Il Museo di Fisica di Sardegna

	Interactive experiments and exhibits News Events F G C
	Introduzione Arago's disk Oersted's experiment Electromagnetic induction The Foucault current Electrodynamic actions Coils, solenoids and other things Measurement of a magnetic field Magnetic properties of materials Levitation experiment And if there's no variation in flow? The rotating magnetic field High frequency fields Photos	INTERACTIVE EXPERIMENTS AND EXHIBITS HIGH-FREQUENCY FIELDS This photograph shows a fluorescent lamp immersed in water with no wires. When placed in a microwave oven, it lights up. Although water is the best absorber of electromagnetic field energy at the frequency of 2.4 GHz, the field inside the oven is so intense that even when immersed in a mass of water, we have avalanche phenomena of the gas inside the bulb. Now let’s witness an extraordinary experiment. Click on the photo. Inside the oven we have placed a wooden toothpick that had been lit with a cigarette lighter. It still had some embers on its tip. The energy of the intense electromagnetic field inside the oven causes an avalanche of the air; balls of plasma form and float around in the oven. This phenomenon immediately suggests the the existence and cause of which are not generally accepted. Do you ever find yourself needing to destroy once and for all a compromising audio CD or CD-Rom? Click on the photo Innumerable other experiments with high-frequency fields can be made inside microwave ovens, but even outside of them. In this photograph we have a small high-tension generator with which we can obtain sparks. It is connected to an isolated wire (the red one). A radio is placed close to it. This is an experiment showing the transmission of signals over electromagnetic waves. The signal at the base of the antenna is shown in the photo below: we can see that it is alternating current at a very high frequency (about 2 MHz) which is greatly damped. What takes place is as follows: the signals travel through the isolated wires at the same speed as light. Thus on the antenna, at the base of which we have the tension mentioned above, there is a perturbation that travels up and down with the same frequency. In a certain sense this perturbation is equivalent to a special cloud of charges that move in an accelerated way. Thus we have irradiation. The fact that the wave is damped is the demonstration that a small amount of energy is lost through the ohm resistance of the antenna, but mostly by irradiation. LET’S MEASURE THE SPEED OF LIGHT WITH THE MICROWAVE OVEN The speed of an electromagnetic perturbation or a signal in bare wires immersed in air is equal to the speed of light c. By powering a line of two parallel wires with a sinusoidal radio frequency signal having known frequency f, there will be definite values for the length of the line for which there will be a system of stationary waves on it. From those values we can immediately deduce wavelength λ, and since in the time of a period T = 1/f the perturbation travels over a distance equal to a λ wavelength,the speed is: V=f^.λ To see it, click on the photo. The neon light comes on when along the line there is a system of stationary waves including a whole number of half wavelengths. The neon light shows maxima at intervals of 6.1 cm. Thus λ= 12,2 cm.. The frequency of the microwave oven is 2450 MHz (it’s written on the plate at the back) from which we have v = 2,98 ^. 10¹⁰ cm/s, which is close to the known speed of light. DETECTION OF THE MICROWAVE FIELD And now, the simplest detector of the very-high frequency of the electromagnetic field generated by the microwave oven: a dipole having a length of 6.1 cm (&lambda/2, a strip of card for printed circuits) a germanium diode with a contact tip and two small inductors (X_l =w^.L = 150 Ohm se L = 10 nH) to stop the radio frequency signal from being short-circuited by the measuring instrument. Click on the photo to see what happens The sensitivity of this system of detection is very high: about 100 μW at the bottom of the scale.