Standing Wave in Metal Rod

An aluminum rod, supported in the middle, rings for a long time in its longitudinal mode.

What it shows:

Longitudinal standing waves in solids.

How it works:

A metal rod is not unlike an organ pipe with both ends open. Holding it exactly in the middle will force the simplest, or fundamental, mode of vibration -- the ends will be free to vibrate maximally and the center will be a node. The fundamental frequency happens to be 2.26 kHz. As with a pipe open at both ends , the rod will vibrate at all the odd as well as even...

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Induced Electric Dipole

What it shows:

Bringing a charged rod close to neutral dielectric polarizes the dielectric's surface charges. Here a pile of Styrofoam puffs are polarized and attracted to a charged rod.

How it works:

The neutral puff experiences a non-uniform electric field from the rod. Although there are polarized charges of both kinds, because (figure 1) the field is stronger near the rod due to the concentration of positive charges, there is a net attraction. On a dry day they'll jump to meet the rod.

figure 1....

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Continuous Spectrum

What it shows:

White light is shown, á la Newton's demonstration of dispersion by a prism, to be composed of a continuous spectrum of colors.

How it works:

A large brilliant spectrum is produced by using a 1 kW carbon arc light source 1 with adjustable slit, a "fast" f/0.9 imaging lens, 2 and a highly dispersive in-line prism. 3 The spectrum easily fills a two meter wide screen with vibrant colors. An alternative (more compact) setup consists of a Beseler slide projector 4 which...

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Periodic Potential

What it shows:

Ball bearings simulate atoms in a lattice sitting at local potential minimums. Giving them energy excites the atoms and they oscillate about their equilibrium positions in these wells; only with large amounts of energy can they be truly dislocated.

How it works:

A piece of wood 100 × 25 × 2cm acts as the ‘potential’ structure of the lattice. The atoms, 3cm diameter ball bearings sit at the bottom of a cosine varying potential cut to about 10cm depth in the wood by a jig saw.The balls are held in the 2-dimensional...

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Saddle Shape Universe

Curved space segment for open universe geometry.

What it shows:

Whether the Universe continues to expand forever or will collapse back in upon itself depends upon the amount of matter it contains. For a density parameter Ω less than unity the Universe will not have enough mass to collapse and will be in a state of perpetual expansion. In general relativity, the curvature of space is dependent upon the density of the Universe, and for Ω<1 the curvature is negative or hyperbolic. It can be represented two dimensionally (see Comments) by a saddle...

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Lead Bell

Dull at room temperature, rings clearly after immersion in liquid nitrogen.

What it shows: 

A lead bell, dull sounding at room temperature, rings brightly when cooled to liquid nitrogen temperatures.

How it works: 

A lead bell at room temperature is dull in more ways than one. But its elasticity is temperature dependant, with an increase in elasticity as its temperature decreases. This increase in elastic modulus narrows the resonance response with frequency and increases the quality Q of the lead as...

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Mercator Projection

What It Shows

As shown in the drawing, the Mercator projection is a cylindrical map projection of the spherical globe. The meridians and parallels of latitude on the globe end up appearing as lines crossing at right angles in the projection. Areas on the globe far from the equator appear to be much larger on the projection. It's not trivial to visualize this projection and the intention of this demonstration is to shed some light on it (literally).


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