Pressure

Pascal's Paradox

What it shows:

Three containers are filled with water to the same depth, and each has the same base surface area (see figure 1). Since the pressure and area are the same in each container, the force should be the same (pressure = force/area). pascalSo how come the scales...

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Magdeburg Hemispheres

When evacuated, held together by bombardment of atmospheric molecules.

What it shows:

Two brass hemispheres are brought together and evacuated, and are held together by the pressure of the atmosphere.

How it works:

Two brass hemispheres fit together to form an air-tight seal. One has a vacuum pump attachment and stop cock; the completed sphere can evacuated using a vacuum pump under a minute. As atmospheric pressure is 105Nm-2, the 11cm diameter hemispheres are held together by a force of 15000N. Invite members of your...

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55 Gallon Drum

What it Shows

With an air pressure of 105 Nm-2 at sea level, even a heavy duty oil drum will be crushed if it has nothing inside to balance the pressure.

How it Works

The screw cap on the drum is fitted with a vacuum pump connector. Simply turn on the Varian SD-200 pump and wait—it usually takes a few minutes to pump down, so you can carry on with what you were doing interrupted by various creaks and bangs as the drum's side walls begin to give. Because drums of this size are ribbed for strength, they can hold up under the strain, but when it...

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Inverted Pascal Experiment

What it shows:

This is a concept question relating to Pascal's cask-bursting experiment. Imagine the experiment inverted—literally! Attach a 20-ft length of tubing to the opening of a can full of water. Turn the can upside down and raise it high. Will the water stay in the can, or will it run out? Will atmospheric pressure hold up the column of water in the tubing? What will happen? Have the class vote.

...

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Golf Ball Atmosphere

A model of molecular motion and pressure using practice golf balls.

What it shows:

The kinetic energy of gas molecules bouncing off a surface causes pressure.

Increasing the molecules' speeds increases the pressure and the volume of the gas.

 

How it works:

Plastic practice golf balls represent...

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Siphon

What it shows:

A siphon is a device that allows the transfer of a fluid from one reservoir to a second at a lower level even though the first part of the journey is up-hill.

How it works:

A siphon is effectively an inverted U-tube with unequal length tubes. The asymmetry means that there is a pressure difference between the ends;

at the upper reservoir: p1 = P - ρgh1
at the lower reservoir: p2 = P - ρgh2
(where P = atmospheric pressure)

so p1 > p2 if h2 > h...

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Double Bubble

What it shows

When two different size soap bubbles are connected together, the smaller diameter bubble will shrink and collapse to blow up the larger diameter bubble. One can use this to demonstrate Laplace's law or the phenomenon of minimizing the surface area of a soap film.

How it works

Laplace's law tells us that the gauge pressure of a spherical membrane is given by 2γ/r, where γ is the surface tension and r is the radius of the sphere.1 For soap bubbles (which have an inside as well as outside surface), the gauge pressure is twice...

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Fakir Physics

The concept of pressure is demonstrated by lying on a bed of nails.

What it shows:

The lecturer (or someone else) lies on a bed-of-nails without discomfort, thus demonstrating the concept of pressure, which is the force per unit area. For added drama the person is sandwiched between two beds of nails with the added weight of a cinder block on top. The cinder block can be broken with a sledge hammer.

figure 1: A supermarket tabloid reports on how macho man pulls off this stunt.
...

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