Prove the existence of electromagnetic wave by experiment It is measured that the speed of electromagnetic wave propagation is the same as that of light The electromagnetic wave is observed to be focused, straight forward, reflected, refracted andPolarization phenomenon It is proved that when atoms are excited by the impact of electrons and emit spectral lines, the energy is discrete
Hertz was born inGermanyhamburg A letter changeChristianityOfjudeafamily.His father was a consultant in Hamburg, and his mother was the daughter of a doctor.
In March 1885, Hertz transferred to the Karlsruhe Institute of Technology at the southwest border of Germany as a professor of the Department of Physics.He began to assemble his electrical laboratory and demonstrated electrical experiments in class.He said: "I don't believe that one can know the reality only by theory."
The experimental funds in the primary school were very small, but he built a precise electromagnetic laboratory bit by bit.Department higher educationTrigonometryProfessor Max Doll admired him very much and knew that this young man had a character that was not compared with others;He invited Hertz to come to his house and introduced his daughter, Elisabeth Doll.
Elisabeth later wrote: "Hertz has an almost proud self-confidence under the starlight. He thinks he is the only person in the world who knows what starlight is. In his opinion, starlight in the sky is different light bodies, regularly emitting different frequencieselectromagnetic waveCome to the ground... In his description, the starry night is not only beautiful, but also accurate. "Hertz's confidence is not wrong. In the 19th century, there were two people in the world who understood electromagnetic wave experiments best. One wasMichael Faraday (Michael Faraday), the other is Hertz.
Elisabeth doesn't understand electromagnetic waves, but she knows that the man who seeks the truth of science is sincere and frank.They married less than four months after they knew each other, when Hertz was 29.
Hertz found the destination of love and launched the most famous research in his life.Because of the success of this experimental research, Morris H. Shamos, a professor of the Department of Physics in New York, recalled that physicists in history, from Galileo to Einstein, believed that the greatest physical experimenter was Hertz.Hertz proved through experiments that the essence of light is electromagnetic wave, which is a mystery of mankind for thousands of years.
scientific research
In 1885, he obtainedUni Karlsruhe He is a professor and discovered electromagnetic waves there.In 1885, Gill University planned to promote Hertz to associate professor, but he was unwilling to obtain a pureTheoretical physicistPosition of.At this time,Universit t Karlsruhe Ready to give Hertz physicsprofessorPosition.Considering that the university has a goodInstitute of PhysicsSo he came to Karlsruhe University.At first, Hertz felt lonely in Karlsruhe and was not sure about his future research.But in the time that followed, Hertz accomplished two great things.In July 1886, after three months of proposing, Hertz married a colleague's daughter, Elizabeth Doll.Later, Hertz started and finally completed the electromagnetic wave experiment that brought him worldwide reputation.
autograph
Hertz at Berlin UniversityHelmholtzEncouraged by Helmholtz when studying physicsmaxwellAt that time, the German physics community was deeply convinced of Weber's theory that electricity and magnetic force could be transmitted instantaneously.So Hertz decided to prove Weber andMaxwell theoryWhose is right.According to Maxwell's theory, electric disturbance kinetic energy radiationelectromagnetic wave。Hertz will be generated according to the capacitor passing through the spark gapOscillation principle, designed a set of electromagnetic wavesGenerator, Hertz will be oneInduction coilThe two ends of are connected to the two copper rods of the generator.When the current of the induction coil is suddenly interrupted, the induced high voltage will cause spark between the spark gaps.Instantly, the charge oscillates between zinc plates through the spark gap, with a frequency of millions of weeks.According to Maxwell's theory, this spark should produce electromagnetic waves, so Hertz designed a simpledetectorTo detect this electromagnetic wave.He bent a small piece of wire into a circle, leaving a small spark gap between the two ends of the wire.Electromagnetic wave should be generated on this small coilInduced voltageAnd spark is generated in the electric spark gap.So he sat in a dark room, the detector was 10 meters away from the oscillator, and