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Magnetic Fields and Forces

  • K1-04: FORCE ON CURRENT IN MAGNETIC FIELD - PORTABLE

    K1-04
    Demonstrate the force on a current-carrying wire in a magnetic field.

    A wire is placed or held between the poles of a small horseshoe magnet. When the ends of the wire are connected to a 6-volt battery, the wire jumps out of the magnet. The current in the wire or the orientation of the magnet can be changed to investigate the directions of the vectors involved.
    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 it around 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 K1-03 in this section for a larger version of this demonstration for classrooms.

  • 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

  • K1-06: CURRENT BALANCE

    K1-06
    Demonstrate the force between two adjacent parallel currents.
    The device uses two parallel currents, the lower one stationary and the upper one which can move to and away from the stationary wire. Current in the wires causes a force between the wires which can be determined by balancing the moving current loop with a movable counterweight. The measured magnitude of the force can be compared with the measured value.
    K1, PS1

  • 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

  • K1-11: CATHODE RAY TUBE - DEFLECTION BY CURRENT

    K1-11
    Demonstrate visually the force between two parallel currents.
    The discharge tube produces an electron beam moving from left to right, which can be seen on the fluorescent screen inside the tube. Activating an electric current in an external wire held along the top of the discharge tube, parallel to the electron beam, deflects the electron beam a small distance up or down. If the direction of the current in this wire is reverse, the direction of the deflection will likewise reverse. This illustrates the vector nature of the force.

    The photographs above show the displacement of the beam when the positive current in the wire is running (a)left-to-right, at left above, and (b) right-to-left, at right above. This is a small effect; a picture with no current in the wire is reproduced at the bottom above for reference. Using the switch to turn the current ON and OFF while watching the beam shows the effect VERY clearly.

    If desired, a video camera may be requested to display this demonstration on the projection screen in the large lecture halls.
    K1, PW1

  • 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

  • K1-13: CATHODE RAY TUBE - DE LA RIVE MAGNETIC EFFECT

    K1-13
    Illustrate bending of an electron beam in a magnetic field.
    The tube contains a point electrode at the top and a ring-shaped electrode at the bottom. A cylindrical bar magnet runs up the center axis. When powered up, an electron discharge starts at the top electrode and trails down to the ring at the bottom. When an additional 5V potential is applied to the bar magnet, as shown in the photo, the discharge precesses around the magnet.
    K1, P2, PS1

  • 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

  • K1-15: OSCILLOSCOPE CRT - DEFLECTION BY ELECTRIC FIELD

    K1-15
    Demonstrate the deflection of an electron beam by an electric field.
    The electron beam starting at the rear of the tube is seen on the front fluorescent screen in the standard way. Applying an electric field on either of the two sets of internal deflection plates causes either horizontal or vertical deflection of the beam.
    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

  • K1-23: TORQUE ON CURRENT LOOP - MODEL

    K1-23
    Model torque on a current loop due to magnetic field.
    A uniform magnetic field is represented by the red vectors. A hypothetical current-carrying coil positioned at an angle with respect to the magnetic field and carrying an electrical current (indicated by the black arrows) experiences a torque due to the magnetic field. The torque is created by the forces on the four sides of the coil, which are shown by the white arrows; the blue arrow is the normal axis of the coil.

    Note: This uses the flux vector framework from J3.
    K1, J3

  • K1-31: MAGNETOHYDRODYNAMIC GENERATOR

    K1-31
    Illustrate magnetohydrodynamic forces.
    A shallow bowl of copper sulfate solution is placed in the (downward) magnetic field of a medium-strength horseshoe magnet. An electron current is created between the negative electrode band around the circumference of the bowl and the positive electrode at the center of the bowl. The resulting vxB force on the electrons creates counterclockwise rotation of the copper sulfate solution. Small pieces of paper are dropped onto the surface of the rotating liquid as an indicator.
    K1, PS1

  • K1-41: BARLOW'S DISC

    K1-41
    Demonstrate how an electric potential difference and current can be generated when free (conduction) electrons move through a magnetic field.
    Rotating a copper disc between the pole tips of a strong C-shaped magnet, to create a vxB force on free electrons in the copper, creates a potential difference which is picked up by brushes at the center and the outside of the disc and displayed on the galvanometer. Reverse rotation of disc to invert current (center photograph).
    K1, D1, ME2

  • 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

  • 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

  • K2-46: 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!

  • K2-64: 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