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Capacitance

  • 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

  • J4-01 PARALLEL PLATE CAPACITOR

    J4-01
    Demonstrates that potential difference across a capacitor is proportional to the plate separation

    This simple parallel plate capacitor consists of two large aluminum plates with an air gap. The parallel plate capacitor is charged to 1000 Volts using a low-current DC power supply by pressing a switch. The plates may then be separated and the voltage observed using the electrometer, demonstrating that the voltage is proportional to the plate separation.
    Engagement Suggestion
    • You can show that the voltage across the capacitor varies with the spacing if the charge is held constant (i.e. the power supply is not connected), or you can show how the capacitance varies with the spacing if the power supply remains connected. Note that this remains linear only within a limited distance regime.
    J/K

  • J4-03: PARALLEL PLATE CAPACITOR - SERIES CAPACITORS

    J4-03
    Demonstrate the effect of capacitors in series.
    The parallel plate capacitor is charged using a low-current DC power supply and separated as shown. A thin metal sheet is then inserted between the two capacitor plates, forming two capacitors in series. The voltage read by the electrometer remains virtually the same, indicating that the capacitance of the series capacitors is the same:

    C = C1 C2 / (C1 + C2),

    where either capacitance C is inversely proportional to the distance between the plates.
    J/K

    j4-03a

  • 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

  • J4-21: COLOR FILTER MODEL OF CHARGED DIELECTRIC

    J4-21
    Represent charge separation in a capacitor dielectric.
    Two color filters, representing the positive and negative charges in a dielectric material, are positioned on an overhead projector. When unpolarized they overlap completely. Separating the transparencies slightly represents the separation of charge in the dielectric which occurs when an electric field is applied across a dielectric sheet, for example, in a capacitor. Use two negative color filters, such as yellow and cyan (as photographed), or magenta, so that the mixture (green) is easily visible.
    J4

    j4-21a

  • J4-22 PARALLEL PLATE CAPACITOR WITH DIELECTRIC

    J4-22
    Demonstrates that inserting a dielectric into a capacitor increases the capacitance
    The parallel plate capacitor is charged by the power supply and the plates are separated, increasing the voltage between the plates. A thick dielectric sheet inserted between the plates of the capacitor results in a decrease in the voltage between the plates. Because the charge on the plates remained constant, this means that insertion of the dielectric has increased the capacitance. This allows more charge to be stored by the capacitor at the same voltage.
    J/K

  • J4-23: POLARIZATION OF DIELECTRIC - DISSECTIBLE LEYDEN JAR

    J4-23
    Demonstrate that energy is stored in a capacitor as charge separation in the dielectric.
    A Leyden jar is charged by a Wimshurst machine. It can then be quickly disassembled as follows: (1) remove the charging leads without discharging the capacitor, (2) using an insulated hook, remove the center conductor, (3) remove the glass jar by hand without touching the outer conductor. You may then feel free to handle all parts individually before re-assembly. To re-assemble: (1) place glass jar in outer conductor without touching outer conductor, (2) using insulated hook, insert center conductor. Finally, short out the inner and outer conductors to obtain a large crack. This experiment is generally used to demonstrate that the charge is stored in the glass dielectric, not on the surfaces of the inner or outer conductors. In fact, the energy is stored as polarization charge. However, there is some controversy regarding this explanation, that can be seen in the reference by Zeleny (https://doi.org/10.1119/1.1990632).
    J4, J1a

    j4-23a

  • J4-24: FORCE ON DIELECTRIC IN ELECTRIC FIELD

    J4-24
    Demonstrate the force on a dielectric in an electric field.
    Two wire electrodes extend into a bath of dielectric oil, as seen in the photograph above. When a potential of 2500 volts is impressed between the wires a force is exerted on the dielectric in that region, causing the oil to rise between the two wires, as seen in the close-up photographs below.

    j4-24aj4-24b

  • J4-31 ENERGY STORED IN A CAPACITOR

    J4-31
    Demonstrates that energy is stored in a capacitor and how that energy may be used
    Three capacitors totaling 205,000 microfarads are charged to 15 volts. Closing the three switches one at a time (a) turn on the light, which decays exponentially, (b) turn on the bell, or (c) activate a motor which lifts the entire system up a few inches
    J4

