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Electromagnetic Radiation

  • E2-49: PULSAR MODEL - RADIOWAVES

    E2-49
    Show the changing field pattern from a rotating dipole.
    Position antenna so that the electric field from the transmitter picked up by the receiving antenna lights the lamp on the antenna. Then rotate the transmitting antenna with its stand, causing the light to turn on and off, dependent on the dipole radiation pattern of the antenna.

    e2-49a

  • H4-51: MODULATION - AM AND FM

    H4-51
    Demonstrate AM and FM signal modulation as an introduction to vibrato and tremolo.
    The Pasco Dual Function Generator is used to produce either amplitude modulation or frequency modulation using various combinations of sine, triangular, and square waves. Frequency modulation is pure vibrato and amplitude modulation is pure tremolo; actual vocal vibrato is a combination of pure vibrato and pure tremolo.
    H4, ME2

    h4-51ah4-51b

  • H4-52: SPECTRUM ANALYSIS OF MODULATION

    H4-52
    Compare and analyze the frequency spectra of various modulated sounds such as tremolo, vibrato, and beats.
    Using the Pasco Dual Function Generator, a 1000 Hz sine wave is modulated by a 100 Hz sine wave and the spectrum of the modulated signal displayed using the spectrum analyzer. The photograph at the center shows the original 1000 Hz sine wave and the photograph at the right shows the case where that wave is amplitude modulated by a 100 Hz sine wave, producing a beat-like wave and a spectrum that has two sidebands around the 1000 Hz carrier. Amplitude modulation, frequency modulation, or double sideband modulation (sometimes called balanced modulation, or ring modulation with synthesizers) can be used. Two sine waves can be added together using the Dual Function Generator to produce beats, and the spectrum of the beats obtained and compared with that of double sideband modulation. The waveform is displayed on one trace and the spectrum on the other.

    Try out some frequency combinations ahead of time, then have students predict the results.
    H4, ME2, ME3

    h4-52ah4-52b

  • H4-58: MODULATION - AM RADIO

    H4-58
    Show amplitude modulation in in AM radio signals.
    The amplitude modulated signal from an intermediate stage in the portable radio is viewed using an oscilloscope. You may leave it freely running or you may freeze the trace using the oscilloscope stop button in order to obtain the clearest picture for your particular need. Please turn off radio when done to save battery.

    Please handle with care, as discrete-component radios are hard to come by!
    J2B, ME2, ME3

  • I2-06 THERMOPILE WITH AUDIO OSCILLATOR

    I2-06
    Observe infrared radiation
    The output from a commercial thermopile is connected to an audio oscillator (as in N1-05) such that the frequency of the oscillator is proportional to the temperature observed: the hotter the object the higher the pitch. Use various sources: ice, boiling water, liquid nitrogen, the floor, people, etc. This is only qualitative; the system is not calibrated.
    N1, I2, PS1

  • I2-07: THERMOPILE WITH DVM

    I2-07
    Observe infrared radiation.
    The output from a commercial thermopile is connected to a digital voltmeter where the voltage is proportional to the temperature observed: the hotter the object the higher the voltage. Use various sources: ice, boiling water, liquid nitrogen, the floor, people, etc. This is only qualitative; the system is not calibrated.
    N1, ME2, I2, PW1

  • I2-08 RADIATIVE HEAT TRANSFER

    I2-08
    Shows radiation from a hot object
    As more voltage is applied to the heater it glows more brightly and emits more heat
    I2, PW1

  • I2-12: RADIATION FROM COLD OBJECT

    I2-12
    Show radiation from a cold object
    If you put a hot object at the focus of one of the concave parabolic mirrors and a thermal probe at the focus of the other mirror, heat from the hot object will heat up the probe, yielding a temperature rise of the thermometer. (Compare the top and center pictures above.) If you put something very cold at the first focus, the temperature will drop. (Compare the top and bottom pictures above.) This demands a rather different explanation - blackbody radiation emitted by all objects - than the rather simple explanation given in the case of the hot object.

    This experiment demands the proper explanation in terms of blackbody radiation emitted by all objects, not just "hot" objects. The historical struggle of physicists to deal with this is documented in an interesting article by Hasok Chang, Lecturer in Philosophy of Science at University College, University of London, entitled Rumford and the Reflection of Radiant Cold: Historical Reflections and Metaphysical Reflexes, in Physics in Perspective Volume 4 Issue 2 (2002), pp 127-169.

    Note that this experiment uses materials from I5-51 and L3-16. If you want to use those demonstrations in the same class, be sure to discuss logistics with Lecture-Demonstration staff in advance.
    I2, I0, I5, L3

    I2-12A

  • J3-23 FARADAY CAGE - RADIOWAVES

    J3-23
    Demonstrates that radio waves cannot penetrate a Faraday cage
    The radio is tuned to a good station so that everyone can hear. Lowering the screen Faraday cage over the radio stops the radio waves, and the sound of the radio ceases.
    K8

  • K2-63: DISPLACEMENT CURRENT MODEL

    K2-63
    Illustrate the geometry for displacement current

    This is a model of the classic displacement current experiment described in general physics textbooks. Displacement current is sensed as oscillating magnetic field between the plates of the capacitor. The oscillator is set to about 15 kHz, and tuned to give the maximum displacement current. Evidence of the displacement current is the existence of an azimuthal magnetic field between the capacitor plates. This is sensed by observing the EMF induced by inserting the search coil (from K2-27) radially into the capacitor with the coil oriented vertically (photo at left). Holding the search coil in the capacitor parallel to the plates should produce considerably less pickup

    This device can be used to demonstrate in three dimensions the geometry of the displacement current experiment. There is some discussion whether the actual pickup displayed is due to displacement current or simply some sort of general electromagnetic pickup, that is obviously filling the area.
    K2, ME2, ME3

  • K7-45: LOW AND HIGH PASS FILTERS

    K7-45
    Demonstrate use of a series RL circuit as a low-pass filter and a series RC circuit as a high-pass filter.
    The circuits above have been mounted on a plastic base for use with an overhead projector. Values of circuit components are shown on the projectual. In each case the input signal is input into the upper trace of the scope and the filtered signal into the lower trace. Complex waves can also be input to see how removal of some harmonics affects the wave shape.

