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PHYS272

  • 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

  • K2-12: SELF-INDUCTION - DEMOUNTABLE TRANSFORMER

    K2-12
    Demonstrate back-EMF in an inductor.
    A 1000-turn transformer coil and a neon bulb connected in parallel are connected in series with a 7.5 volt battery and a key switch, as shown in the circuit below. When the switch is closed current flows in the inductor. When the key is released the field of the inductor collapses, inducing an EMF sufficient to light the neon bulb.
    K2, K3, PS1

  • K2-21: RUHMKORFF INDUCTION COIL

    K2-21
    Demonstrate induction of a very high voltage using a small voltage source.
    The Ruhmkorff coil is a classical transformer that uses a vibrating interrupter mechanism to create high-voltage pulses from a low-voltage direct current. They were widely used in industry and research in the late 19th and early 20th centuries, but are now largely used for educational purposes. Interestingly, automotive spark plugs are a descendant of this technology.

    In this experiment, a 7.5 volt battery is connected to the input of a high-voltage Ruhmkorff induction coil. The induced voltage will produce a 1"-2" spark. Note: Please be careful not to touch the electrodes until after the coil is fully discharged.

  • K2-25: MUTUAL INDUCTION - DEMOUNTABLE TRANSFORMER

    K2-25
    Demonstrate mutual induction and to show the effect of various core materials.
    Two matching 500-turn coils from the demountable transformer set are positioned adjacent to each other. Switching the current in the primary on and off induces short spikes of current in the secondary. Rods of various core materials are available; experiment with the class to determine their effect on the induced current. While holding down the key so that current is flowing in the primary coil, insert and remove various cores. Have the class make predictions as to the effect of different materials.
    K3, PS1, ME2

  • K2-28: DEMOUNTABLE TRANSFORMER - 10 KV ARC

    K2-28
    Demonstrate that a large voltage increase is attainable using a transformer.
    The 250-turn primary coil is plugged into 110 VAC. The turn ratio of nearly 100 produces a large secondary voltage which arcs across electrodes placed on the output of the 23,000-turn secondary coil.
    K3, PS1

  • K2-41: LENZ'S LAW - ROLLING RODS

    K2-41
    Demonstrate eddy currents and Lenz's law
    Various rods and tubes, metallic or non-metallic, magnetic or non-magnetic, are rolled down a wooden ramp into the magnetic field of a strong horseshoe magnet. Wooden rollers are not affected by the magnetic field. Magnetic rollers (such as a steel tube) are immediately sucked into the magnet gap when they get close enough to the pole tips. When non-magnetic metal rollers roll into the magnetic field, eddy currents are generated in the rollers which oppose the motion. The roller will slow down dramatically as it enters the field, but eventually pass through.
    K2

  • K2-42 LENZ'S LAW - MAGNET IN ALUMINUM TUBE

    K2-42
    Demonstrates Lenz's law

    Two arrays of magnets, containing five strong disc magnets each with small aluminum spacers between the magnets, are dropped through a vertical aluminum tube. One set, having its poles North-to-South, has very little external field, and falls very quickly through the tube. The other set, having its poles arranged North-to-North, then South-to South, etc., has a large external field. A solid aluminum bar of the same size is also available for comparison.

    Background

    As the magnetic array falls, it induces large currents in the aluminum tube. According to Lenz's law, these currents interact with the falling magnet array so as to oppose its (falling) motion, and the array takes several seconds to fall about two meters through the tube. By comparison, the aluminum rod falls much more quickly. For advanced students, compare the two different magnetic arrays, to show the relationship between the amount of slowing and the changing flux. For the simpler form of the demonstration, just use the aluminum rod and the North-North South-South array (marked with a red dot) to maximize the difference.

