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Will sit on a bench or a cart and will be fairly clear to the audience by itself.
Microscope Resolution Tuesday, December 6, 2016

What it shows:  The wave nature of light limits our ability to see the very small. Application of the Rayleigh limit of resolution tells us that the size of the smallest objects one can resolve under a microscope is approximately equal to the wavelength of light. The optical limits of a microscope are demonstrated as one attempts to resolve 1 μm diameter spheres (about twice the wavelength of light) — one sees spots of light surrounded by diffraction rings rather than sharply defined spheres, similar to the 3rd image (from: Cagnet/Francon/Thrierr, Atlas of Optical...

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Cannon Ball Boat Puzzler

What it shows

Does the level of the ocean rise or fall when a cannon ball is tossed overboard? A question of displacement.

How it works

A difficult effect to see at sea, but it becomes clear by taking some parameters to extremes. Reducing the ocean to 12L in volume, and the boat to practically no mass by using a plastic bowl, a cannon ball of 1kg mass suddenly becomes substantial. With the cannon ball in the boat, its weight is distributed throughout the boat; the lowered density increases the amount of water displaced (by the fraction of the boat submerged), raising...

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Buoyant Force Measurement

What it shows

We have three 20 oz. soda bottles, one filled with water, one filled with sand, and one filled with air. A spring scale shows the water-filled bottle to weigh approximately 6N in air, and nearly 0N when it is fully submerged in a large container of water. Since gravity is still acting on the bottle when it is submerged in the water, there must be a force of 6N pushing up on it. This is the buoyant force.

We can do the same experiment with the bottle of sand. This bottle weighs roughly 13N in air, but when it is fully submerged in water it weighs 6N less. Even...

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Archimedes' Principle

What it shows

Archimedes' principle states that the buoyant force or upthrust is equal to the weight of fluid displaced. An object with equal mass but a lower density occupies more volume so displaces more water; it therefore experiences a greater upthrust.

How it works

This demo compares the buoyant force acting on two 1kg masses, one of aluminum and one of brass. Each in turn is lowered into a beaker of water using a spring balance (figure 1). The aluminum, having the lower density, experiences the greater upthrust and a reduction in weight from 10N to about...

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Bernoulli Beach Ball

What it shows

Bernoulli's principle shows the velocity dependence of pressure in a fluid. Here, fast flowing air creates a zone of low pressure that holds a beach ball aloft.

beach ball held aloft by fast flowing air from a blower

How it works

Here we have a beach ball held in the air stream from a...

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Viscous Fluid

What it shows

For a body to reach terminal velocity when falling through a fluid, the drag force (given by Stoke's Law) coupled with the buoyant force (from Archimedes' principle) need to balance the falling object's weight. Leaving derivations to other great texts you end up with

...

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Pulse Reflections in a Coax Cable Thursday, February 25, 2016

What it shows:  A voltage pulse, injected into a long coaxial cable, will travel down the length of the cable and undergo a reflection at the other end. The nature of that reflection depends on how the cable is terminated at the other end. Shorting the cable at the far end produces an inverted reflection. With no termination (an "open" end), the reflected pulse is not inverted. When the impedance of the termination matches that of the cable, there is no reflection.

Knowing the length of the cable and noting the amount of time it takes the pulse to come...

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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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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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Mixing Air and Water

Three clear containers, about 10% full of water, and three immersion blenders are on the bench. Three students volunteer to mix air into water. To one container is added an egg white, and to another is added xantham gum. The students are met with varying levels of success.

Good containers are 1500 ml beakers. The xanthan gum is best hydrated before the demo, and added as a gel to the water. An equal mix of lecithin and xanthan gum also works.

Cooking Ice vs. Cooking Eggs

An egg size piece of clear ice is dropped into a hot frying pan, with hissing and melting and steaming from solid to liquid to gas . An egg is carefully dropped into another hot frying pan, and it transforms from liquid to solid.

A small water bottle in the freezer overnight will freeze solid.  Cutting off the plastic and breaking the ice with a hammer will generate the egg size piece of ice.

Thermocouple Brownie

A brownie pan with two food safe thermocouples, one in the brownie batter and one in the air next to the pan, is put in a pre-heated oven, and the temperature profiles recorded and displayed.

Clean copper wire is used to make an armature for the thermcouple wires. Crimp the center of a 20 cm piece of 14 ga wire on the side of the baking pan. Bend loops at the ends of the copper wire to hold the thermocouple wire.

The oven temperature thermocouple should be about five centimeters away from the pan, at the same level as the center of the pan. The brownie thermocouple is...

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Specific Heats of Oil and Water

A volunteer puts her hands in oil and water in large beakers on thermostated hot plates, at about 60°C. The water beaker hand is removed almost instantly. The oil beaker hand can remain indefinitely.

The heat capacity of oil is about half that of water. Oil is thought of as hotter because it can be heated to higher temperatures than boiling water, but at the same temperature, water moves more heat into your hand than oil does.

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