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PHYS122

  • 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.

  • K2-22 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.

  • K2-24: MUTUAL INDUCTION

    K2-24
    Demonstrate mutual induction.
    Two pairs of concentric coils are coupled by an iron core. Connect 9-volt battery to start and stop current in any one of the coils. The current induced in any of the other coils can be seen using the galvanometer. Connecting battery to coil with meter will damage the meter.

  • 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

  • K2-43: LENZ'S LAW - PERMANENT MAGNET AND COILS

    K2-43
    Demonstrate Lenz's law
    Thrust one pole of a strong horseshoe magnet into a coil, or quickly withdraw it from inside the coil. A current is induced in the coil that opposes the motion of the magnet, and the reaction force on the coil results in the coil being either pushed or pulled in the same direction that the magnet was moved. Move the magnet in and out of the single turn aluminum coil. Disconnect the hook for the single turn coil. No current can flow so no force is created. Videos:
    K2

  • 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

  • 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.

  • K5-33 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

  • K5-36: RESISTORS AT LN TEMPERATURE - LIGHT BULB INDICATOR

    K5-36
    Demonstrate materials with both positive and negative temperature coefficients of resistance.
    Copper and carbon resistors are mounted on plastic tubes so that they can be inserted into liquid nitrogen. When the copper resistor is wired in series with a light bulb across 12 VDC, the bulb becomes brighter when the resistor is cooled to the temperature of liquid nitrogen, indicating a positive temperature coefficient of resistance for copper (first set of photographs). When the carbon resistor is wired in series with the light bulb across 12 VDC, the bulb becomes dimmer when the resistor is cooled to the temperature of liquid nitrogen, indicating a negative temperature coefficient of resistance for carbon (second set of photographs).
    K5, I0

  • K5-44: NON-OHMIC DEVICE - LIGHT BULB

    K5-44
    Show the change in resistance of a light bulb with temperature.
    A 60 watt incandescent light bulb is connected to a switch so that it can be quickly disconnected from the 110 VAC power to an ohmmeter. The resistance of the 60 watt bulb in operation at a high temperature is R = V^2/P = 110^2/60 = 200 ohms. The resistance cold is about 18 ohms. Turn the bulb on, then switch it to the ohmmeter. The resistance starts high and drops quickly as the bulb cools.

  • K6-01 SERIES AND PARALLEL LIGHTS - TWO BULBS

    K6-01
    Demonstrates the effect of series and parallel connections of two identical light bulbs
    Two light bulbs can be wired in series or in parallel across 110 VAC circuit. Usually uses either 75 watt and 150 watt incandescent bulbs, or a pair of 40 watt bulbs. The voltage of the device can be reduced with a variac if desired.

    Important note: Turn off device before connecting or disconnecting wires! The bulbs can be wired either in series or parallel by swapping the wires, but this must not be done while powered.

    K6

  • K6-03 SERIES AND PARALLEL LIGHTS - BATTERY AND CLIP-ON WIRES

    K6-03
    Shows voltages and currents in series & parallel circuits
    Series and parallel combinations of light bulbs can be connected to the 7.5 volt battery source. Meters indicating current and voltage can be inserted in the circuit as required.

    Note that due to the aging of our large display galvanometers, this is better performed now with digital multimeters. A camera can be used to show them on the lecture hall screen if desired.
    K6, ME2

  • 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

  • K7-11 RC CIRCUIT - RC TIME CONSTANT - OSCILLOSCOPE

    K7-11
    Observe the shape of the charging and discharging curves. Measures an RC time constant.
    An RC circuit with meters is mounted on an overhead projector projectual (the same setup as in K7-12). The voltage across the capacitor and the charging current are displayed. Either the voltage across the capacitor or the charging current (voltage across the series resistor) can also be displayed individually on the oscilloscope. Various combinations of resistance and capacitance can be plugged into the circuit.
    K7, ME2, PS1

  • K7-15: CURRENT IN RC CIRCUIT?

    K7-15
    Demonstrate charging of a capacitor in a possibly counterintuitive way.

    A switch is closed to charge a 1 farad (yes, one FARAD) capacitor with a 3 volt battery (actually two 1.5 volt batteries in series). The capacitor has 1.5 volt light bulbs on each side of it in the circuit, as shown in the circuit drawing above.

    Consider the following series of questions:

    Q: What will happen when the battery is connected (switch turned to left position):

    (a) both bulbs will light and stay lit,

    (b) both bulbs will go on momentarily,

    (c) only one bulb will light and stay lit (if so, which one?),

    (d) only one bulb will go on momentarily (if so, which one?),

    (e) neither bulb will go on at all,

    (f) something else will happen (if so, what?).

    A: (b), Both bulbs will go on momentarily as the capacitor charges, then they will fade out. Click below for video

    Q: What will happen when the switch is opened (center switch position)?

    A: Nothing: the capacitor remains charged and no current flows.

    Q: What will happen when the switch is closed to the right, completing the circuit including the capacitor and the two light bulbs?

    A: The capacitor will discharge through the bulbs, turning them on momentarily while current is flowing, with the intensity decreasing as the current falls to zero. Click below for video.
    K7

  • K8-01 ELECTROMAGNETIC WAVE - MODEL

    K8-01
    Shows the relationship between the electric and magnetic field vectors in a plane-polarized traveling electromagnetic wave
    Red pegs represent the electric field vector and blue pegs represent the magnetic vector. The spatial relationship between these vectors and the direction of propagation can be seen. By moving the model along its axis the temporal aspect of the wave can be shown. This wave has a wavelength of 0.81 meters, and as an EM wave would have a frequency of 370MHz
    FS1

  • K8-45 RADIO WAVES FROM SPARK

    K8-45
    Demonstrates that a spark contains radio waves
    Turn the radio on to a frequency where there is no station. Hold the battery near the radio and short it out by quickly contacting and releasing the contact using a banana wire cable. A clicking sound will readily be heard on the radio.

    Compare J3-23: Faraday Cage - Radio Waves, which can use the same radio to illustrate a related phenomenon.
    K8