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Resistance

  • I7-21: SUPERCONDUCTOR - MAGNET LEVITATION

    I7-21
    Demonstrate levitation of a magnet above a high-temperature superconductor
    A one-inch diameter superconducting disc is set on a conducting base in a bath of liquid nitrogen. A cubic samarium cobalt magnet levitates above the superconductor. Note that to show the Meissner effect you must place the magnet on the disc before cooling it down. When the superconductor passes through its transition temperature the magnet rises up by itself and levitates. For large groups, a camera can be provided.
    I7, I0

  • I7-23: Magnetic Track and Superconductor

    I7-23
    To illustrate levitation of a superconductor and magnetic pinning
    A chilled superconducting puck is levitated above a magnetic track. Despite the curve and slope of the track, the puck will remain above the track as it moves.

    This is an illustration of the diamagnetic and magnetic pinning effects of a superconducting material. When setting up, be sure to chill the puck in the position you want it above the track for maximum efficiency.

    The University of Cambridge has made available a helpful video lecture on magnetic pinning: https://ascg.msm.cam.ac.uk/lectures/fundamentals/pinning.php.

  • J7-12: CURIE POINT OF IRON

    J7-12
    Show the Curie point of iron.
    An iron wire is normally ferromagnetic, and therefore strongly attracted to the magnet as shown in the photograph. When heated so that it glows red hot by passing an electric current through it, the iron rises above its Curie point and loses its ferromagnetism, so springs at either end of the wire pull it away from the magnet.
    J7

    j7-12aj7-12b

  • K4-07 BICYCLE GENERATOR

    K4-07
    Demonstrates a 110 VAC magnetoelectric generator, and the relationship of work to power output

    Pedaling the bicycle generates 110 VAC, which can be used to light an array of five 110 volt 150 watt lights. The sum, totaling 750 watts or about one horsepower when fully lit, can be verified using the voltmeter on the generator housing.
    K4, FS1

  • 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.

  • K5-31 OHM'S LAW

    K5-31
    Demonstrates relationship between current, voltage, and resistance

    This simple circuit consists of a variable voltage power supply and a socket that can hold one of three modular resistor units, with a ammeter measuring the current through the resistor and a voltmeter measuring the voltage across the resistor. The whole circuit is mounted on a transparent plate that can be placed on an overhead transparency projector to show the wiring and the meters.

    The voltage can be varied to show how the voltage and current change together in a linear relationship to the resistance. Both two 1,000 Ohm resistors and one 2,000 Ohm resistor modules are available; the two 1,000 Ohm modules can be placed in parallel if desired.

    It can be valuable to ask students to make predictions about how the results will change when you change the resistance, then afterwards have them discuss their predictions and compare them to the results.
    K5

  • 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

  • K5-34: THERMAL COEFFICIENT OF RESISTANCE IN COPPER

    K5-34
    Show that the resistance of copper changes linearly with temperature.
    Measure the resistance of the copper coil at room temperature (295K), at the temperature of a dry ice and methanol mixture (195K), and at the temperature of liquid nitrogen (77K). Plot resistance versus temperature to demonstrate the linearity.
    K5, I0, ME2

  • K5-35: RESISTORS AT LN TEMPERATURE

    K5-35
    Illustrate materials with both positive and negative temperature coefficients of resistivity.
    Approximately equal copper and carbon resistors are mounted on long leads to a plastic mount, allowing them be inserted into a small liquid nitrogen bath. When cooled from room temperature to the temperature of liquid nitrogen, the resistance of the copper resistor decreases dramatically (first set of photos), while the resistance of the carbon resistor increases (second set of photos).
    K5, ME2, I0

  • 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

  • K5-41: V-I CURVES FOR OHMIC AND NON-OHMIC DEVICES

    K5-41
    Illustrate resistive properties of resistors and diodes.
    A signal generator set to 500 Hz is used as the source of AC current feeding a series circuit with a shunt resistor and one other "test" circuit component, which can be either a resistor or a diode. The current is displayed on the vertical axis of the oscilloscope as the voltage across a shunt resistor. The voltage across the second element (the element under study) is displayed on the horizontal axis. As seen in the photograph above, the diode is non-ohmic. The breakdown potential of the diode can be observed. The polarity of the vertical input must be inverted in this demonstration due to grounding of the point between the two circuit elements.

