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Electromagnetic Induction

  • K2-45: EDDY CURRENTS - MAGNET AND SOFT DRINK CAN

    K2-45
    Demonstrate eddy currents and Lenz's law.
    A soda can is suspended by a nylon filament over a horseshoe magnet that can rotate on a plastic disc. When the magnet is rotated, the force on the can due to Lenz's law causes the can to rotate in the same direction as the magnet.

  • K2-46: MAGNET LEVITATION ABOVE SPINNING DISC

    K2-46
    Demonstrate Lenz's law in a dramatic way.
    A small rare-earth magnet is glued onto a short plastic strip, and held above a rapidly spinning aluminum disc. Eddy currents in the disc create a magnetic field which levitates the magnet. Let the magnet hang loosely on the disc at rest, then start the rotator.

    Note: When using, be careful to avoid the center of the spinning disc. The axle is steel, which causes a very different result in a magnetic field!

  • K2-47: SPEEDOMETER MODEL

    K2-47
    Demonstrate operating principle of some speedometers.
    Two conductive discs are placed parallel and coaxial to each other, one mounted to a manual roator, the other suspended from a wire. The lower disc, on which a magnet is mounted, rotates when the handle is cranked. Induction causes the upper disc to rotate through an angle which is dependent on the torsional constant of the wire support and the rotational speed of the lower disc. The upper disc is damped by a magnet held in place on a tuning fork. The lower photographs above show details of the assembly. This idea is used in some auto speedometers, where the wheels are attached to the lower disc and the speedometer indicator is attached to the upper disc.
    K2, K1, D1

  • K2-48: EDDY CURRENT MOTOR`

    K2-48
    Show the use of eddy currents in producing a motor with few moving parts and no electrical brushes.
    An aluminum soda can is mounted on a rotating bearing, with the sawn-off cores of transformers mounted adjacent to the center of the can. When 110 VAC power is connected to the transformers, eddy currents cause the can to rotate.
    K2

  • K2-49: MAGNETIC STIRRER

    K2-49
    Illustrate how a magnetic stirrer works.
    A magnetic stirrer has a small rotating magnet under the top plate. When the magnet rotates, another magnet placed in the material to be stirred rotates and stirs that material. The outer case has been removed so that the mechanism can be seen. An aluminum plate positioned on top of the device will rotate due to eddy currents.
    K2

  • K2-61 THOMSON'S COIL

    K2-61
    Demonstrates a number of concepts in magnetic induction
    A large vertical induction coil with a fixed iron core rests on a power supply base. The coil can be activated by a momentary switch, and a variety of induction effects can be shown.

    Some demonstrations that can be performed with this apparatus: (1) JUMPING RINGS: Placing a ring over the extended primary coil core and switching it on causes the ring to jump. A smaller ring will jump higher. Cool the ring in liquid nitrogen to get a really great jump, but be careful about hitting the rear projection screen. Broken metal rings and wooden rings are unaffected. (2) RESISTIVE HEATING: Verify that there is resistive heating in the secondary ring by having a student hold it down until it gets too hot to touch! (3) A light bulb on a small coil lights up when the coil is moved over the extended core. (4) A secondary coil with small light bulb placed in a beaker on top of the secondary coil will remain lit when it is covered by water in the beaker.

    To understand the force on the jumping ring one must account for its self-inductance, which causes an extra phase lag of the induced current. The AC current in the coil produces an alternating magnetic field, which induces an alternating current in the ring. The ring thus experiences an alternating vertical magnetic force, due to the radial component of the magnetic field. (One can also think of this as a force between the two currents, repulsive when they are parallel and attractive when they are opposite.) Without self-inductance of the ring, the induced current would lag the magnetic field by a quarter cycle, and the time averaged vertical force would vanish. The self-inductance causes an additional phase lag, hence a repulsive average force. See Jeffery & Amiri, "The Phase Shift in the Jumping Ring," TPT 46, 250(2008), for a detailed explanation.

    An interesting historical note: This device is named for its inventor, electrical engineer Elihu Thomson, not for his better known contemporary J. J. Thomson, whose work with CRTs led to the discovery of the electron.

    Water, liquid nitrogen for cooling rings, and related accessories can be available upon request.

    Thanks to Prof. Ted Jacobson for assistance with this explanation.
    K2

  • K2-62 CAN SMASHER - ELECTROMAGNETIC

    K2-62
    Blasts a soda can into two pieces using electromagnetism

    A 400 microfarad capacitor is charged to 3000 volts (1.8 kilojoules) and discharged through a three-turn coil into which an aluminum soft drink can has been positioned. With the circular windows open, the two pieces of the can will be blasted over thirty feet to the sides of the large lecture hall. Charging the capacitor to less voltage results in a can with a "waist."

