Reverse Sprinkler Friday, December 18, 2015:

What it Shows

Inspired by Richard Feynman's story in his 1985 book (pp 63-65), Surely You're Joking Mr. Feynman, the demonstration answers the question "which direction does a lawn sprinkler spin if water enters the nozzle rather than being expelled from the nozzle?" The reverse sprinkler spins in the opposite direction of a "normal" sprinkler. "Dissipative effects" has been the hand-waving reason for the past 30 years, but the real reason why it spins in the reverse direction is far from obvious (see Comments, below). It turns out that a sprinkler designed to be "truly...

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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.


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Standing Wave on Long Spring

Obtain as many harmonics as your arm can handle.

What it shows:

Generation of a standing wave by reflection from a fixed end.

How it works:

A two person demonstration using a 2m (2cm diameter) steel spring. 1 One party acts as the fixed end, standing holding the spring rigidly at chest height. The other sends the pulses down the spring by vigorous up-and-down movements. The frequency is adjusted to set up a standing wave from the fundamental up to whatever you're capable of (see Comments). Amplitudes of...

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Bird on a High-Voltage Transmission Line

What it shows:

Why doesn't a bird sitting on a high-voltage wire get electrocuted? This demonstration addresses that question and serves as a model of the situation.

How it works:

The important concept conveyed is that there needs to be a voltage difference across a conducting medium for current to flow through the medium. In this situation the conducting medium is a bird sitting on a high-voltage wire. The voltage on the wire is the voltage of the whole length of wire with respect to the ground. Although the bird on the...

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Microwave Tunneling analog

3 cm microwaves and prisms made of plastic beads demonstrate total internal reflection in one prism, and coupling of the evanescent wave to a second prism. An audio signal corresponds to the one kiloHertz modulation of the microwaves.

The prisms are made of foam core board, cut and hot glued, then filled with small pony beads.


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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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Rock Samples

Selection of igneous, sedimentary and metamorphic rocks.

What it shows:

A selection of rocks and minerals available for lecture demonstrations

How it works:

The following samples are available for show-and-tell in lecture. The samples vary in size, and only those marked with a 4 are big enough to be seen.

1. A selection of elements in their natural (unrefined) states:
native copper Cu 4
diamond C iso
gold Au
realgar (silver ore) 4
graphite C hex 4

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