lecdem.physics.umd.edu

ASTR101

I6-01 GAS PRESSURE - MODEL

I6-01

Illustrates the molecular nature of gas pressure

A vibrator motor is activated causing chaotic motion of a group of ball bearings in a clear plastic container. The upward motion of the ball bearings pushes a black plastic plate upward, indicating the upward force of "air pressure" on the plate. Increasing the speed of the motor by turning up the variac increases the average speed of the balls and pushes the plate up further, modeling a greater pressure.

I6, PW1
J1-24 ELECTROSTATIC HAIR RAISING

J1-24

Demonstrates electrostatic repulsion

While standing on a large styrofoam insulating block, touch your hands to the top of the Van de Graaff dome, then have someone turn it on. The fact that your hair stands on end is a result of the repulsion between charges of the same sign that collect on your hair.

J1a, OS2
K2-62 CAN SMASHER - ELECTROMAGNETIC

K2-62

Blasts a soda can into two pieces using electromagnetism

A 400 microfarad capacitor is charged to 3000 volts (1.8 kilojoules) and discharged through a three-turn coil into which an aluminum soft drink can has been positioned. With the circular windows open, the two pieces of the can will be blasted over thirty feet to the sides of the large lecture hall. Charging the capacitor to less voltage results in a can with a "waist."

This device can be explained in two distinct ways:
(1) The rapidly rising current creates a rapidly rising magnetic field along the axis of the coil, which in turn induces an electric field going in circles inside the coil. The induced electric field causes an electron current in the can which experiences a vxB force in the magnetic field of the coil, causing the can to break into two pieces which are blown to the opposite sides of the lecture hall.
(2) A type of "theta pinch" phenomenon. More information on this is available from Wikipedia. Another way to understand this is that the induced current around the can is opposite to the current in the primary coil, since it is opposing the change in flux. These concentric opposite currents repel each other, so the can is pinched and torn apart and ejected out the sides.

This is an UNFORGETTABLE DEMONSTRATION. A must when you cover electromagnetism.

This video, from the Video Encyclopedia of Physics Demonstrations, shows the operation of the can crusher with an animation illustrating (1) the electron current in the coil, (2) the vector magnetic field that it creates, (3) the induced electric field within the coil created as the coil current rapidly rises, (4) the electron current circling in the can created by that induced electric field, (5) and the vxB force on the electrons moving around the can.

Following a description of the crusher electronic components, the animation is displayed. The animation may be stopped so that the directions can be studied in detail for the five (5) quantities listed above. Using the left hand rule (for electrons) the directions can be verified; note that according to Lenz's law the direction of the electron current induced in the can must be in the opposite direction to the electron current in the coil.

Note that the magnetic field at either end of the coil possesses both an axial and a radial component; the electron current in the can is entirely azimuthal. Using the left hand rule to determine the direction of the cross product of the electron velocity and the magnetic field, it can be seen that the axial component of the magnetic field leads to an inward force, crushing the can, while the radial field component leads to an axial force, away from the plane of the coil at both ends of the can, causing the two parts of the can to move rapidly away from the coil. (In the large lecture hall the two parts of the can will be blown to the sides of the lecture hall.)

The web site http://hibp.ecse.rpi.edu/Can_Crusher/home.html contains a drawing and animation showing how the RPI electromagnetic can crusher works.

FS1
K8-01 ELECTROMAGNETIC WAVE - MODEL

K8-01

Shows the relationship between the electric and magnetic field vectors in a plane-polarized traveling electromagnetic wave

Red pegs represent the electric field vector and blue pegs represent the magnetic vector. The spatial relationship between these vectors and the direction of propagation can be seen. By moving the model along its axis the temporal aspect of the wave can be shown. This wave has a wavelength of 0.81 meters, and as an EM wave would have a frequency of 370MHz

FS1
L3-33: VANITY MIRROR

L3-33

Show the image produced by a cocave mirror.

A standard hand-held vanity or shaving mirror, flat on one side, concave on the other. Pass it around for the class to examine, or simply use it to admire yourself.
L4-06 REFRACTION IN CLOUDY WATER

L4-06

Demonstrates a light ray bends when it enters a different medium at an oblique angle.

The ray from the laser refracts when entering the surface of the cloudy water. The path of the laser beam in the water may be rendered more visible by adding a touch of powdered creamer to the water.
L4-23 BENDING OF LASER BEAM IN SUGAR SOLUTION

L4-23

Demonstrates a medium with a continually varying index of refraction

Placing sugar along the bottom of a long, narrow water tank, as the sugar dissolves it creates an index of refraction gradient with the greater index of refraction nearer the bottom of the tank. A laser beam bends continuously in the sugar solution and reflects off the bottom of the tank, as shown in the photograph. Observation of the path of the laser beam is enhanced by adding a pinch of powdered coffee creamer.

