Physics and Astronomy Demonstration Resources

          Demonstration Index                          

27. ELECTRIC CURRENTS

27a. A Whimshurst or Van der Graaff generator is connected to a long wooden rod attached to which, at equal intervals, is a series of 3 or 4 electroscopes. After the generator is started, there is a delay until the first electroscope deflects, a further delay until the next one moves and so on. Explaining this odd time dependence is difficult. If rod away from the generator is grounded, then steady state behavior is ohm's law-like with huge resistors.

27b. Transient currents: A sensitive galvanometer is set up, one terminal connected to ground, the other being connected to an insulated sphere. When a charged electrophorous plate is brought close to the sphere, the galvanometer will be seen to deflect only for that time that the charge is moving. This may be compared with connecting a small battery in series with a large resistance between the sphere and the ground, which gives a steady current flow.

27c. Ohmic and non-ohmic resistors: The voltage current characteristics of ohmic and nonohmic resistors may be shown using large demonstration meters. The current through the conductor is plotted as a function of applied voltage. Two types of nonohmic resistors are available; the simple light bulb whose resistance grows as the current increases, or a semi-conductor diode whose high impedance when reverse biased can be shown.

27d. I-V plotter on CRTO: An alternating current from a transformer controlled by a variac is used to drive a current through a resistor. The voltage across the resistor is applied to the vertical deflection plates on the oscilloscope. A second small resistor is placed in series with the test resistor, the voltage across this resistor being a function of the current through the test resistor. This voltage is applied to the horizontal plates of the oscilloscope. With a suitable applied alternating voltage, the I- V characteristic under test can be observed. This form of the apparatus is extremelysuccessful indemonstratingthe characteristicsofsemi conductordiodes.

27e. Temperature variation in resistance: A length of nichrome wire is wrapped around a ceramic form and connected in series with a lightbulb and battery. When the wire is heated with a Bunsen burner, the light will be seen to glow more dimly. If, however, the wire is dipped into a dewar of liquid nitrogen, the light will glow very much more brightly.

27f. R=ρL/A: The current through wires of different lengths, materials, and diameters for a given potential difference between the ends of the wires is used to show the relationship R = ρL/A.

28. ELECTRIC CIRCUITS

28a. Internal resistance: A volt meter is connected across the terminals of a small C or D cell and the change in terminal potential as a current is drawn from the cell is noted. The magnitude of the current being drawn may be measured using an ammeter.

28b. Lightbulb brightness explored for various wattage bulbs connected singly, in series or in parallel. Students asked to predict which bulb will be brighter or dimmer.

28c. A number of single loop circuits may be shown using any number of resistors and up to three volt meters or ammeters.

28d. A large Wheatstone Bridge is available wherein the current in each branch may be measured, the voltage across each resistor may be measured, and the condition for null demonstrated.

28e. Equivalent resistance of a number of resistors in series or parallel may be shown using a simple battery volt meter, and ammeter combination, the equivalent resistance being calculated from the voltage and current reading. Three 1,000 ohm resistors in series can be shown to be equivalent to one 3,000 ohm resistor, and so one.

28f. Joule heating in a resistive material may be demonstrated by placing across two mails connected to a terminals of a twelve volt car battery a large lump of steel wool. The wool will glow brightly and possibly burst into flame.

28g. Hot dog cooker: The heat generated by passing a current through a resistance may be graphically demonstrated by arranging for two stainless steel nails to be connected directly to the 115 V AC line. A hot dog impaled upon the nails will start to bubble and steam and will be found to be very edible after approximately one minute.

28h. RC - Circuit: The very slow rise in terminal potential in a large capacitor as it is charged through a large resistor may be demonstrated using a three-thousand volt power supply, 100 M resistor and two microfarad capacitor. The voltage at the terminals of the capacitor may be shown using an electrostatic volt meter.

28i. Square wave RC circuit: The square wave output of an oscillator is used to charge and discharge a capacitor through a resistor. The voltage across the capacitor and the voltage across the resistor are applied to separate traces on the dual beam oscilloscope and projected by TV. The voltage across the resistor is a measure of the charging current, the voltage across the capacitor following the exponential build up and decay associated with such circuits. Both the resistance and the capacitance are variable.

28j. Many multi-loop DC circuits are possible using meters up to three volts and to three amperes. Circuits constructed in the past have consisted of as many as three sources of EMF and five resistors. The voltage across each resistor and the current through each resistor can be measured.

29. MAGNETISM

29a. The Oersted demonstration that an electric current exerts a force on a magnetic compass can be repeated. The first version consists of a horizontal wire connected through a reversing switch to a 12 volt car battery. Large compasses are situated above and below the wire; the compasses point in opposite directions when the current is on, the directions then reverse when the current is reversed.

    A second version uses a small horizontal wire above projection compasses on an overhead projector. Repositioning the current bearing wire causes the projector compasses to move so as to keep the compass pointing perpendicular to the wire. A third version has a vertical current carrying wire above the overhead projector; small projection magnets surrounding the wire all become tangent to a circle when current flows.

