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PHYS411

  • J1-27: QUALITATIVE EXISTENCE OF ELECTROSTATIC FORCES

    J1-27
    Demonstrate the existence of electrostatic forces.
    The Van de Graaff is first turned on, charging the dome, and the ground sphere by induction; sparks between them indicate the existence of high voltages. The Van de Graaff is then turned off, and conducting aluminum foil cylinders hanging from strings are inserted between the dome and the ground sphere. The cylinders move back and forth between the dome and the ground sphere, demonstrating the existence of electrostatic forces.
    J1a, FS2

  • 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

  • J4-22 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

  • J5-16 MAGNETIC FIELD OF WIRE, LOOP, SOLENOID

    J5-16
    Visualize magnetic field lines for simple current configurations

    A portable power supply and switch mechanism are used to provide a brief, strong current through any of three transparency-mounted conductor configurations. Connect current leads to the conductor configuration desired; single wire, single loop of wire, and multiple-turn coil are available. While current is on, sprinkle iron filings on plastic sheet passing through the sample and gently tap the plastic sheet to make the iron filings align along the magnetic field lines.

    For safety and to preserve the lifespan of the apparatus, do not turn current on for more than a second. Please be careful not to touch leads or conductors while current is on.
    J/K

  • J5-30: OMEGA LEVITRON

    J5-30
    Show an interesting demonstration of magnetism.
    The Levitron floats in mid-air by the forces exerted by two opposed permanent magnets. Once set in motion, it's stabilized in space through the gyroscopic effect that the spinning produces, and will stay levitated for a minute or more (if you're lucky).

  • J6-04: LOW-POWER HIGH-FORCE ELECTROMAGNET

    J6-04
    Show that a small amount of energy can produce large magnetic forces
    A magnet and keeper are held together by energizing the magnet with a flashlight battery. It usually takes more than one person pulling on each side to separate the magnet and keeper.
    J6

  • J6-31: FORCE ON SOLENOID CORE

    J6-31
    Demonstrate the force exerted on the core of an electromagnet.
    When the switch is on, the force between the coil and the iron core suspends the coil and base a few centimeters off the table.
    K2, PW1

    j6-31a

  • J6-33: ELECTROMAGNETIC GUN

    J6-33
    Demonstrate the force on a solenoid core, and to contrast forces on magnetic and non-magnetic cores.
    This demonstration uses the coil from demonstration K2-22, but in an unusual configuration:

    (1) Insert iron core, displace it as far as possible toward the base, and apply current until the core reaches the center of the coil. The core will eject a few meters. (2) Insert copper core (hollow) about one inch forward of the center of the coil. Apply current to slowly eject the core. (3) Insert both cores about an inch forward of the center of the coil. Predict the results. Dramatic. Stand clear!!

    K2, PS1

  • J7-10: DIAMAGNETIC REPULSION OF WATER

    J7-10
    Demonstrate diamagnetism of water
    Water is repelled by a magnet. The effect is called diamagnetism. However, the effect is very weak. The orientation of the magnet doesn't matter; with either pole toward the water, the water is repelled. Atoms and molecules in which all of the electrons are paired with electrons of opposite spin and in which the orbital currents are zero, such as in helium, water, and bismuth, are diamagnetic. Bring a magnet toward a diamagnetic material, you will induce an electric current in the atoms of that material which make the atoms magnetic in a way that will repel the approaching magnet. (This is the same result as predicted by Lenz's law.) The glass is also diamagnetic and contributes to the repulsive effect.

  • J7-11 PARAMAGNETISM AND DIAMAGNETISM

    J7-11
    Demonstrates paramagnetic and diamagnetic materials
    A sample of copper sulfate, a paramagnetic material, is slightly attracted to a magnet. A sample of bismuth, a diamagnetic material, is slightly repelled by a magnet. Samples of copper sulfate and bismuth are balanced on a light dowel rod hanging by a strip of plastic audio recording tape. The 2-kilogauss horseshoe magnet is used to push and/or pull the samples around, illustrating the small paramagnetic and diamagnetic forces.
    J7

  • J7-12: CURIE POINT OF IRON

    J7-12
    Show the Curie point of iron.
    An iron wire is normally ferromagnetic, and therefore strongly attracted to the magnet as shown in the photograph. When heated so that it glows red hot by passing an electric current through it, the iron rises above its Curie point and loses its ferromagnetism, so springs at either end of the wire pull it away from the magnet.
    J7

    j7-12aj7-12b

  • J7-21: HYSTERESIS

    J7-21
    Demonstrate the hysteresis loop for an iron transformer core
    This demonstration shows the standard hysteresis curve with the magnetic induction B plotted on the vertical axis and the applied field H plotted on the horizontal axis. The value of H is obtained by using the voltage across a 1 ohm resistor in series with the 250 turn primary coil of the transformer. The value of B in the core of the transformer is obtained as the voltage across a capacitor in series with a resistor and the 500 turn secondary coil of the transformer. A variac adjusts the primary voltage so that the interesting parts of the hysteresis loop can be seen.