    This device can be explained in two distinct ways:
    (1) The rapidly rising current creates a rapidly rising magnetic field along the axis of the coil, which in turn induces an electric field going in circles inside the coil. The induced electric field causes an electron current in the can which experiences a vxB force in the magnetic field of the coil, causing the can to break into two pieces which are blown to the opposite sides of the lecture hall.
    (2) A type of "theta pinch" phenomenon. More information on this is available from Wikipedia. Another way to understand this is that the induced current around the can is opposite to the current in the primary coil, since it is opposing the change in flux. These concentric opposite currents repel each other, so the can is pinched and torn apart and ejected out the sides.

    This is an UNFORGETTABLE DEMONSTRATION. A must when you cover electromagnetism.

    This video, from the Video Encyclopedia of Physics Demonstrations, shows the operation of the can crusher with an animation illustrating (1) the electron current in the coil, (2) the vector magnetic field that it creates, (3) the induced electric field within the coil created as the coil current rapidly rises, (4) the electron current circling in the can created by that induced electric field, (5) and the vxB force on the electrons moving around the can.

    Following a description of the crusher electronic components, the animation is displayed. The animation may be stopped so that the directions can be studied in detail for the five (5) quantities listed above. Using the left hand rule (for electrons) the directions can be verified; note that according to Lenz's law the direction of the electron current induced in the can must be in the opposite direction to the electron current in the coil.

    Note that the magnetic field at either end of the coil possesses both an axial and a radial component; the electron current in the can is entirely azimuthal. Using the left hand rule to determine the direction of the cross product of the electron velocity and the magnetic field, it can be seen that the axial component of the magnetic field leads to an inward force, crushing the can, while the radial field component leads to an axial force, away from the plane of the coil at both ends of the can, causing the two parts of the can to move rapidly away from the coil. (In the large lecture hall the two parts of the can will be blown to the sides of the lecture hall.)

    The web site http://hibp.ecse.rpi.edu/Can_Crusher/home.html contains a drawing and animation showing how the RPI electromagnetic can crusher works.

    FS1

  • K2-63: DISPLACEMENT CURRENT MODEL

    K2-63
    Illustrate the geometry for displacement current

    This is a model of the classic displacement current experiment described in general physics textbooks. Displacement current is sensed as oscillating magnetic field between the plates of the capacitor. The oscillator is set to about 15 kHz, and tuned to give the maximum displacement current. Evidence of the displacement current is the existence of an azimuthal magnetic field between the capacitor plates. This is sensed by observing the EMF induced by inserting the search coil (from K2-27) radially into the capacitor with the coil oriented vertically (photo at left). Holding the search coil in the capacitor parallel to the plates should produce considerably less pickup

    This device can be used to demonstrate in three dimensions the geometry of the displacement current experiment. There is some discussion whether the actual pickup displayed is due to displacement current or simply some sort of general electromagnetic pickup, that is obviously filling the area.
    K2, ME2, ME3

  • K2-64: UNIPOLAR GENERATOR

    K2-64
    Demonstrate unipolar generation of DC voltage, which may involve an explanation other than electromagnetic induction.
    A strong (over 10 kilogauss) cylindrical magnet is rotated about its axis at 1725 RPM. Brushes positioned on the axis of rotation and the "equator" of the bar magnet (midway between the two poles) are attached to a digital voltmeter. An electrical potential of about 15 millivolts is measured. Reversing the direction of rotation or reversing the ends of the magnet causes the DC voltage to reverse in sign. An aluminum bar can be substituted for the magnetic one and rotated in the CW or CCW directions. Note that there is a small (about 0.1-0.3 mV) potential developed with this arrangement, probably due to contact potentials between the various materials in the system and the wires, similar to the potential developed in a thermocouple. The observed potential is the same for either rotational direction of the aluminum rod; it would likely be opposite in sign for opposite rotation direction if it were due to some sort of induction effect. The explanation of this device is perhaps problematic. Many people believe that because there is no change in flux in the wire loop this cannot be an electromagnetic induction effect; the only explanation lies in special relativity. Other theoreticians disagree.
    K2, ME2

  • K2-65: INDUCTION AND CAPACITOR FLASHLIGHT

    K2-65
    Demonstrate a weird but high-tech method for constructing a flashlight.
    This flashlight has a low voltage LED lamp but uses no battery. Instead, the power is generated by moving the flashlight back and forth along its length, causing a strong rare-earth magnet to pass back and forth through a coil. A low voltage AC current is created by this action. This current then passes through a bridge rectifier and charges a high-tech capacitor, which is used to power the light.
    K2