he found that there was a small spark between the electric spark gap of the detector.Hertz covered the wall at the far end of the darkroom with a zinc plate that can reflect radio waves,incident waveOverlapping with the reflected wave should produce standing wave, which is also separated by the detectoroscillatorDetect at different distances to confirm.Hertz first calculates the frequency of the oscillator, and then uses the detector to measure the wavelength of the standing wave. The product of the two is the propagation speed of the electromagnetic wave.As Maxwell predicted.Electromagnetic waves travel at the speed of light.In 1888, Hertz's experiment was successful, and Maxwell's theory gained great glory.Hertz pointed out in his experiment that electromagnetic waves can be reflected, refracted andvisible light、Thermal waveThe same waspolarization。The electromagnetic wave emitted by his oscillator is a planePolarized waveThe electric field is parallel to the conductor of the oscillator, while the magnetic field is perpendicular to the electric field, and bothvertical transmissionDirection.In a famous speech in 1889, Hertz clearly pointed out that light is a kind ofElectromagnetic phenomenon。The first time a message was transmitted by electromagnetic waves was in 1896ItalyOfMarconi Started.In 1901, Marconi successfully sent the signal toAtlanticAmerica on the other side.Twentieth CenturyRadio communicationIt has also made extraordinary development.Hertz experiment not only confirmed Maxwell'sElectromagnetic theoryAnd found a way for the development of radio, television and radar.With Michelson's experiment in 1881 and Michelson Morey's experiment in 1887 overturning the existence of light ether, Hertz rewrote Maxwell's equations and incorporated new findings into them.Through experiments, he proved that electrical signals can travel through the air as predicted by James Maxwell and Michael Faraday, which is the basis of the invention of radio.He noticed that when charged objects areultravioletIt will lose its charge quickly when irradiated. I foundphotoelectric effect(Later byAlbert EinsteinExplain)。
die young
On January 1, 1894, 37 year old Hertzsepticemiastaybonn die young.His nephew Gustav Ludwig Hertz isNobel PrizeWinner, Gustav's son Carl Helmos Hertz founded ultrasoundImaging medicine。
Commemorative Stamp[3]
In 1892, Hertz was diagnosed with infectionWegener granuloma(You will experience severe headache when you get sick)And he tried to cure the disease.On January 1, 1894, Hertz died in Bonn, Germany at the age of 37. After his death, he was buried in the Jewish cemetery in Ohlsdorf Hamburg.After Hertz died, he left his wife Elizabeth Hertz (original name: Elizabeth Doo) and two daughters Joanna and Mathilde.His wife did not remarry after his death.In the 1930s, when Hitler rose, his wife and two daughters also moved from Germany to England.In 1960, Charles Sa Scand visited Mathilde Hertz, asked about her father, and soon published a book about Heinrich Hertz.According to Charles Sa's book, Hertz's two daughters are not married, so he has no descendants.
Main contributions
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Hertz experiment
Hertz made great contributions to human civilization. Just when people expected more of him, he died of blood poisoning on New Year's Day in 1894 at the age of 37.In order to commemorate his achievements, people use his name to name the units of various fluctuation frequencies, called "He" for short.Hertz, tooInternational System of UnitsA unit of medium frequency, which is a measure of the number of periodic change repetitions per second.Hertz's name comes from German physicist Heinrich Rudolph Hertz.Its symbol is Hz.Electricity (voltage or current) can be divided into DC and AC.In communication applications, signal transmission is generallyalternating current。Sinusoidal ACelectrical signalWith the change of time, its amplitude is positive and negative, and propagates forward at a certain energy and speed.Usually, we put the abovesine waveThe number of repeated changes with amplitude within 1 second is called the "frequency" of the signal, which is represented by f;The time required to change the signal waveform once is called "cycle", which is represented by T and is expressed in seconds.The distance traveled by a wave for one cycle is called "wavelength", which is expressed in λ and is expressed in meters.f. T and λ have the following relationship: f=1/T, v=λ. f, where v iselectromagnetic waveOfpropagation velocity, equal to 3x10 ^ 8 meters/second.The unit of frequency is Hertz, which is referred to as Hertz, and is represented by the symbol Hz.