    I: Low-pass filter with cutoff frequency f=R/L: The signal is input to the series RL circuit and the output is taken across the resistor.

    II: High-pass filter with cutoff frequency f=1/RC: The signal is input into the series RC circuit and the output is taken across the resistor.

    In this demonstration the experimental crossover frequency is about 5 kHz, so the biggest change in the amplitude of the signal is between 1 kHz and 10 kHz. In the sequences of photographs above the effect of the low pass filter (first sequence) and the high pass filter (second sequence) are shown at 1 kHz and 10 kHz respectively.
    K7, ME2, ME3

  • K7-61: TESLA COIL

    K7-61
    Demonstrate a tesla coil, including how magnetic induction and a resonant RLC circuit is used in the production of high-voltage high-frequency sparks.
    Our Tesla coil, circuit above, uses a 5000 volt transformer to charge a large oil capacitor. When the potential across the capacitor reaches the breakdown potential of the spark gap, breakdown across the gap occurs. The spark gap then becomes a conducting part of the RLC circuit, which resonates at a frequency of about 200 kilohertz. The large coil in the resonant circuit is the primary of the final transformer and the very fine coil is the secondary, producing about 200,000 volts at 200 kilohertz.
    Identify the components of the coil from the close-up of the figure at the right above.
    In the photograph the wires known as JACOB'S LADDER have been attached to the output terminals of the Tesla coil. A fluorescent light held by one end with the other end near the secondary coil will light by induction.
    DANGER: THE LOW VOLTAGE SECTIONS OF THIS DEVICE HAVE LETHAL CURRENTS.

  • 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

  • K8-03 LIGHT NANOSECOND

    K8-03
    Shows the distance light travels in one nanosecond
    This stick of wood is slightly less than 30cm. This length is the distance light travels in one billionth of a second
    K8

  • K8-04: SPEED OF LIGHT

    K8-04
    Measure the speed of light

    A laser light pulse a few nanoseconds long is emitted by a light-emitting diode and immediately strikes a partially silvered mirror. The reflected light returns to a phototransistor to give the first pulse of the oscilloscope trace shown in the picture. The light transmitted through the partially silvered mirror reflects off a distant front-surface mirror, in the foreground of the picture, returning to the phototransistor to create the second pulse. The distance the laser beam travels, measured using a metric tape, and the elapsed time, determined from the oscilloscope trace, are used to calculate the speed of light.

    Please Note: Care must be used to position the light reflected from the distant mirror onto the phototransistor. This needs to be carefully monitored while in use, as environmental changes can affect it. Also, if there are issues of signal stability, check the trigger level on the oscilloscope to confirm that it is compatible with the trigger signal.

    K8, ME2

    K8-04A

  • K8-05: ELECTROMAGNETIC PLANE WAVE MODEL

    K8-05
    Illustrate the geometry of a polarized electromagnetic plane wave.
    The model illustrates a polarized electromagnetic plane wave, expanding on the simpler version of demonstration K8-01.
    K8

  • K8-11: MICROWAVES - STANDING WAVES

    K8-11
    Demonstrate microwave standing waves.
    A standing wave is set up between the source and an aluminum sheet. The receiving antenna can be moved back and forth between the transmitter and the reflector to locate nodes (bottom) and antinodes (top). The wavelength is 12 cm, so the nodes (or antinodes) are 6 cm apart.
    K8

  • K8-12: RADIOWAVES - STANDING WAVES

    K8-12
    Demonstrate standing waves using radio waves.
    This demonstration uses a simple low-power RF transmitter with vacuum tubes, of a type developed by P. E. Klopsteg. A handheld dipole antenna with an indicator lightbulb at the center serves as the receiver. Set transmitter so that the antenna is parallel to the blackboard. (The blackboard is made of a painted metal sheet.) Hold the receiving antenna parallel to the transmitting antenna between you and the transmitter, and walk up the steps in the lecture hall. The bulb will produce maximum glow about every two meters, illustrating standing waves. This demonstration is only usable in classrooms PHY1410 and 1412.
    FS1

  • K8-13: RADIOWAVES - LECHER WIRE STANDING WAVES

    K8-13
    Illustrate standing electromagnetic waves in a high-impedance transmission line.
    Slide the fluorescent tube along the wires as shown. It will light up at the antinodal regions and remain dark at the nodal points of the standing wave along the transmission line.

  • K8-14: RADIOWAVES - STANDING WAVES IN AN ANTENNA

    K8-14
    Show standing waves in a dipole antenna.
    Electrons oscillate in a half-wave dipole antenna such that a standing wave is set up along the antenna with voltage antinodes at the ends of the antenna and a current antinode at the center. A fluorescent lamp can be used to sense the distribution of potential along the dipole, as seen in the photograph. Touch one end of the tube gently on the antenna while holding the other end. The bulb will light near the voltage antinodes.