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Bucket of Light

What it shows:

A beam of laser light can be trapped inside a stream of water by suffering total internal reflection—the aquatic equivalent of a fiber optic cable.

How it works:

A stream of water flows from a hole in the side of a soda bottle (figure 1). The critical angle of 49° is such that...

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Faraday Induction

What it shows:

The mathematical description of electromagnetic induction as formulated by Maxwell and Faraday requires two different sets of equations to calculate the induced voltage, depending on whether the coil is stationary and the magnet moving or vice versa. In fact, as this demonstration shows, the voltage is the same as predicted by the two sets of equations.

How it works:

The apparatus is identical to demonstration Faraday's Law, and is described in detail there. Briefly, it consists of a galvanometer hooked up to a...

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Mixing and Unmixing

Food coloring in glycerine is mixed by turning a drum, then unmixed by reversing. Has entropy decreased?

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 insuring that the mixing/unmixing is done without turbulence.

How it works:

The space between two, transparent and concentric...

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Binary Star

Two small closely mounted bulbs simulate resolution problems.

What it shows:

The ability to resolve two closely separated stars depends upon the aperture size of the observing instrument. Here two tiny bulbs represent stars that are barely resolvable by human eyes across the lecture hall.

How it works:

The light collected from two stars by the eye (or by a telescope mirror) are themselves geometric point sources but are circular diffraction fringes

The light from a distant star is not detected as a geometric point...

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Random Walk Model

What it shows:

A random walk is a mathematical model for the movement of a particle that is under the influence of some random or stochastic mechanism that affects its direction of movement. Physical situations that can be described by random walks include diffusion and Brownian motion.

How it works:

The board is a two dimensional random walk model consisting of a hexagonal array of corks, 1 11 to a side (331 corks in all), with each point of the hexagon given a number. The random walk begins from the center cork and...

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Loaded Beam

loaded beamsBeam supported at ends with platform scales and toy truck as load to demonstrate moment arms.

What it shows:

The concept of moment arms is exemplified by this model of a truck on a bridge.

How it works:

A beam (board) supported...

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Reversible (Kater's) Pendulum

A physical pendulum with two adjustable knife edges for an accurate determination of "g".

What It Shows

An important application of the pendulum is the determination of the value of the acceleration due to gravity. By adding a second knife-edge pivot and two adjustable masses to the physical pendulum described in the Physical Pendulum demo, the value of g can be determined to 0.2% precision.

How It Works

Using a simple pendulum, the value of g can be determined by...

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Tail Wags Dog

Lecturer tries to swing baseball bat while standing on turntable.

turntable

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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