Presentations

Reversible Fluid Mixing

What it shows:

Ink is squirted into a fluid and mixed in until it disappears. By precisely undoing the motions in the reverse direction, the ink becomes unmixed! The demonstration seems to defy thermodynamics in that it appears that entropy decreases, but in actuality the reversible mixing is made possible by ensuring that the mixing/unmixing is done without turbulence.

How it works:

The space between two transparent and concentric cylinders is filled with a viscous fluid (glycerine or Karo™ syrup). One or more lines of...

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Barton's Pendulum

Ten coupled pendulums of different lengths; shows resonance and phase.

What it shows:

All objects have a natural frequency of vibration or resonant frequency. If you force a system—in this case a set of pendulums—to oscillate, you get a maximum transfer of energy, i.e. maximum amplitude imparted, when the driving frequency equals the resonant frequency of the driven system. The phase relationship between the driver and driven oscillator is also related by their relative frequencies of oscillation.

How it works:

Barton's Pendulum...

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Ring of Fire

ring of fire

What it shows:

In explaining the electron orbits in the Bohr atom, de Broglie's principle of particle wave duality allows you to treat the electrons as waves of wavelength nλ = 2πr where r is the radius of the orbit. Then the only orbits allowed are those which are integer...

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Semimusical Blocks of Wood

What it shows:

Drop a piece of wood on the floor and listen to the sound it makes. It may sound like noise, but it also makes a "semimusical" sound which is so poor in quality that one would be hard pressed to call it musical. Yet it is not pure noise because the sound contains a series of regular impulses that have a pitch. This may be demonstrated by dropping wood bars (one by one) onto the floor — a musical scale or tune is easily recognized.

How it works:

The tuned wood (oak) bars are 6½" long and 1" wide with thicknesses...

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Capacitance of Human Body

What it shows:

Determine the capacitance of the human body as follows. Charge a person of unkown capacitance to 1000 volts. The person is subsequently connected (in parallel) to an external capacitor of known capacitance. The voltage measured across the capacitor combination allows one to determine the unknown capacitance of the person (typically between 180 — 200 pF).

How it works:

A 1000 volt power supply (output is in the microamp range) is used to put charge on a person. We assume that the amount of charge transferred to the...

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OHP Magnetic Lines of Force

What it shows:

The magnetic field lines of the Earth can be represented by the field lines of a bar magnet.

How it works:

The Earth's magnetic field is basically a magnetic dipole. It can therefore be represented to first approximation by the field of a bar magnet. The shape of the field lines can be highlighted by the sprinkling of iron filings, or by the use of plotting compasses. The latter method has the advantage of showing the variation of dip angle with latitude, with the lines of force running parallel to the surface of...

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Skin Depth

What it shows:

The depth to which electromagnetic radiation can penetrate a conducting surface decreases as the conductivity and the oscillation frequency increase. This demo compares the skin depth of AM and FM radio frequencies, and shows just how small these distances are.

How it works:

An electromagnetic wave entering a conducting surface is damped and reduces in amplitude by a factor 1/e in a distance ∂ given by 1

...

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Disappearing Prism

What it shows:

Light is refracted as it passes between two transparent materials of different refractive indices. If the materials are different, but the refractive indices are not, then the light rays are undeviated and the materials are optically indistinguishable.

How it works:

"And if you put a sheet of common white glass in water, still more if you put it in some denser liquid than water, it will vanish almost all together, because the light passing from water to glass is only slightly refracted or reflected or indeed...

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Hear the Wall Bend

What it shows:  A room-size laser interferometer with audio signal output. A standing wave is produced whenever a wave is reflected back on itself. A resonant cavity requires a second reflection so that the twice reflected wave has the opportunity to be in phase with the original wave. Here, laser light is reflected from a half-silvered mirror (mounted on a wall) so as to return to the laser and be reflected again by the laser. Movement of the wall by half a wavelength is sufficient to change the cavity formed between laser mirror and wall mirror from one resonant...

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