  • K5-43: NON-OHMIC DEVICE - V VS. I

    K5-43
    Demonstrates the non-linearity of the V vs I curve for a diode
    A zener diode is connected in series with a resistor across the output of a power supply. Observe the current vs. voltage as the voltage is varied. The voltage and current are read from projection meters when the device is placed on an overhead projector. The shunt with the zener diode has a series limiting resistor, so you will see a current in the milliamp range when the zener becomes conducting at about 7 volts.

    Note that this uses the same apparatus from K5-31: Ohm's Law, and can be taught in conjunction with that topic. The Zener diode can be used as an extension of the discussion of Ohm's Law, inviting students to hypothesize other non-ohmic devices.

  • 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.

  • K5-45: SEMICONDUCTOR MODEL

    K5-45
    Model of semiconductor.
    Nails are driven into the board in straight rows, so that the balls can roll down the slope relatively unimpeded. In this case, the apparatus simulates the motion of charge carriers in semiconductors under the action of an electric field. The field is increased by changing the tilt of the board.

  • K6-21: HEATING IN CURRENT-CARRYING WIRE

    K6-21
    Show the conversion of electrical energy into heat.
    Push to attach 110 VAC to wire, heating the wire and causing it to become longer and sag. The marker hanging in the center of the wire indicates the sag.

  • K6-22: ENERGY CONVERSION - IMMERSION HEATER

    K6-22
    Demonstrate quantitatively the conversion of electrical energy into heat.
    This 300-watt immersion heater is used to heat approximately 300 ml of water in a borosilicate beaker. Measure the initial water temperature with a digital thermometer, allow it to heat for a fixed time, then measure the final temperature. Compare the temperature change calculated for the energy conversion (as per Q=mcT where ! is the energy transferredm m is the mass of water, c is the specific heat, and T is the change in temperature) to that measured, and invite students to talk about the meaning of the difference (heat loss through the sides of the beaker, etc.).

    Note that the heater will (obviously) get hot! Do not allow it to burn your hand or the power cord.
    K6, I0

  • K6-32: POTENTIOMETER

    K6-32
    Find an unknown EMF using a voltage divider.
    A battery of voltage V is connected across a resistive wire of length L. A second, "unknown" lesser voltage battery is connected through a meter to the slide tap of the potentiometer. Find the distance L1 along the wire where the clip lead draws no current. The unknown voltage Vu is then: Vu=V L1/L External batteries can be used to provide a different battery configuration if desired.
    K6, ME2

  • K6-33: WHEATSTONE BRIDGE

    K6-33
    Demonstrate operation of a Wheatstone bridge.
    A Wheatstone bridge is connected as in the circuit below using two (approximately) 400 ohm resistors, a 0-90 ohm variable resistor and a 60 watt light bulb (16.7 ohms cold) with a 6 volt battery and a 0-5 mA ammeter (large lecture meter). The unknown resistance Rx (the light bulb) is: Rx = Rs (R2/R1), where Rs is the resistance of the active part of the variable resistor (equal to the fraction of its length used multiplied by 90 ohms).

  • K6-35: VOLTAGE DIVIDER

    K6-35
    Show how a voltage divider can be used to produce any voltage up to the maximum of the source.
    The circuit is assembled as shown in the diagram above: a 7.5 volt battery is connected across the entirety of an exposed resistive coil, with a voltmeter connected from the negative terminal of the battery to the adjustable slider that runs along the length of the resistor. This allows a variable voltage to be measured on the display meter.