  • K3-01: INDUCTION IN A TRANSFORMER

    K3-01
    Demonstrate that a change in the primary current induces a secondary voltage.
    A simple transformer is built from two demountable coils and an iron core. The 46-turn primary coil of the transformer is connected through a variable resistor to a 1.5 volt battery, and a large galvanometer or other measuring device is connected directly across the 500-turn secondary, as shown in the circuit diagram above. The primary current in this simple transformer circuit is changed by sliding the variable resistor contact, inducing a secondary potential which is indicated by the galvanometer. Motion of the resistor slider and current in the galvanometer are correlated.
    K3, ME2, PS1

  • K3-02: DEMOUNTABLE TRANSFORMER - SINGLE SPARKS

    K3-02
    Demonstrate induction of large voltages by a large secondary-to-primary turn ratio.
    A six-volt primary source is switched on and off to a five-turn transformer primary coil. The 23,000-turn secondary produces enough voltage to cause a significant arc between wires connected to the transformer secondary

  • K3-03: DEMOUNTABLE TRANSFORMER - V VS N - PROJECTION METER

    K3-03
    Demonstrate the effect of changing the number of turns in the secondary of a transformer.
    A "Variac" variable transformer steps down 110VAC into the primary coil of the demountable transformer. Use the variac to put 75VAC at 60 Hz onto the 250-turn primary coil of the transformer. The secondary coil is created by looping a wire around the transformer core and connecting it to the AC voltmeter. The voltage produced is approximately proportional to the number of turns in the secondary coil; the photographs above show the cases of 4 turns (top) and 8 turns (bottom) in the secondary coil. Be sure all wires are connected before adding power, and do not disconnect or reconfigure before cutting power.
    K3, ME2, PS1

  • K3-04: DEMOUNTABLE TRANSFORMER - V VS N - OSCILLOSCOPE

    K3-04
    Demonstrate the role of turns in the secondary of a transformer
    A "Variac" variable transformer steps down 110VAC into the 250-turn primary coil of the demountable transformer. Use the variac to put 50VAC at 60 Hz onto the primary coil of the transformer. The secondary coil is created by looping a wire around the transformer core and connecting it to the oscilloscope. The voltage produced is approximately proportional to the number of turns in the secondary coil. Be sure all wires are connected before adding power, and do not disconnect or reconfigure before cutting power.

  • K3-05: DEMOUNTABLE TRANSFORMER - WELDER

    K3-05
    Show that very large currents can be produced in the secondary of a step-down transformer.
    A 500-turn primary coil operated at 140 VAC and 5-turn secondary coil form a transformer (using a demountable iron core). This is used to produce large secondary current. Holding two nails together tip-to-tip across the secondary produces over 100 amps in the secondary (stepped up from 4-5 amps in the primary), welding the two nails together.
    K3

  • K3-06: TRANSFORMER - PRIMARY CURRENT VS LOAD

    K3-06
    Counterintuitive demonstration of how the primary current in a transformer increases when the secondary load increases.
    A 15-watt light bulb in series with the 500-turn primary of a transformer is used as the "sensor" of primary current. Switching on a small bulb across the 46-turn secondary makes the primary sensor bulb glow more brightly. Before switching on the secondary bulb, ask your students what will happen to the bulb in the primary circuit. The picture at the right shows the secondary circuit switched on (switch at lower left) so the small secondary bulb is glowing and the primary bulb glowing more brightly. Ask your students what will happen to the primary sensor bulb when the secondary bulb is removed. After re-connecting, ask your students what will happen to the secondary bulb when the primary bulb is removed.

  • K4-01: AC/DC GENERATOR

    K4-01
    Demonstrate a simple AC/DC generator.
    The generator consists of a coil which rotates through an appropriately shaped magnet, which is powered by a 1.5 volt battery. Turning the crank generates either an AC voltage, using a double commutator, or DC voltage, using a split ring commutator. The commutator must be set manually for each case. The output is displayed by the meter. Shown in the photographs above are connections for AC (left) and DC (right) commutators. The button adjacent to the batteries must be held down to activate current in the magnet coil.
    K4, ME2

  • K4-02 MAGNETOELECTRIC GENERATOR WITH LAMP

    K4-02
    Demonstrates a small 110 VAC magnetoelectric generator
    A small magneto-electric generator is connected across a simple incandescent light bulb. Crank the handle to light the bulb.
    K4

  • K4-03: MAGNETOELECTRIC GENERATOR WITH BELL

    K4-03
    Demonstrate a simple hand-cranked generator.
    A simple generator is made up of a rotating coil arrangement inside a set of horseshoe magnets. This is connected to a traditional electromechanical alarm bell. Turn the crank to ring the bell.
    K4

  • K4-04: MAGNETOELECTRIC GENERATOR WITH AC INDICATOR BULB

    K4-04
    Show explicitly a generator producing AC output.
    A neon discharge lamp is used to indicate the polarity of the output of a hand-cranked generator. As the generator is turned, alternate sides of the neon bulb flash, indicating that the electrons are coming from alternating poles of the generator.
    K4