Hertz's laboratory[2]
H. Hertz is a famous German physicist. In 1887, he confirmed the existence of electromagnetic waves through experiments.In memory of him, later generations set "Hertz" as the unit of frequency.Common frequency units includeKilohertz（KHz）、Megahertz（MHz）、Gih（GHz）Etc.stayCarrier beltInformativeelectrical signalSometimes it contains multiple frequency components;Mark the position of all these components on the frequency axis,And indicate thatThe "spectrum" of the signal is the size of each component in power or voltage.What it occupiesfrequency range It's called signalingfrequency bandRange.For example, inTelephone communicationIn, the frequency range of voice signal is 300~3400 Hz;stayfrequency modulationIn (FM) broadcasting, the frequency range of sound is 40 Hz~15 kHz,Television broadcastingThe frequency range of the signal is 0~4.2 MHz, etc.
wave equation
Heinrich Rudolf Hertz first verified Maxwell's theory through experiments between 1886 and 1888.He proved that radio radiation has all the characteristics of waves and found thatelectromagnetic fieldThe equation can be usedpartial differential equation Expression, commonly referred to aswave equation 。On November 5, 1887, Hertz summarized this important discovery in a paper entitled "On Induction Phenomena Caused by Electrical Processes in Insulators" sent to Helmholtz.Then, Hertz confirmed that the electromagnetic wave istransverse wave, has similar characteristics to light, such as reflection, refraction, diffraction, etc., and has experimented with the interference of two rows of electromagnetic waves, and confirmed thatRectilinear propagationThe propagation speed of electromagnetic wave is the same as that of light, which fully verifies Maxwell'sElectromagnetic theoryIs correct.And further improvedMaxwell's equations, making it more beautiful and symmetrical, and obtaining the modern form of Maxwell's equations.In addition, Hertz did a series of experiments.He studiedultravioletyesSpark dischargeThe photoelectric effect is found, that is, the object will release electrons under the irradiation of light.This discovery was later established by EinsteinLight quantaTheoretical basis.In January 1888, Hertz summarized these achievements in the article "On the propagation speed of electrokinetic effects".Hertz experimentAfter the announcement, the world's scientific community was shocked.The electromagnetic theory initiated by Faraday and summarized by Maxwell has won a decisive victory until now.In 1888, it became modernHistory of ScienceA milestone on.Hertz's discovery has epoch-making significance. It not only confirms the truth of Maxwell's discovery, but also, more importantly, creates the radio electronic technologynew era。With Michelson's experiment in 1881 andMichelson Morey experimentOverthrewLight etherHertz rewrote Maxwell's equations to include new findings.Through experiments, he proved that electrical signals like James Maxwell andMichael Faraday The theory that it can pass through the air as predicted is the basis of the invention of radio.He noticed that when charged objects areUV irradiationIt will lose its charge soon when I findphotoelectric effectLater explained by Albert Einstein.
photoelectric effect
Light irradiation on certain substances causes changes in the electrical properties of substances.This kind of phenomenon of light calling changes is collectively calledphotoelectric effect。Metal surface is brightIrradiationThe effect of downward emitting electrons is calledphotoelectron。Light wavelength is less than a certaincritical valueThe electrons can only be emitted when the wavelength is limited, and the corresponding light frequency is calledLimiting frequency。The critical value depends onMetallic materialsAnd the energy of emitted electrons depends on the wavelength of lightlight intensityIt doesn't matter. This can't be used upVolatilityExplain.Another point contradicts the fluctuation of light, that is, the photoelectric effectInstantaneous, according to the volatility theory, ifincident lightWeak, the irradiation time should be longer, so that the electrons in the metal can accumulate enough energy and fly out of the metal surface.But the fact is that as long as the frequency of light is higher than the limit frequency of metal, no matter how strong or weak the brightness of light is, the generation of photons is almost instantaneous, not more than ten negative ninth power seconds.The correct explanation is that light must be composed of strictly specified energy units (i.e. photons or photons) related to wavelength.This explanation was put forward by Einstein.The photoelectric effect was discovered by German physicist Hertz in 1887Quantum TheoryIt plays a fundamental role. Under the irradiation of light, the phenomenon that electrons in objects are separated is called photoelectric effect.Photoelectric effect is divided intoPhotoelectron emission、Photoconductive effectandPhotovoltaic effect。