29b. The force on a moving charge in a magnetic field can be demonstrated by using a cold cathode tube wherein a ribbon electron beam produces a trace on a fluorescent screen placed the length of the tube. The deflection of the beam when a magnet is brought into the vicinity of the tube can be shown. B parallel, or opposite, to beam direction gives no deflection.

29c. Electron beam deflection uses Welch Scientific lecture- table CRTO (large screen). Remove cover, by-pass safety switch (careful, high voltage!) and bring magnet toward electron beam from various directions. Can also show electric deflection using zero shift controls for vertical and horizontal motion. Very good for F = v × B demo.

29d. Circular path of electrons shown using a region of homogeneous magnetic field produced by a pair of Helmholtz coils. As the tube contains a small quantity of hydrogen gas, the circular path of the electron beam is visible. The magnetic field and the velocity of the electron beam may be varied. The electron beam may be tilted slightly with respect to the magnetic field, the resultant helix being quite apparent. The apparatus can be used to measure e/m.

29e. Two versions of magnetic force on a wire are available. The first version consists of a wire stretched loosely between two stands and connected to a car battery. A large magnet produces a magnetic field near the center of the wire. When current is allowed to flow, the wire flies in one direction or the other in the region of the magnetic field. A more dramatic second version consists of a U-shaped piece of wire dipping into two cups of mercury, connected via a switch to a car battery. The wire is placed between the poles of a large magnet. When a current flows, the wire flies out of the magnet, frequently going up as high as 8 feet.

29f. Faraday's motor consists of a pool of conducting liquid from the center of which projects the pole of small magnet. From a hook above the magnet is suspended a copper wire that dips into the liquid. A current is made to flow through this wire and the pool of liquid. Due to the interaction of the current with the radial component of the magnetic field, the wire rotates in a circle about the bar magnet. This is supposed to be the first motor wherein a current produced a continuous motion.

29g. Barlow's wheel consists of a copper disc mounted vertically and free to rotate. A current is made to flow from the center of the disk to its outer edge, and the interaction of this current with an external magnetic field causes the disk to rotate. Contact with the wheel is maintained by a conducting liquid (mercury?) and by use of axis of rotation.

29h. The torque on a current loop in a magnetic field may be demonstrated, the apparatus consisting of a circular coil of wire suspended by a torsion fiber between the poles of a magnet. The direction of the torque and its magnitude may be varied by varying the magnitude and direction of the current flowing through the coil.

29i. Thomson's e/m experiment can be performed. The apparatus consists of a tube wherein a ribbon electron beam is produced and made to collide with a phosphor coated screen at grazing incidence. A known electric field is established in the region of the screen and the deflection of the beam noted. A magnetic field is produced in the same region with a pair of helmholtz coils. The magnetic field is varied until the beam is deflected back to the original path. From the magnitude of the electric and magnetic fields, the ratio e/m may be calculated. Results with precisians better than 50% are not to be expected due to inhomogeneities in both fields.

29j. Hall effect magnetic field probe used to detect size and direction of B. (In preparation)

29k. Ion drag --- a small glass dish has a magnet placed at its center and a current is made to flow radially from this magnet to a ring electrode at the outer edge of the dish through salinated water. Small plastic arrows are floated on the surface of the water and are seen to rotate about the magnet with the water, which rotates due to ion drag. (In preparation)

29l. A quantitative measure of the force on a wire in a fixed magnetic field can be made by having a rigid conductor suspended from two spring balances and passing through the region of the poles of the magnet. A current, whose magnitude can be varied is passed through the wire and a plot of force vs current may be made.

29m. Magnetic mirroring can be demonstrated using an inhomogeneous magnetic field. Use apparatus of 29d, but replace Helmholtz coil with strong bar magnet. Electron beam can be reflected back toward source. Best seen after lecture.

29n. Two electron motors are available. The first is formed in the shape of a classic demonstration motor, with both commutator and slip-ring connections. The second is based on the old 'St. Louis' motor design, the shaft being vertical with the pole pieces on a large rectangular frame. Only commutator connections are available. The field and rotor coils are very visible. The phase of the commutator may be varied to show the importance of having the current change direction at the correct instant.

29o. A wooden loop is available for demonstrating the projection of a square loop onto a plane. This is often an aid in describing the electric flux or magnetic flux through an area. Also useful to illustrate torque on a current loop.

29p. A version of the shaded pole AC motor can be shown. A large coil is activated by alternating current from the line and a rotating magnetic field is produced by placing above one half of the top pole piece a sheet of copper. Above the copper sheet a metal sphere is floated in a beaker of water. Due to the interaction of the rotating field and that produced by the currents induced in the sphere, the ball is seen to rotate. See file card for details.

29q. A model synchronous motor may be constructed by driving two coils placed at right angles to one another with alternating currents of differing phase. In the region of rotating magnetic field so produced a magnetic needle is placed, and is seen to rotate rapidly if spun at or near the synchronous frequency.