Physics and Astronomy Demonstration Resources

          Demonstration Index                          

33. MAGNETIC MATERIALS

33a. The large domain model consists of a rectangular array of compass needles that are arranged between two sheets of plexiglass. The needle orientations are projected by an overhead projector. Slight vibration causes the needles to form into areas of like orientation ("domains") that can be made to orientate parallel to one another by an external magnetic field. If the field (produced by either bar magnets or a Helmholtz pair) is made to increase slowly, the progressive switching of the domains will be seen. A very effective demonstration.

33b. Magnetized filings. A glass tube filled with iron filings is magnetized with a bar magnet, and the field shown with a compass. The tube is then shaken, and the lack of poles noted. The filings are analogs of domains.

33c. Broken magnet. Small bar magnets are broken into pieces progressively and the nature and location of the poles for the pieces shown by presenting either end of the section to a compass needle. It will be found that no matter how many sections the magnets are broken into, there will always be a north and a south pole.

33d. Curie temperature. An iron wire formed into a pendulum is held to one side of its neutral point by a magnetic field. A bunsen burner plays on the wire at this point, and just as the wire gets red hot, it is seen to swing away from the magnet. Upon cooling, the wire will once again swing towards the magnet. The cycle repeats as long as heat is supplied.

33e. B-H curves. Samples of ferromagnetic substances are placed inside a coil connected to an AC source. The voltage across the coil is taken as a measure of B, and applied to the Y plates of a scope. Another coil around the first is connected to the x plates via an integrator (capacitor resistor pair), the voltage representing a measure of magnetization. The curves plotted out by the oscilloscope are the characteristic B-H curves, showing the linear and saturated regions clearly. The sample may be changed at will.

33f. Magnet fields. Various magnets are placed under a sheet of clear plastic and iron filings sprinkled onto the sheet. The force line pattern so produced is projected. Bar, horseshoe, annular and many other forms of magnets are available.

33g. Barkhausen effect. A sample of iron or any other ferromagnetic material is placed inside a coil of many turns, an audio amp being connected to the coil. With the gain of the amp very high and a speaker connected to the output of the amp, bringing a magnet into the vicinity of the sample will cause a rushing noise to be heard in the speaker. The effect is due to the domains in the sample changing orientation and thus inducing small currents in the coil. If the gods are really with you, you might hear individual clicks from the speaker for small changes in field as single large domains switch.

33h. Magnetic launcher. A graphic, if somewhat odd demonstration of solenoid action (meaning here the action of a device having a coil and an iron slug which is movable inside the coil....the slug is drawn into the coil when a current flows) is available wherein a 4uF capacitor charges through a high resistance until it reaches a potential sufficient to break down a spark gap. In series with the gap is a coil with a metal core placed asymmetrically inside it. When the gap fires, the huge impulsive current pulls the core rapidly towards the center. However, the current stops almost immediately, and the core flies out of the other side of the coil, often flying 10 feet. A metal ball similarly placed will go much further.

33i. Magnetic interactions. Bar magnets are suspended by fine threads and the attraction of unlike and the repulsion of like poles is demonstrated. Large needle mounted compasses may be used in place of magnets.

33j. A coil of wire is connected to a small battery and the power of the magnet so produced to pick up objects is demonstrated. An iron core is then placed in or around the coil and the dramatically increased lifting capacity is shown by lifting 2 or 4 Kgm, as opposed to the (max) 100gms lifted without the core.

34. LCR CIRCUITS

34a. LC Circuit Oscillations. The large inductor of 32c is connected in parallel with a large capacitor, the current through the inductor and the voltage across the capacitor being measured by meters. The capacitor is charged from a battery and allowed to discharge through the inductor. The oscillations so produced have a frequency of about 1/5 Hz and last for several

cycles before losses reduce the readings to zero. The phase relationship between the energy stored in the capacitor (as measured by the voltage across it and that stored in the inductor (as measured by the current through it) can be easily seen.

34b. LCR Circuit. A circuit consisting of a small capacitor, an inductor and a resistor, all of which are variable, is fed with impulses (actually differentiated square waves) from an oscillator. Each impulse causes the circuit to oscillate, the rate of decay of the oscillations being set by the resistance setting. The voltage across the capacitor and that across the inductor are fed to two channels of a dual-trace 'scope. With this arrangement, the effects of changing the capacitance or inductance on the resonant frequency can be shown, as can the phase relationship between the two voltages.

34c. Spring-mass Oscillations. A mass on the end of a vertical spring is occasionally useful in discussing the energy transformations in an L-C circuit. A damping system may be attached to give an analogy to the resistance of a circuit.

34d. LCR Resonance. The circuit of 34b is driven with a sinusoidal oscillator, and the response of the circuit to varying driving frequencies is shown. As the drive approached the natural frequency of the system, a huge response may be seen. The resistor can be varied to show the effect on the 'Q' of the system. (See experiment 18h for analogous mechanical resonance.)

34e. 60 Hz Resonance. A large variable inductor is connected in series with a fixed capacitor and a 100 watt light bulb. At some setting on the inductor, the system will resonate at 60 Hz, hence, if the system is connected across the AC line, a position of maximum brightness will be found on running the inductor through its range of values.

34f. Circuit coupling. Coupling between resonant circuits of variable frequency may be shown using the Unilab RF system. One resonant circuit is used as a feedback network for a vacuum tube oscillator. A second is used as a detector, a light bulb being used to indicate current in the system, which may be arranged either as a parallel or series circuit. The variation in the intensity of the lightbulb as the frequencies of the resonant circuits are varied is shown.