The former phenomenon occurs on the surface of objects, also known asExternal photoelectric effect。The latter two phenomena occur inside the object, calledInternal photoelectric effect。In the photoelectric effect, the emission direction of electrons is not completely directional, but most of them are emitted perpendicular to the metal surface, independent of the light direction. Light is electromagnetic wave, but light isHigh frequency oscillationOrthogonal electromagnetic field,amplitudeVery small, it will not affect the direction of electron emission.hυ=（1/2）mv^2+I+W。Where (1/2) mv ^ 2 is the initial kinetic energy of the photoelectron detached from the object.There are a lot offree electronThis is the feature of metal, so for metal, item I can be omitted,Einstein equationIt becomes h ν=(1/2) mv ^ 2+W.If h νPhotomultiplier tube。The photomultiplier tube can convert each flash into magnifiedElectric pulseAnd then send it to the electronic circuit to record it.ArithmeticIn the Einstein wayQuantitative analysisThe following formula is used for photoelectric effect:Photon energy=The energy required to remove an electron+the kinetic energy of the emitted electron.Algebraic form: hf=φ+Em φ=hf0 Em=(1/2) mv ^ 2.Where h isPlanck constant, h=6.63 × 10 ^ - 34 J · s, f is the frequency of the incident photon, φ is the work function, the minimum energy required to remove an electron from the atomic bond, and f0 is the photoelectric effectthreshold Frequency, Em is the maximum kinetic energy of the emitted electron, and m is the maximum kinetic energy of the emitted electronStatic mass, v is the velocity of the emitted electron. If the energy (hf) of the photon is not greater than the work function (φ), no electron will be emitted.work functionSometimes it is marked with W.When this formula is inconsistent with the observation (that is, no electrons are emitted or the kinetic energy of the electrons is less than expected), it may be because the system is not completely efficient, and some energy is lost by turning into heat energy or radiation.Einstein was awarded 1921 for successfully explaining the photoelectric effectThe nobel prize in physics。
Contact mechanics
Contact mechanicsIt is a subject to study how the objects in contact with each other deform.In 1882, Hertz published the famous article on contact mechanics "AboutElastic solidOn the contact of elastic solids ", Hertz's original intention in this research is to understand how external forces cause changes in the optical properties of materials.In order to develop his theory, Hertz placed a glass ball on a prism. He first observed that the system formed an ovalNewtonian ring, soexperimental observation Hertz assumes that the pressure exerted by the glass ball on the prism is also elliptical.Then he decidedpressure distribution The displacement of the prism caused by the glass ball is calculated and the Newton ring is inversely calculated, which is then compared with the experimental observation to verify the correctness of the theory.Finally, Hertz obtained the contact stress andNormal directionLoading forceRadius of curvature, andModulus of elasticityRelationship between.Hertz's equation deals with fatigue, friction and the interaction between any contact bodybasic equation 。
The main disadvantage of Hertz contact theory is that it does not consider theBinding force。In 1971, K50. Johnson K. Kendall and AD. Roberts solved the problem. They put forward the JKR contact theory named after three people.In JKR theory, they considered theSurface energyDue to the existence of surface energy, a binding force will be introduced between the solids in contact with each other. Finally, according to the principle of energy balance, they get an equation describing the relationship between the contact stress distribution, the curvature radius of the contact body, the elastic modulus and the surface energy of the material.In the JKR model, when the surface energy is zero, the equation naturally transits to the Hertz equation.One of the preconditions for deriving the JKR model is that all interactions between two contact bodies occur within the contact radius. Later, it was proved that different conclusions would be obtained if different assumptions were adopted.In 1975, B.V. Derjaguin, V. M. Muller and Y. P. Toporov et al. assumed that the interaction between contact bodies could occur outside the contact radius, and based on this assumption, they proposed the so-called DMT model to try to consider the influence of binding force.According to JKR and DMT models, different (pull off) separation forces can be obtained (the maximum required to separate two contact bodiesForce）This different result has caused a lot of controversy. Finally, Muller et al. pointed out that JKR and DMT models have their ownScope of applicationThe JKR model describes the materials with large particles, high surface energy and low elastic modulus well.The DMT model is the opposite.
