Physics and Astronomy Demonstration Resources

          Demonstration Index                          

30. MAGNETIC FIELD

30a. Magnetic lines of force around a single wire, a half turn, a solenoid and a toroid may be shown with an apparatus that consists of plastic stands into which these wire configurations are formed. A large alternating current is passed through the wires and iron filings are sprinkled over them. The field line patterns emerge as expected and are projected using overhead projector. An alternating current is used instead of a direct one as the vibration of the filings by the alternating fields causes them to align very rapidly. Also, easier to generate large AC currents by using a step-down transformer.

30b. Magnetic forces between parallel and antiparallel currents are easily demonstrated with an apparatus consisting of two sets of wires arranged so that the above current orientations may be produced. The direction of the current in each wire is indicated with a set of arrows. A changeover switch allows for rapid comparison of the two forces.

30c. Roget's spiral demonstrates that currents flowing in the same direction cause the current carriers to be attracted to one another. A spring is held firmly at its top end and dipping into a pool of mercury at its bottom. A large current is made to flow through the spring causing it to contract, lifting the lower end out of the mercury. As the current path is broken, the current ceases to flow and the spring extends once again allowing a current to flow. Good sparks are produced each time the end lifts out of the mercury.

31. FARADAY'S LAW

31a. Induced currents (1) in a coil when a magnet is moved in and out of the coil, are observed with a galvanometer. The magnet may be mounted in a stand and the coil moved instead, if so desired.

31b. Induced currents (2) in one coil when the current in an adjacent coil is varied. A second coil identical to the first is connected through a switch to a battery and the switch is opened and closed. Impulsive currents will be seen in the secondary. With the switch closed, the second coil is moved towards and away from the first, and again the induced currents noted.

31c. Induced current (3) --- a bar magnet is moved in and out of coils of 250 or 500 or 1000 turns connected in series with a galvanometer. If the velocity is kept constant for the three cases, the induced currents will be in the ratio of the number of turns.

31d. Induced current (4) --- a large single turn of wire is connected to a galvanometer. A very strong bar magnet is moved in and out of the turn, the induced EMF being noted. Knowing the deflection direction for a given polarity at the terminals of the galvanometer the direction of the induced EMF can be determined and thus Lenz's law deduced.

31e. Induced DC current in one coil due to a current whose magnitude increases linearly with time flowing through a primary coil on a transformer. The secondary coil is connected to a galvanometer used to indicate the induced current. The rate of increase of current is variable. The current source, a generator that puts out a linear ramp function from 0 to 100 volts, can be made to ramp in 1, 2, 5, 10 or 20 seconds, hence, the induced current in the secondary can be made to increase by changing the ramp rate. While the current in the secondary should be absolutely constant for the entire ramp, nonlinearities in the core and generator characteristics introduce slight changes.

31f. The levitated ring--- a metal ring is placed on the core of a large solenoid which is suddenly activated with an alternating current. The interaction of the magnetic field and the current induced in the ring causes the ring to fly off. If the ring is replaced with one that has a slot cut in it, no effect is seen. The reason for the ring's jumping is, in fact, quite complicated and has to do with the phase lag of the induced current in the ring produced by the rings inductance and resistance. If the ring is dipped into a dewar of LN₂, the ring will jump much higher. The ring can be levitated if the primary current is properly adjusted.

31g. Eddy current (1). A sheet of copper is suspended between the poles of a magnet from a freely swinging pendulum. When the bob is drawn to one side and allowed to swing into the region of intense magnetic field, the eddy currents induced in the bob cause the oscillations to die out very rapidly. Replacing the copper sheet with one that has slots in it will cause the damping to be greatly reduced. An electromagnet may be used in place of the permanent one, and the solid bob allowed to swing freely for a while until the power is applied to the magnet.

31h. Eddy current (2). A sheet of aluminum is allowed to fall between the poles of a pair of magnetron magnets hanging off the edge of a bench. Due to the large eddy currents induced in the sheet, it will be seen to fall very much more slowly than expected. Plexiglass and wooden sheets show no effect.

31i. Magnetic circuit. The apparatus of 31b is used with the addition of an iron core that is progressively built up around the coils. With each additional section of the magnetic circuit being added, a larger induced current is seen in the secondary coil.

31j. A small magneto is connected to a 10 watt light bulb. The amount of work necessary to turn the generator with and without the load can be compared. The demonstration is best set up at the front of the lecture hall and the students asked to try it after class. If a neon lamp is used in place of the incandescent one, the alternating nature of the generated current can be seen.