34g. Mechanical resonance. A meter stick is tightly clamped at one end, the free end with a disk magnet attached about 50 cm from the clamped end. An AC magnet is placed near the disk magnet and driven by an oscillator. The stick will undergo a maximum deflection at around 10-30 Hertz. Projection of the stick shadow on the screen will make the deflection very clear.

35. ELECTROMAGNETIC WAVES

35a. Energy transmission by electromagnetic waves can be shown using four transmitters: the Welch 1.5 meter transmitter and light bulb detector, the microwave transmitter and diode detector, the Uni-Lab oscillator which (it is claimed) will range in frequency from very low frequency to about 300 MHz, the Leybold transmitter system (not checked out yet).

35b. Polarization of radio waves using any of the transmitters may be shown. Perhaps the clearest is the transmitter of 35a, as the dipole receiver and transmitter have very clearly defined orientations. Characteristic of these setups is the fact that both transmitter and receiver are polarized. Only in the case of the microwave setup is it easily possible to rotate the transmitters axis of polarization.

35c. Polarization of visible light. Two long strips of polaroid are placed on the platen of an overhead projector. With the room lights off, one is placed on top of the other and rotated. The overlap region will be seen to get progressively darker, as the two sheets cross.

35d. The three polarizers demonstration can be performed with any of the radio transmitters or the optical polaroids. The experiment is begun by having the transmitter and receiver crossed so that no energy is received at the receiver (or transmitted to the screen in the case of optical polaroids). A third polarizer is inserted between the transmitter and receiver and slowly rotated. When the orientation of this is at about 45 degrees to the other two, energy will be received. (In the case of the polaroids, a bright band will appear where the third polaroid is placed.)

35e. Microwave interference. Standing waves may be set up between a microwave transmitter and a reflector placed about 1 meter away from the horn. The standing wave pattern is detected using a microwave diode in a tuned cavity attached to an audio amp and speaker. The microwaves are modulated with a rough 500Hz tone so intensity fluctuations in the microwave signal are heard in the speaker. Can measure wavelength by finding distance between nodes.

35f. RF standing waves can be set up on Lecher wires. A light bulb hanging from the wires shows clear maxima and minima as the bulb moves along the wires. The system works well for the Welch 1.5 m transmitter. We also have a Leybold system at 0.6 m which has Lecher wires, but this is in preparation.

35g. RF Antenna. The radio system of 35a is set up with the transmitter and receiver separated by about 2 meters. The change in the intensity of the detecting light bulb as reflectors are brought up behind the transmitter is shown. A piece of metal rod, slightly longer than the transmitter dipole is used as a reflector. As it is moved towards the transmitter, a distinctive maximum will be seen in the received power when the rod is about 1/4 wavelength away. The new equipment system has a number of ways of demonstrating this, though they have not yet been tried. The effects of a director, a rod shorter than the transmitter dipole, can be shown also. The reflector is first positioned, then the director moved until a maximum is reached. With the director and reflector in place, the system forms a very crude Yagi antenna.

35h. Birefringence. A sheet of polarizing material is placed on the platen of an overhead projector, another one being placed some distance above it, orientated at 90 degrees to the first. Plastic models, sheets of cellophane etc. are introduced between the two and the patterns in the sheets or models are seen on a screen. Plexiglass models can be subjected to stresses and the stress lines observed due to induced birefringence.

35i. Malus Law. Polaroids are mounted on a base and separated by about 10 cm. The polarizers are circular in shape and thus may be orientated at any angle with respect to each other. With bright illumination and a photocell system, Malus' law can be verified.

35j. Optical activity. A long glass tube is filled with a concentrated sugar solution and a ribbon beam of light projected into one end through a window. The light is arranged to be plane polarized by a rotatable sheet of polaroid. As the sheet is rotated, dark and bright bands are seen to translate along the tube, becoming progressively more colorful as they get further away from the polaroid. Another version of this is to have two polaroids separated by about 30 cm, and shine a bright light through them. Tubes filled with sugar solutions of different concentrations are put between the two and the angle for zero (or minimum) light transmission is measured. The change in angle is proportional to the change in concentration in preparation.

35k. Wave plate. Circularly polarized light may be produced by passing plane polarized light through a 1/4 wave plate, and any changes in the polarization may be observed by passing the light through a second 1/4 plate and polarizer. Stress models etc (see 35h) can be introduced and the patterns seen.

35l. Inverse square law (1). A metal wire model showing how, as one gets further from a point source, the intensity falls off as an inverse square is available. At distances d, 3d & 4d, areas are marked off in units of d2 and show clearly the 1:9:16 relation. For some reason the 2d area is missing from the model.

35m. Inverse square law (2). A bank of lights, switched so that one, four or nine may be turned on at any time, are arranged at some distance from a photocell and meter. The single light is turned on and the meter reading noted. Four lights are then illuminated, and the photocell moved until the meter reads the same as before. It will be found that the distance is twice what it was before, and repeating with 9 lights will give a distance three times the original one. From this, an inverse square law may be deduced. Can also use arc lamp and a photocell: find I = Kr^-2.

35n. Zeeman effect. A glass sample is placed between the poles of an electromagnet and a beam of light shone through holes bored in the pole pieces, crossed polaroids being placed at the light source and the end of the magnet from which the light is emerging. With no magnetic field, no light is seen in the analyzer; however, if the magnetic field is slowly increased, a progressively brighter spot will be seen. Frequently a laser is used as the light source. (In preparation)

35o. There is a modulated laser available (from 200 AJAH) that has the capacity for being modulated by any external signal. Video signals, or sound signals from (say) a record player may be used to modulate the device, a photomultiplier reconverting the signal after the light beam has passed across the lecture hall. Mirrors may be used to increase the path length, or optical fibers may be inserted in part of the path.