
A person lies on the bed of nails (a board with many nails punched through it), and is not skewered. Another person carefully stands on a board on their chest, and even though the nails now support the weight of two people, the bottom person is still not skewered.
Science:
Coming soon!
Special Requirements:
None.