  • J7-22: MAGNETIC DOMAINS WITH TV

    J7-22
    Demonstrate the existence of magnetic domains and magnetic bubbles in a ferrimagnetic material.
    This device uses a commercial magnetic domain demonstrator, viewing it with a TV camera. The device uses polarized light to view the magnetization of domains in a ferrimagnetic garnet, which has the property of rotating the plane of polarization of the light by an angle dependent on its magnetization. Magnetic domain structure in the garnet is viewed as shown in the photograph. If the magnetic field in which the sample is positioned is increased the domain alignment becomes more complete, and the lines vanish, sometimes leaving a "magnetic bubble," a single small group of domains aligned opposite the surrounding area. Viewing is accommodated using a microscope objective lens on the end of a long tube attached to the TV camera, so that the image is created on the videcon of the camera. The analyzing polaroid, also contained in the tube, is rotated to the correct orientation for optimal viewing.

    j7-22aj7-22bj7-22cj7-22d

  • K1-03 FORCE ON CURRENT IN MAGNETIC FIELD

    K1-03
    Demonstrates force on a current-carrying wire in a magnetic field

    A wire (reinforced by a plastic tube for safety) passes between the pole tips of a strong magnet. When the key is pressed so that current flows in the wire, the wire jumps out from between the pole tips.
    Engagement Suggestion
    • Once students have seen what happens, encourage them to predict the results of reversing the direction of the flow of current. Then swap the leads and show what happens. Have them discuss the results.
    • What if you flip the magnet itself over? Again, have them predict what will happen, then try the experiment and discuss.
    Background

    This illustrates the Lorentz force, or Laplace force, as predicted by Maxwell’s equations. A current flowing through a magnetic field experiences a force determined by the cross product of the current vector and the magnetic field.


    See demonstration K1-04 in this section for a more portable version of this experiment.

    K1

  • K1-05: FORCE BETWEEN CURRENT-CARRYING COILS

    K1-05
    Demonstrate that current-carrying coils produce magnetic fields and interact like bar magnets
    Two coils are aligned with their currents moving around in the same direction, so their magnet fields will be North-to-South. They will attract each other when the current is started by pushing the switch (click here for video). Reversing the wires on the top can put any combination of magnetic poles adjacent, getting either attraction or repulsion (click here for video). A small bar magnet, included with the demonstration, can be used to see the forces between one of the coils and the magnet, again using any combination of magnetic poles (click here for video).
    K1

  • K1-07: Interacting Coils

    k1-07
    Demonstrate that current-carrying coils produce magnetic fields and interact like bar magnets
    Two coils are aligned with their currents moving around in the same direction, so their magnet fields will be North-to-South. They will attract each other when the current is started by pushing the switch. Flipping the polarity switch will reverse the direction of the current in one of the coils, this will cause the magnetic field to align North-to-North, causing the coils to repel.

    Invite students to predict how the interaction will change when you change the polarity. Or, for advanced students, approach it from the other direction: Before powering it on, only tell them which direction the current flows, and invite them to predict the direction of motion. (We recommend testing this in private first to make sure your own prediction is correct.)
    K2, PS1

    coil set with power supply

  • K2-02 INDUCTION IN A SINGLE WIRE

    K2-02
    Demonstrates magnetic induction
    A single wire is connected to a projection galvenometer. Passing the wire quickly between the pole tips of a strong permanent magnet induces electric current, which is seen on the meter.
    K2, K1

  • K2-03 FARADAY'S EXPERIMENT ON INDUCTION

    K2-03
    Demonstrates the induction between two coils
    A primary coil is connected to a battery by a key switch, so that closing the switch causes current to flow in the coil and releasing the switch stops the current. Three secondary coils are connected in series with a galvenometer. The primary coil is positioned inside the secondary coil and the current in the primary turned on and off. When the current in the primary coil is turned on, a sharp spike of current appears in the secondary coil. There is no secondary current while the current in the primary remains on at a constant level. When the key is released the current in the primary coil ceases, creating a sharp current spike in the secondary coil of opposite sign to that produced when the primary current is started. The induced current is greater for a secondary coil with more turns. The experiment can be repeated with copper, aluminum, and iron cores. This uses the same coil and meter setup as K2-04; consider using them together to compare permanent magnets and electromagnetic coils.
    K2

  • K2-04 FARADAY'S EXPERIMENT - EME SET - 20, 40, 80 TURN COILS

    K2-04
    Shows that the induced current is proportional to the number of turns in the secondary coil
    Three coils are connected in series with a projection galvanometer on an overhead projector. A bar magnet is thrust through one of the coils, inducing current in the coil which is shown on the meter. Three coils are included on the device: 20, 40, and 80 turns; the bigger the coil the greater the induced current.

    Have students try to predict the relationship between coil size and current strength before performing the experiment.
    K2, J5a