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PHYS260

  • I6-52: ENTROPY - FOUR BALLS IN GAS DIFFUSION MODEL

    I6-52
    Demonstrate that an ordered state is statistically possible.

    Place two balls of each of two different colors in the diffusion apparatus. To start, either place all four one the same side of the apparatus; or place two of one color on one side of the apparatus, and two balls of another color on the other side. Start the machine going with the hole between sides open.
    Engagement Suggestions
    • Encourage students to notice that although statistically it is less probable, the arrangement of both orange balls on one side and both green balls on the other side will happen occasionally.
    • Ask: Is this as likely to happen using three balls of each color? With four?
    Background
    This is essentially showing the exception to the general principle of demonstration I6-21. With a small enough number of balls in the model, it is statistically possible to “reverse entropy” to a limited extent – that is, occasionally, the balls will randomly organize themselves so that they are once again sorted by color.
    FS1

  • J1-01 TRIBOELECTRICITY - CHARGING BY FRICTION

    J1-01
    Demonstrates "charging by friction"
    Rubbing silk on a glass rod makes the glass positive and the silk negative. Rubbing fur on a hard rubber rod makes the hard rubber negative and the fur positive. This effect is known as "triboelectricity," from the Greek "tribein," or to rub. The positively charged glass rod and the negatively charged hard rubber rod can then be used (1) simply to illustrate that electrical charge exists using an electroscope or (2) to perform other electrostatics experiments.
    J1b

  • J1-11 ELECTROPHORUS

    J1-11
    Demonstrates an electrophorus
    An electrophorus is a device which retains its charge so that it can act as a continuing source of charge for experiments. Charge the acrylic electrophorus plate negative by rubbing it with fur. Placing the aluminum plate on top of the plastic plate and grounding it charges the aluminum positive by induction. Existence of these charges can be verified using the electroscope. The charge remains on the surface of the plastic plate, so the aluminum plate can be charged by induction a number of times before the charge on the plastic leaks away
    J1b

  • J1-12: INDUCTION - ELECTROSCOPE

    J1-12
    Demonstrate charging by induction.
    A charged rod (black rubber in the photograph is negative) is held near the top plate of the electroscope, causing the electroscope to deflect. While the rod is in this position, the plate is touched by a grounded banana wire, and the electroscope returns to the uncharged position. When the charged rod is pulled away, the electroscope is charged positive, and deflects. This experiment can also be done using a positive glass rod to charge the electroscope negative by induction. The sign of the charge on the electroscope can be checked as follows: a rod with the same charge as the electroscope will cause further deflection of the electroscope when held close to the top plate, but a rod with charge opposite that of the electroscope will cause less deflection of the electroscope when brought close to the plate.
    J1b

  • J1-21 ELECTROSTATIC ATTRAC AND REPULS - CHARGED CYLINDERS

    J1-21
    Demonstrates electrostatic attraction and repulsion
    Charge the glass cylinders positive by rubbing with silk, and charge the hard rubber cylinder negative by rubbing with fur. The two positive glass cylinders repel each other, but both are attracted to the negative hard rubber cylinder.
    J1b

  • J1-22 ELECTROSTATIC ATTRAC AND REPULS - WIMSHURST MACHINE

    J1-22
    Demonstrates electrostatic attraction and repulsion
    Two conducting balls suspended from the two arms of the Wimshurst machine are oppositely charged and attract each other. The same two balls suspended from a single arm of the machine will repel each other.
    J1a

  • 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

  • J1-41: CONDUCTORS AND INSULATORS

    J1-41
    Demonstrate the difference between materials that conduct electricity and those that do not.
    Two electroscopes are charged with equal and opposite charges. The electroscopes are then connected by a clean rod of acrylic (left), that does not conduct, and of metal (right) that will discharge both electroscopes when it contacts both.
    J1b

    j1-41a

  • J2-03 VAN DE GRAAFF GENERATOR WITH GROUND SPHERE

    J2-03
    Demonstrates the operation of a Van de Graff generator and illustrates electrostatic concepts
    The ground sphere is positioned a few inches away from the Van de Graaff dome and grounded to the base of the Van de Graaff. When the machine is turned on, the dome becomes charged negative and the ground sphere becomes charged positive by induction. The ground sphere is attracted to the dome, as can be easily seen. After the spark, the two spheres lose their charges, and the ground sphere relaxes to its original position, whereupon the cycle repeats. See below for a paper by Dr. R. Berg on the fabrication and maintenance of the belt.
    J2a