Geometrical Optics
L7-16: GALILEOSCOPE

l7-16

Demonstrate optics of a telescope

The Galileoscope is a refracting telescope, or refractor: a long tube with a big lens (the objective) at the front end and a small lens (the eyepiece) at the back end. Light is refracted when it goes through the big lens, and then reach the eyes through the eyepiece. The scope can be disassembled to see the lenses.
M7-31 TYNDALL'S EXPERIMENT - COLLOIDAL SUNSET

M7-31

Colloidal sunset demonstration

The collimated white light from the bright point source passes through the empty tank and hits a nearby screen. Chemicals previously prepared are then mixed in the tank: 2.5 ml sodium thiosulfate solution in 100 ml water, and 2.5 ml concentrated HCl 1:4 dilution in 650 ml water. When the chemicals mix they begin to form a suspension of sulfur particles which act as scattering centers for the light, especially blue light at first. This leaves the light on the screen with a yellowish tint. As time passes the sulfur particles grow larger and scatter more light and light of a longer wavelength, changing the light on the screen to a bright red. Ultimately most light is scattered, leaving no light on the screen.

M7, LS1, OM1
N1-01: PRISMATIC SPECTRUM OF WHITE LIGHT - POINT SOURCE

N1-01

Demonstrate continuous spectrum

This is a convenient setup for showing the visible spectrum. A bright point source is used to provide a continuous white light spectrum. Light from the point source is focused first by an integral condenser lens and iris and then a 20cm focal length convex cylindrical lens onto an adjustable slit. A 20cm focal length convex spherical lens then images the slit through an equilateral flint glass prism onto a screen. For mechanical drawings of the original point source, see lecdem.physics.umd.edu/images/Demos/point%20source%20plans.pdf

FS1, LS1, OM1
N1-02: PRISMATIC SPECTRUM OF WHITE LIGHT - INCANDESCENT

N1-02

Demonstrate continuous spectrum

An incandescent bulb source with reflector is used to provide a continuous white light spectrum.
N1-05 SPECTRA - VISIBLE AND INVISIBLE

N1-05

Demonstrates continuous spectrum

The carbon arc lamp is used to provide a continuous white light spectrum. Light from the arc lamp is focused by a condenser lens with iris and a 20 cm focal length cylindrical lens onto a slit. A 20 cm focal length convex lens then images the slit onto the screen through an equilateral prism. A fluorescent screen (with fluorescein) is used to show that there is ultraviolet radiation, including a strong UV line, in the carbon arc spectrum. A thermopile is used to sense infrared radiation, where the heat measured by the thermopile causes an audio oscillator to rise in pitch, so a hotter source produces a higher tone. (see I2-06 for more on this apparatus) Aiming the thermopile from the spectrum back toward the prism, it is observed that the hottest part of the spectrum is just off the red color, in the infrared.

N1, OM1, LS1
N2-32: ABSORPTION SPECTRA OF GLASS

N2-32

Demonstrate absorption spectrum of glass doped with various chemicals.

A bright point source with condenser lens and iris is focused by a 10 cm focal length cylindrical convex lens onto a slit, which is in turn focused onto a distant screen by a 20 cm focal length spherical convex lens. A white light spectrum is obtained by inserting an equilateral prism just after the spherical lens. A sample of glass doped with a chemical is placed in the beam just before the slit. The system now shows the absorption spectrum of the glass.

The glass selections available are neophane glass (left), holmium oxide glass (left center), and dydimium glass (right center). The spectrum of white light, without any absorbing glass, is shown at the right.
P3-61: FLUORESCENT LIQUIDS

p3-61

Show fluorescence of different chemicals.

A set of liquid vials containing fluorescent materials is illuminated by an ultraviolet light source. The radiation is absorbed by the chemicals and emitted at a visible frequency, causing them to glow. Identification of the chemical is printed above each vial. The bottle at the right contains quinine water. It's almost enough to make you give up drinking quinine water.

The second photograph was taken with normal fluorescent lighting.

P3
P3-62: FLUORESCENT CHALK

p3-62

Demonstrate fluorescence in different materials.

Colored chalk is doped with various fluorescent chemicals so that when illuminated by ultraviolet light it glows brightly. Write on the blackboard with different colors, then make them come alive with a black light.

P3