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Electric Fields and Potential

  • A2-16: SADDLE POINT MODEL

    A2-16
    Illustrate the geometry of a saddle point

    Large model shows equipotentials and lines of force. Small model shows force vectors and the "neutral point."

    This demonstration can also be useful in gravitation and electrostatics, or in other areas to discuss potential surfaces.

    Please handle with care, as these models are old and fragile.
    A2

  • A2-17: CLOSED CURVES AND CAPPING SURFACES

    A2-17
    Illustrate various complicated closed curves

    Distorted pasta strainers illustrate possible choices for closed surfaces.

    These three-dimensional models can be used. if so desired, to illustrate some variety in choice of closed surfaces when referring to Gauss' Law or Stokes' Theorem.

    Invite students to handle them and experiment with different distortions of the form; show that the surface area remains (approximately) invariant.
    A2

  • J1-26 VAN DE GRAAFF - REPULSION OF PIE PANS

    J1-26
    Demonstrates electrostatic repulsion

    A group of aluminum pie pans is placed on top of the Van de Graaff dome and the Van de Graaff is turned on. The pie pans are pushed off the top of the dome one at a time by the electrostatic repulsion. Use this as a way to argue that electrostatic forces might be stronger than gravitational forces.

    Engagement Suggestion:
    • Before turning the generator on, encourage students to predict what is going to happen. Challenge them to explain their hypotheses in terms of what they have learned about the behaviour of electrical charge.
    • Feel free to invite students to collect the scattered pans, but remind them not to get close to the Van de Graaff while it is turned on.
    J1a

  • 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

  • J2-14 LIGHTNING ROD SIMULATOR

    J2-14
    Demonstrates how lightning rods really work
    A ground sphere is positioned adjacent to the Van de Graaff generator so that the ground sphere is charged by induction and creates large sparks. While this system is working, a grounded point is aimed at the Van de Graaff dome from a distance of several times the distance between the dome and the ground sphere. The grounded point discharges the dome at a much lower potential, preventing buildup of charge on the ground sphere and the concomitant spark discharge.
    J2a

  • J2-16: ELECTRODELESS DISCHARGE

    J2-16
    Demonstrate electrostatic discharge.
    A small fluorescent tube is held near the high-voltage electrode of the Tesla coil, lighting the tube.
    OS1

  • 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

  • J3-04: ELECTRIC FIELD LINES - SOAP BUBBLES

    J3-04
    Show the shape of electric field lines for a large dipole by observing soap bubbles move along the lines.
    A Wimshurst machine produces a high potential difference, one side of which is connected to a conducting sphere and the other side connected to a person blowing bubbles, forming an electric dipole. The bubbles attain the charge of the person blowing them and follow electric lines of force to the sphere, which has the opposite charge. A few bubbles will actually touch the sphere, pick up the charge on the sphere and return to the bubble blower. Because of the volume and high ceiling required to do this experiment, it works best in the lecture halls.
    J1a

    j3-04a

  • J3-05: VAN DE GRAAFF - INDUCTION WITH SPHERES AND NEON BULB

    J3-05
    Demonstrate the existence of electric fields and to identify the polarity of the charge on a sphere.
    A neon lamp is mounted on a long plastic pole with one side of the lamp grounded. The neon lamp lights when held in the vicinity of the Van de Graaff dome, even without direct contact, indicating the existence of an electric field. The side of the neon bulb which lights is the negative side, so the direction of the electric field can be determined by noting which side of the neon bulb is lighted, if you know how the internal connections of the neon bulb are made. The magnitude of the glow on the neon bulb is proportional to the strength of the field.
    J3a

  • 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

  • J3-07: VAN DE GRAAFF - DISCHARGE TO VARIOUS RADII

    J3-07
    Demonstrate that for a charged conductor a smaller radius produces a higher electric field.
    The Van de Graaff discharges to the ground sphere at the left in the absence of the smaller elliptical conductor (from J3-06) at the right. When the grounded elliptical conductor is positioned near the Van de Graaff dome, the end with the smaller radius of curvature discharges the dome with less of a spark, indicating a smaller electric field. Discharge to the larger end is more similar to the ground sphere.
    J3a, J3b

  • 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

  • J4-04 PARALLEL PLATE CAPACITOR - IONIZATION OF AIR

    J4-04
    Demonstrates the mobility of ions
    Charge the capacitor and separate the plates. Bring a lighted match under the volume between the two plates. The ionization of the flame creates free positive and negative charges which migrate to the capacitor plates, quickly discharging the plates.
    J/K