  • J2-13 PLASMA MACHINE - EYE OF THE STORM

    J2-13
    Demonstrates electrostatic discharge
    This device is a commercial apparatus often used in magic shows or to enhance the look of a laboratory in a science fiction movie. When the machine is turned on a discharge occurs between the inner electrode and the outer glass; placing your hand on the glass draws the discharge but does not create a shock. The spark can also be controlled by ambient sounds. Prof. Dennis Papadopoulos has calculated that the operating voltage of this device is approximately 6 kV in the range of 20-40 kHz.
    J2b

  • J3-01: EXISTENCE OF ELECTRIC FIELDS

    J3-01
    Demonstrate the existence of electric fields and to map them semi-quantitatively.
    A small conducting sphere on an insulating pole is connected electrically to an electroscope. The strength of the electric potential can be mapped out by observing the deflection of the electroscope as the metal ball sensor is moved about in the region of the Van de Graaff dome. After drawing a spark the electroscope must be grounded to reset it to neutral potential.
    J3a, J3b, LS1

  • J3-02: ELECTRIC FIELD OF RING OF CHARGE - MODEL

    J3-02
    Aid in the visualization of the electric field of a ring of charge
    This is a non-working model for the electric field of a ring of charge. It shows explicitly the fields of two opposite segments of the charged ring, along with the vector sum of the two fields. This shows how two opposite sides cancel, leading to the field vector shown on the model.
    J3b

  • J3-06 ELLIPSOIDAL CONDUCTOR

    J3-06
    Demonstrates that charge distribution on a conductor depends on the surface curvature
    The ellipsoidal conductor is charged, either positive or negative, using a charged rod from J1-01. Simultaneously touch proof planes to the larger and the smaller ends of the figure, and then to the corresponding electroscope. A few transfers show that there is more charge on the smaller end than on the larger end.
    J3b, J1b

  • J3-08 VAN DE GRAAFFS - INTERACTING FIELDS

    J3-08
    Shows field lines for two identical charges
    The two Van de Graaffs are turned on briefly to charge them to the same potential. (They may be touched together to assure that the potential is the same magnitude.) The tissue streamers indicate the paths of electric field lines, which are radially outward for isolated charges. When the two domes are brought together, the inner field lines are pushed away from each other
    J3a

  • J3-11: EQUIPOTENTIALS/LINES OF FORCE - ONE CHARGE

    J3-11
    Aid in visualization of equipotentials and lines of force.
    A positive point source is located at the center of the model. The vertical height of the surface represents the potential; the white lines are equipotentials. The blue lines represent lines of force.
    J3b

  • J3-13 POTENTIAL SURFACE MODEL WITH E FIELD VECTORS

    J3-13
    Illustrates the relationship between the electrostatic potential and the electric field
    The white styrofoam surface represents a potential surface, with the closed lines encircling it being the equipotentials. The electric field vectors at various points on the surface are represented by the vectors sticking out from the surface. These vectors are quantitatively correct for this surface.
    J3b

  • J3-14 FLUX MODEL - ELECTROSTATICS

    J3-14
    Aid in visualizing flux at an angle through a surface
    Shows red flux lines and blue plane, oblique to the flux lines, represented by the blue vector area
    J3b

  • J3-21 FARADAY CAGE

    J3-21
    Demonstrates that the electric field within a closed conducting surface is zero
    When the cage is charged, the aluminum foil "pith balls" move away from the outside of the cage, indicating the presence of an electric field. However, on the inside of the cage the pith balls do not move away from the wire cage, indicating that there is no charge and therefore no electric field on the inside of the conducting surface.

    Note that this is most easily charged with the charging materials from J1-01.
    J3, J1

  • J3-22: FARADAY CAGE - ELECTROSCOPE

    J3-22
    Demonstrate that the electric field within a closed surface is zero.
    Charge the two wire strainers with triboelectric materials (J1-01), which are connected electrically to the electroscope indicator. With the two meshes separated, the electroscope deflects. When they are placed together, forming a closed sphere, the electroscope deflection is zero, indicating zero electric field within the closed cage. The charge that was originally on the electroscope has flowed to the outside of the conductor. Note that if you then re-open the two hemispheres, the electroscope again deflects as the charge flows back into it. An alternative way to use this demo is to bring the wire strainers near to a charged van de Graaff, first open, then closed. The difference in this case is that the electric field being shielded is fully external to the device, in contrast to the first method where it was an internally generated field.

    This is best used with a light source to project the shadow of the electroscope needle through the mesh onto the wall.
    J3, J1

    j3-22a

  • J3-23 FARADAY CAGE - RADIOWAVES

    J3-23
    Demonstrates that radio waves cannot penetrate a Faraday cage
    The radio is tuned to a good station so that everyone can hear. Lowering the screen Faraday cage over the radio stops the radio waves, and the sound of the radio ceases.
    K8