[t+]

Takes about 10-15 miutes to set up.
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...

Read more about Microscope Resolution
Bernoulli Wind Tunnel

What it shows

Bernoulli's Principle states that the pressure exerted by a fluid is velocity dependent; the faster the velocity the lower the pressure. This can be demonstrated by a form of Venturi tube, a pipe with a narrow constriction which forces an increase in fluid velocity.

How it works

The varying cross-section of the wind tunnel forces air to travel its length at different velocities, with the highest velocity being at its most constricted part. From equipartition of energy, the increased energy in one degree of freedom (the kinetic energy of the flow)...

Read more about Bernoulli Wind Tunnel
Bernoulli's Principle

What it shows

The relative velocities of two sides of a spinning ball to an oncoming wind creates a pressure difference and therefore a net force on the ball perpendicular to the air flow.

figure 1. Direction of motion of ball due to pressure difference

diagram of air flow and spinning ball
...

Read more about Bernoulli's Principle
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

...

Read more about Viscous Fluid
Poiseuille's Law

What it shows

Poiseulle determined that the laminar flow rate of an incompressible fluid along a pipe is proportional to the fourth power of the pipe's radius. To test his idea, we'll show that you need sixteen tubes to pass as much water as one tube twice their diameter.

How it works

Poiseulle's law states that the flow rate Q is also dependant upon fluid viscosity η, pipe length L, and the pressure difference between the ends P by

...

Read more about Poiseuille's Law
NMR model

What it shows:  Using the classical description of the motion of a spin in an external magnetic field, the demonstration helps visualize NMR in the time domain. The nuclear magnet and its classical vector model are represented by a spinning ball with magnets attached. A rotating mass is characterized by its angular momentum L, which is the analog of the magnetic moment mu, which characterizes a rotating charge distribution. The spinning ball mimics protons in that it has both angular momentum as well as an "intrinsic" magnetic moment. The torque...

Read more about NMR model
Air Table Center-of-Mass Motion Monday, May 2, 2016

What it shows:  Two bodies, rotating about each other, rotate about their common center-of-mass (COM). The COM exhibits uniform motion (or none at all) regardless of what the two bodies are doing.

How it works:  The "bodies" are 4-1/2" diameter acrylic disks that float on a cushion of air on a large air table.1 Presently we have three versions ready to go. (1) The first version has two disks connected by means of a 12"- long plastic ruler. A large "dot" at the center of the ruler marks the COM. The disks can be made to simply...

Read more about Air Table Center-of-Mass Motion
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...

Read more about Pulse Reflections in a Coax Cable
Viscous Flow of Bread Dough

Bread dough is stiff but still flows. A big blob of foodstuff that slumps over time, like Silly Putty but large and edible.

Make bread dough enough for a couple loaves, and knead it stiff enough that a round ball of dough takes half an hour to slump to half its original height. Place on a plate, put a camera on it. Project the image at the beginning, just as the dough ball is released, and again some time later, after viscous flow.

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.

Potato Chip Calorimeter

A soda can with 50 ml of water is held over a burning potato chip of known mass. A thermocouple reads the temperature of the water, and its change, to estimate the energy content of the chip.

Other burnable foods include nuts, e.g. brazil nut, and other kinds of fried snack chips.

The heat transfer from burning chip to soda can bottom is only so good, and the temperature rise does not match that expected in the ideal case.

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.

Liquid Nitrogen Marshmallows

A big insulated bowl is filled with liquid nitrogen and marshmallows, which when frozen, are eaten in spectacular fashion.

Regular size marshmallows. Use wooden spoons, big bowl with holes for draining. Push down the marshmallows in the liquid nitrogen and mix to evenly freeze. Serve to volunteers. 

Eaten with open mouth and exhaling slightly gives the effect of dragon's breath.

Reaction of Propane and Oxygen

An oxy-propane torch is lit with just propane, and the flame examined, before oxygen is added, changing the flame character and temperature.

The torch head is gaffed to the table top with enough slack to turn the gas valves. The propane tank and the oxygen tank are on separate dollies. The torch valves are closed, and the tank regulators set so there is about three pounds of pressure behind each gas. (The precise settings of the regulator are a good thing to check out in a practice session before doing this demo.)

With a friction lighter in one hand, open the propane valve on...

Read more about Reaction of Propane and Oxygen

Pages