    Optionally, a smaller portable handheld of this demonstration is available upon request, suitable for small groups.
    FS2

  • K2-44 EDDY CURRENT PENDULUM

    K2-44
    Shows the damping of pendula due to eddy currents

    Pendula with bobs of different materials and geometries are swung through the poles of a strong horseshoe magnet. The amount of damping is greater for those bobs in which strong eddy currents can flow. Bobs include, solid copper, copper loop, broken copper loop, laminated copper, copper with central hole, aluminum, and wood.
    Engagement Suggestion

    After showing the swing of the nonconductive (wood) pendulum, encourage students to make a prediction about what the copper disc will do.

    Ideas to ask them about as discussion prompts:
    • • Will it swing just the same,
    • • stop immediately in the magnetic field,
    • • slowly slow down after a couple of swings,
    • • or gain energy and swing higher/faster?
    Background
    As a conductive pendulum swings into the magnetic field, the changing magnetic flux induces electrical eddy currents in the metal. Some shapes (e.g. solid disc) offer more opportunity for these currents to form and grow. Outside of the magnetic field, these currents disappear at the magnetic flux does, but each pass through the magnet creates the currents again. This causes a gradual loss of kinetic energy in the pendulum.
    K2, K1

  • 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

  • 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

  • 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

  • K5-12 BATTERY AND CURRENT - WORKING MODEL

    K5-12
    Model of battery with circuit attached
    Though originally built as a toy, this device can be used as a model of an electric circuit attached to a battery. The "battery" raises the penguin "electrons" to a high potential. where they then progress through a "circuit" as they lose their potential. This model might indicate that a battery EMF provides energy which the carriers dissipate against resistance - the carriers do not speed up as they lose potential energy.
    K5

  • K5-13: ELECTRIC CURRENT - MODEL

    K5-13
    Indicate how electrons really flow through a conductor.
    Nails are driven into one side of an inclined plane in an orderly pattern, representing the lattice of a crystal, and into the other side in a random fashion, representing the polycrystalline structure of a metal. Ping pong balls represent free electrons traveling through the material. In spite of the much larger number of nails on the structured side, the balls move more quickly than through the random array of fewer nails. The slope of the inclined plane models the potential difference, and the interaction of the balls with the nails models the interaction of the free electrons with the ion lattice of the material.

  • K5-14: ELECTRIC CELL

    K5-14
    Demonstrate how an electric cell is formed.
    A container is filled with a dilute solution of HCl or diluted vinegar, and and a pair of electrodes is inserted into the electrolyte solution. The voltage is measured using the lecture meter or digital interface. The standard electrode pair is copper and zinc.
    K5, ME2

  • K5-23: ROTATING TWO-COLOR LED

    K5-23
    Demonstrate that what comes out of a 110 VAC plug is in fact alternating current.
    The bi-color LED is wired across the 110 VAC line. When current flows one direction the red side glows, and when current flows the other direction the green side glows, so if it is viewed from a distance the color appears yellow. When the device is swung in a circle the alternating colors become easily visible, indicating that the current in the line is alternating direction. This new and improved version of the device was created by Prof. Steven Rolston.

    Try showing the class the light while it is stationary, then explain how it works and have them predict what it will look like in motion.

  • K5-32: RESISTANCE VS DIAMETER AND LENGTH

    K5-32
    Determine how resistance varies as a function of diameter and length for the same material.
    The resistance of two wires of the same length but different diameter can be compared. The resistance as a function of length can be determined by sliding the clips along the wire. Use of high-resistance nichrome wire keeps stray resistance (in contacts, etc.) small compared with the resistance being measured.
    K5, ME2

  • K5-33 CONDUCTIVITY OF SALT SOLUTION

    K5-33
    Shows that pure water is not conducting, but a solution of an electrolyte is conducting
    A 110 VAC lead is connected to a series arrangement of a light bulb and two parallel plates. Shorting the plates will light the bulb. Inserting the plates into distilled water does not light the bulb. Inserting the plates into tap water lights the bulb dimly, and inserting the plates into salt water lights the bulb fully. An electrolyte solution allows passage of electric current.
    K5