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Magnetic Fields and Forces

  • 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).

  • J5-31: QUADRUPOLE MAGNETIC FILED

    J5-31
    Visualize the magnetic field of a quadrupole magnet.
    Two large horseshoe magnets are arranged with their poles N to S and S to N, forming a "quadrupole" as shown in the photograph at the right. Quadrupole magnets are used in focusing of charged particle beams, where the beam moves through the field perpendicular to the plane of the magnets, as shown in the photograph. A single quadrupole focuses in one direction and defocuses in the other direction, so quadrupole magnets are generally used in pairs as beam focusing magnets.

    j5-31a

  • J5-32: MAGNETIC LEVITATION - PERMANENT MAGNET AND ELECTROMAGNET

    J5-32
    Demonstrate magnetic levitation.
    A ring magnet levitates above an electromagnet, held in place horizontally by a plastic tube. Turn the power supply on and off for controlled levitation.

  • J5-33 TORQUE ON A BAR MAGNET

    J5-33
    Demonstrates the torque on a bar magnet in a magnetic field
    A small bar magnet is suspended on a rotating pivot between the poles of a horseshoe magnet. If the bar magnet is turned and released it rotates back to its original position due to the torque applied by the magnetic field of the horseshoe magnet.
    J5b

  • J5-35: MAGNETS INTERACTING ON PIVOTS

    J5-35
    Show the interaction between two bar magnets
    Start the larger bar magnet rotating to watch the reaction of the lighter magnet.
    J5b

  • J5-36: MAGNETIC SPINNER

    J5-36
    Demonstrate magnetic levitation.
    Axle with two magnets mounted on it levitates above a magnetic base as it spins.

  • J5-41: FLUX MODEL - MAGNETOSTATIC

    J5-41
    Illustrate flux through plane at an angle.
    Magnetic field lines are red vectors. The area of the blue surface, oblique to the magnetic field, is represented by the blue vector. The model is useful in viewing the geometry of the flux through the surface, which is the dot product of the field vector B and the area vector dA: FLUX = B dA cos a, where a is the angle between the field and the area vectors.

  • J5-51: GAUSSMETER

    J5-51
    Demonstrate use of a gaussmeter
    This is a Hall probe gaussmeter of research quality. It can measure fields from a fraction of a gauss to over 10 kilogauss (1 Tesla). Radial (the flat one) and axial (the cylindrical one) probes are included. Ask for magnets (Demonstration J5-04: MAGNETS) and measure their fields with a gaussmeter.
    J5, K1

  • J6-01: ELECTROMAGNET WITH BANG

    J6-01
    Illustrate the force which can be exerted by a small electromagnet.
    An electromagnet operated by a single flashlight battery holds up an iron brick as in the photograph. Flip the switch off to allow brick to fall, awakening your students.
    J6

  • J6-02: ELECTROMAGNET WITH JUNK

    J6-02
    Demonstrate an electromagnet.
    Close switch to pick up junk with coil. Release switch to let junk fall. Some light items may cling to the magnet after the switch is released due to residual magnetism of the core.
    J6, PS1

  • J6-03: ELECTROMAGNET WITH JUNK - WITHOUT CORE

    J6-03
    Demonstrate electromagnetism.
    This shows the functioning of J6-02 without the iron core. The absence of a core makes the field weaker, but the physics a bit simpler.
    J6, PS1

  • 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

  • J6-21: MAGNETIC FIELD IN GAP OF ELECTROMAGNET

    J6-21
    Semi-quantitatively measure the magnetic field in the air gap of a magnet.
    Using a magnet with a square yoke, separate the top yoke member, creating two gaps in the yoke. Into one gap place a small search coil; into the other place a non-magnetic shim. As the gap length h is increased the field B(gap) decreases:

    B(gap) = mu(0) N i / h, where N is the number of turns and i is the current in the coil.

    j6-21aj6-21bj6-21cj6-21d

  • 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

  • J6-32: FORCE ON SOLENOID CORE - SMALL

    J6-32
    Demonstrate the force exerted on the core of an electromagnet.
    Turn on magnet and feel the force with which the coil is pulling the core into the center of the solenoid. Turn on magnet and feel the force with which the coil is pulling the core into the center of the solenoid.

  • 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.

  • J7-23: MAGNETIC DOMAINS - MODEL

    J7-23
    Model the behavior of a magnetic material in a magnetic field.
    The array of compasses, representing magnetic domains in a sample of magnetic material, is positioned in the center of the Helmholtz coils on the overhead projector. Using a small bar magnet the compasses are de-aligned into a state of relative disorder. As the field of the Helmholtz coils is increased, more of the small magnets flip into alignment with the applied field. This is a model of the domains flipping into alignment in a sample of magnetic material as the magnetic field is increased. This effect, known as the Barkhausen effect, is shown directly in Demonstration J7-24: BARKHAUSEN EFFECT.
    OS9, J5, PS1

    j7-23aj7-23bj7-23c

  • K1-02: FORCE BETWEEN CURRENT-CARRYING WIRES - PROJECTION

    K1-02
    Demonstrates the force between two adjacent parallel current-carrying wires
    A pair of wires are mounted in an overhead projectual, as seen without any current flowing in the photograph above. The projectual can be wired to carry parallel currents or antiparallel currents. Connect power supply to the desired wires, one set at a time. With the red and black connectors (at right in the photo above) hooked up in series, the current will be antiparallel. With a shunt joining those two connectors, the two wires can be run in parallel, with one cable connected to each end.

    Students should clearly see that parallel currents attract and antiparallel currents repel.

    Note that it is very important not to run the power for more than a second, less if possible! The device can easily overheat and be damaged.
    J/K

  • 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