31k. Transformers of a wide range of turns ratios may be made from the coils and cores available in the store room. Output voltages for line voltage input can be anywhere in the range of 0.5v to 10000v. Usual demonstrations with these transformers are spot welding (low voltage, high current) and Jacobs Ladder (31m). The transformers are often used to power other demonstrations.

31l. Two induction coils are available for demonstrating large induced EMF's from rapidly changing magnetic fields. The smaller induction coil produces sparks from 1 to 5 cm long, while the larger one produces 6 - 10 cm sparks. Both induction coils have power and reversing switches to select electrode polarity, the larger coil also having the capacity for varying the phase of the interrupter.

31m. Jacob's Ladder is usually used as a demonstration of high voltages and arcing. Two horn-shaped electrodes are connected to the secondary of a 110 to 15000 v transformer. The gap at the bottom of the electrodes is narrow enough that the 15000v causes an arc. Due to the heat created in the arc, the ions rise, carrying the arc with them. The flame thus rises up the electrodes until it reaches the top where it blows itself out. The demonstration has the minor disadvantage that the voltage and current levels available from the transformer are lethal, so great care must be taken.

31n. Deformed loop. A long wire is connected to the terminals of a galvanometer. The induced current as the wire is changed in shape and orientation with respect to a large magnetron magnet are observed on the galvanometer.

31o. Orientation of secondary coil. A 1000 turn coil is driven from an electronic oscillator. The induced EMF's in a second coil connected to the oscilloscope as the orientation and separation of the two coils is varied is shown.

31p. Power line losses. Two thin nichrome wires are stretched the width of the lecture bench and at one end connected to a 100 watt light bulb and AC Voltmeter. The other end of the wires are connected directly to the AC line. Due to transmission losses the potential difference across the lightbulbs is less than 50v. If, however, transformers are inserted into the system at either end of the nichrome wires, arranged to increase the voltage to the wires to 10000v at the input end, and reduce it accordingly at the other, the losses will be far less, the bulb having about 105v across it, thus glowing brightly. The power losses for the two cases can be calculated.

31q. Motor as generator. The sinusoidal EMF generated by a coil turning in a magnetic field can be shown using one of the demonstration motors of 29n connected to a galvanometer. A small battery is connected to the field windings to produce a constant magnetic field or permanent magnets can be used. Angular velocities of less than one radian/sec may be used.

31r. Back EMF. The current through a small DC motor as a function of shaft speed is measured. As the shaft velocity increases, the back EMF from the coil rotating in a magnetic field increases, and the input current decreases. The demonstration may, of course, be considered from the point of view of conservation of energy.

31s. A large toroidal coil is connected directly to the AC line. A single turn of wire is passed through the coil and connected to an AC voltmeter. The induced EMF in this turn is compared with the induced EMF's for passing the wire twice, three times, etc., through the coil. If turn is a closed loop, large current is induced and a wisp of smoke emerges as the insulator melts.

31t. Reflected impedance. The coil of 31s is connected to the AC line via an ammeter, a single turn being passed through it and connected to another ammeter in series with a variable resistor. The change in primary current as the secondary current is varied is pointed out.

32. INDUCTANCE

32a. Inductor back EMF (1). A neon light bulb is connected to the terminals of 10 H inductor which is also connected to a 1.5v battery via a switch. When the switch is closed, no effect will be seen. However, on opening the switch, the large back EMF from the coil causes the neon to light. A small 1.5v bulb can be connected in series with the inductor to indicate that a current is flowing.

32b. Inductor back EMF (2). A large electromagnet is set up in series with a large switch and connected to a car battery. With current flowing in the circuit, the switch is thrown open as quickly as possible, producing a very large spark.

32c. L-R Circuit (1). A 45000H (that is not a typographical error) inductor is connected to a battery via an ammeter, a voltmeter being placed across its terminals. As the inductor has an internal resistance of 5000 ohms, the time constant is of the order of 9 seconds. The very slow rise in current in this case can be compared with the time taken for an equivalent resistance taking the place of the inductor. At any time, the battery may be shorted out and then disconnected. With the inductor in place, a reverse current will be seen to flow for up to 20 seconds.

32d. L-R Circuit (2). A small inductor is connected in series with variable resistor and a square wave oscillator. The voltages across the R & L are fed to two channels on an oscilloscope. As the oscillator appears to the circuit as alternately a source and a short, the exponential waveforms of a "charging" & "discharging" inductor can be seen. The voltage across the resistor is used as a measure of inductor current. The time constant of the circuit can be varied over a wide range.