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PHYS142

  • J5-04: MAGNETS

    J5-04
    Show various magnets
    A varied collection of magnets is presented, including bar magnets, horseshoe magnets, disc magnets, ring magnets, and rare earth magnets. Ask about other types of magnets.
    J5

  • 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

  • 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

  • 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

  • K1-12 CATHODE-RAY TUBE - DEFLECTION BY MAGNET

    K1-12
    Demonstrates the force on an electron beam by a magnetic field
    The cathode ray discharge tube produces an electron beam moving from left to right, which can be seen on the fluorescent screen inside the tube. Holding a bar magnet close to the tube, parallel to the tabletop so that it produces a horizontal magnetic field inside the tube, causes the electron beam to deflect up or down. If the directions of the magnet's poles are reversed, the direction of the deflection should also reverse, illustrating the vector nature of the force.

    If desired, a video camera may be requested to display this demonstration on the projection screen in the large lecture halls.
    K1

  • K1-14 OSCILLOSCOPE CRT - DEFLECTION BY MAGNET

    K1-14
    Demonstrates the force on an electron beam by a magnetic field
    The beam of an oscilloscope CRT is viewed at the front on the fluorescent screen in the standard way. Holding a magnet near the CRT such that the magnetic field is perpendicular to the path of the electrons creates a magnetic force on the electron beam which deflects the beam, as can be seen on the fluorescent tube.
    K1

  • K1-21 TORQUE ON CURRENT LOOP IN MAGNETIC FIELD

    K1-21
    Demonstrates the torque on a current loop in a magnetic field
    A few-turn coil is positioned in the magnetic field of a small horseshoe magnet, as shown in the photograph. Pushing the switch connects the battery to the coil, passing electrical current through the coil and creating the torque, which is visible as a small rotation of the coil about its axis. Reversing the coil leads reverses the direction of the torque.

    A video camera can be made available upon request for displaying this demonstration in large lecture halls.
    K1

  • K1-22 TORQUE ON 500-TURN COIL IN MAGNETIC FIELD

    K1-22
    Demonstrates the torque on a current loop in a magnetic field
    A large coil sits between the poles of strong magnets, with the plane of the coil parallel to the magnetic field lines. A large current pulse can be applied to the coil by charging and then discharging a capacitor. When the current pulse is applied to the coil, a torque is exerted on the coil by the magnetic field which rotates the coil so that the magnetic field is perpendicular to the plane of the coil. This effect can be quite dramatic; be sure to keep fingers clear of the magnets.

    Charge capacitor to no more than 25V.
    K1

  • K2-01 EARTH INDUCTOR

    K2-01
    Induces an emf by moving a coil through Earth's magnetic field
    A large wire coil is connected to a projection galvanometer. Motion of the coil through the magnetic field of the earth induces an emf which is indicated on the meter. Alignment of the coil relative to the earth's magnetic field lines can be found which produces a maximum deflection of the coil, or almost no deflection. Optionally, a bar magnet (available upon request) can be thrust in and out of the coil to induce a larger voltage, illustrating the relatively low strength of the Earth's field.
    K2

  • 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

  • K2-22 INDUCTION COIL WITH LIGHT BULB

    K2-22
    Demonstrates megnetic induction with 110 VAC
    Closing the switch puts 110 VAC on the primary coil, which is coupled to the secondary coil (on top) by a ferromagnetic core. The induced current in the secondary coil lights the 110 VAC light bulb.

  • K2-40: MAGNETIC ACCELERATOR

    K2-40
    Demonstrate magnetic potential energy
    A slightly curved track holds a series of steel balls. One ball, superficially similar to the others, is a magnet. To use: Fill the track with nonmagnetic balls, and release a single ball from one upper end of the track. It rolls down and across the track until is collides with a stationary line of balls. As expected, the last ball in the line moves out with slightly less speed than the incoming ball. Now repeat the demonstration, replacing the first stationary ball in the line with the magnetic ball and the result is quite different. The attractive force of the magnet adds energy to the system, in a quite dramatic fashion.

    A similar effect can be seen with C7-19: Gaussian Gun
    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

  • K2-62 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

  • K6-11 CIRCUIT PARADOXES

    K6-11
    Series-parallel circuits to encourage dicussion about DC circuits
    Two circuits, with identical batteries and identical light bulbs, are connected with a switch through one branch of the circuit. (When using, be sure to check main power switch on underside.)
    K6

  • K7-03 INDUCTOR DELAYING A LAMP

    K7-03
    Demonstrates the timing of current in a switched inductor circuit
    An inductor and light bulb are connected across a battery with a switch. In parallel with this inductor and light bulb is a second identical light bulb. The separate light bulb will light immediately, but the light bulb in series with the inductor will be delayed in lighting until the current reaches a high enough level. The delay time is related to the L/R time constant of that leg of the circuit.
    K7, PS1

  • K7-04: RL CIRCUIT - FIELD COLLAPSE WITH FLUORESCENT BULB

    K7-04
    Demonstrate the magnitude of induced voltage
    A 9-volt battery is connected across a large inductor; pushing the red button disconnects the inductor from the battery and immediately connects it across a fluorescent light tube. Collapse of the magnetic field in the inductor induces a voltage large enough to cause the fluorescent bulb to flash - well over 100 volts, or over ten times the voltage of the battery.
    K7

  • K7-26: RLC CIRCUIT - 0.3 HZ RESONANCE

    K7-26
    Plot a graph of resonance behavior in a very low frequency resonant circuit.
    A series RLC circuit, containing a very large inductor, a 0-50 kilohm resistor, and a 100 microfarad capacitor is driven by a sine-wave oscillator as shown in the circuit above. As the frequency of the oscillator is varied between about 0.1 Hz and 0.5 Hz, the resonance in the system is observed to be about 0.3 Hz. The voltage across the capacitor is shown at various frequencies on an oscilloscope. The scope shows both the signal from the oscillator and the signal across the capacitor.