  • J4-32 DISCHARGE OF CAPACITOR WITH BANG

    J4-32
    Demonstrates that capacitors store electrical energy
    A 3500 microfarad capacitor is charged to 100 volts using the battery pack. Touch the capacitor terminals to the copper contacts on the battery pack; check that the polarity is correct, this is an electrolytic capacitor. Discharging the capacitor with the large screwdriver produces a very loud BANG.
    J4

  • J4-41: CAPACITORS

    J4-41
    Display a variety of capacitors
    Just a bunch of different kinds of capacitors. Just lying there.
    J4

  • J4-42: CAPACITORS IN SERIES AND PARALLEL

    J4-42
    Demonstrate the voltage/charge/capacitance relationships in series and parallel capacitor circuits.
    A commercial bank of four identical capacitors can be connected in various series and/or parallel combinations and charged using an attached battery. The digital voltmeter is used to probe the voltage on any individual or group of capacitors and displayed for the class using a TV camera and video projector in the lecture halls (monitor in smaller classrooms).
    J4, ME2

    j4-42a

  • J4-43: CAPACITORS IN SERIES AND PARALLEL WITH PROJECTION METER

    J4-43
    Illustrate voltage relations in series and parallel capacitor circuits.
    Two capacitors can be connected in series or in parallel and charged with the battery, or they can be charged individually and then connected. The projection meter is used to read the voltage across either or both capacitors. BE CAREFUL: this uses large capacitors charged to 50 volts!
    J4

    j4-43a

  • J4-51: THEREMIN

    J4-51
    Demonstrate the theremin
    A theremin is a musical instrument, invented in the early twentieth century by Russian scientist Dr. Theremin, which uses capacitance to change the pitch and the loudness of the sound. It was popular in dance bands in the first half of the twentieth century, and even used by The Beach Boys in the 1960s. By moving your hands up and down over the triangular capacitor plates on the top of the box, the frequency and loudness of the sound can be varied to produce a musical tune. Perhaps one of the most elegant examples of theremin music is the Rachmaninoff "Vocalise" performed by Clara Rockmore, the most well-known theremin artist ever, with Nadia Reisenberg on the piano. This music is on a CD, The Art of the Theremin, which will be found in our library of CDs in the "MUSIC" section of the demonstration storage.
    J4, ME3

  • K4-06: MAGNETOELECTRIC GENERATOR WITH CAPACITOR

    K4-06
    Demonstrate that the generator is producing electrical energy, and that the capacitor stores electrical energy; also that a generator can run in reverse as a motor.
    The capacitor is charged up by cranking the generator. The generator is then run as a motor by energy stored in the capacitor.

    Ask your students the following brainteaser question: If you charge the capacitor by cranking the generator, what will happen when you stop cranking and release the handle of the generator? (a) It will continue to rotate in the same direction, (b) It will rotate in the opposite direction, (c) It will remain at rest.

    For discussion: Have students decide for themselves what form of energy is being stored here. Rotational energy? Electrical? Magnetic?

    K4

  • K7-15: CURRENT IN RC CIRCUIT?

    K7-15
    Demonstrate charging of a capacitor in a possibly counterintuitive way.

    A switch is closed to charge a 1 farad (yes, one FARAD) capacitor with a 3 volt battery (actually two 1.5 volt batteries in series). The capacitor has 1.5 volt light bulbs on each side of it in the circuit, as shown in the circuit drawing above.

    Consider the following series of questions:

    Q: What will happen when the battery is connected (switch turned to left position):

    (a) both bulbs will light and stay lit,

    (b) both bulbs will go on momentarily,

    (c) only one bulb will light and stay lit (if so, which one?),

    (d) only one bulb will go on momentarily (if so, which one?),

    (e) neither bulb will go on at all,

    (f) something else will happen (if so, what?).

    A: (b), Both bulbs will go on momentarily as the capacitor charges, then they will fade out. Click below for video

    Q: What will happen when the switch is opened (center switch position)?

    A: Nothing: the capacitor remains charged and no current flows.

    Q: What will happen when the switch is closed to the right, completing the circuit including the capacitor and the two light bulbs?

    A: The capacitor will discharge through the bulbs, turning them on momentarily while current is flowing, with the intensity decreasing as the current falls to zero. Click below for video.
    K7