The theory of contact mechanics, which began with Hertz's pioneering work and was subsequently perfected by others, is one of the indispensable tools in various scientific and engineering studies involving contact bodies.Therefore, Hertz's contribution to the field of contact mechanics should not beelectromagneticsThe outstanding achievements in the field are ignored.
Hertz's main contribution is to use experiments to prove the existence of electromagnetic waves and measureElectromagnetic wave propagationThe speed of the electromagnetic wave is the same as that of light. It is further observed that the electromagnetic wave has the properties of focusing, straightforwardness, reflection, refraction and polarization.
(1) Hertz experiment to prove the existence of electromagnetic waves
Hertz is a student of Helmholtz. Under the influence and requirements of his teacher, he studied electromagnetic theory in depth.1879, GermanyBerlin Academy To ask the scientific community for informationmaxwellThe electromagnetic theory was experimentally verified, which prompted the young Hertz to germinateelectromagnetic waveThe ambition of the experiment.
Some Hertz experimental devices are as follows.AA 'is two 40 cm square copper plates welded with a diameter of 0.5 cm and a length of 70 cmCopper rod, connect a small copper ball on each head and place it relatively, leaving a gap of about 0.75 cm in the middle of the ball.The surface of the copper ball is carefully polished, and the two rods are respectively connectedInduction coilWhen the two ends of the rod are powered on, discharge occurs between the two rods to form oscillation.Then take a 2mm thick copper bar to make a ring with a radius of 35cm, such as B in.The gap f and width of the ring can be adjusted from a few tenths of a millimeter to a few millimeters with precision screw.When placed in an appropriate position, gap f will generate spark discharge along with AA ', and the spark can be 6-7 mm long.B ringCan revolve around the plane parallel to AA 'normalWhen mn rotates to different positions, the spark length of f discharge is different.When f is at a or a ', there is no spark at all;Turn a little, and sparks will start to appear;When turning to b or b ', the spark is maximum.
(2) Hertz measures electromagnetic wave speed
Hertz's most convincing experiment is to directly measureelectromagnetic waveThe propagation speed of.The device he uses is as follows: conductor AA '(Hertz calls it primary conductor) generates electromagnetic waves under the excitation of induction coil.The AA 'plane is perpendicular to the floor. In the figure, Hertz marks a baseline rs, and the following is the distance mark from the AA'Center point45 cm.
The experiment was conducted in a 15 × 14 meter large classroom without any furniture within 12 meters of the baseline.The whole room is blackened to observe the discharge spark.The secondary circuit is the circle C with a radius of 35cm or the square wire frame B with a side length of 60cm.
according tomaxwellIn theory, it has been known that the speed is about 300000 kilometers per second. It is very difficult to directly measure such speed.Hertz remembered that his teacher Kundt usedstanding wavemeasuresound velocityWe designed a plan ingeniously.He pasted a piece of zinc foil 4 meters high and 2 meters wide on the wall of the classroomGas pipeline, water pipe, etc., so that electromagnetic waves are reflected on the wall.Forward waveandreflected waveThe superposition results will form standing waves.according towave theory , the pitch of standing wave is equal toHalf wavelengthThe wavelength can be known by measuring the position of the node.Hertz moves along baseline rsDetection coilAs expected, the length of spark gap is different at different positions.Some places are the strongest. This isAntinode；Some places are the weakest, even without sparks. This isNodal。
according tocapacitorObtained from the oscillation theory HertzElectromagnetic oscillationPeriod of.The period can also be calculated from the assumption that the speed of light is the speed of electromagnetic waves and the measured wavelength, with a difference of about 10%. Hertz confirmed that the speed of electromagnetic waves is the speed of light.
(3) The electromagnetic wave is observed to be focused, straight forward, reflected, refracted andpolarizationphenomenon
In order to further investigate the properties of electromagnetic waves, Hertz designed a series of experiments, including focusing, straightforwardness, reflection, refraction andpolarization。He bends a 2m long zinc plate into a parabolacylinderThe focal length of the cylinder is about 12.5 cm.He firedVibratorAnd the receiving vibrator are respectively installed on the focus line of two cylinders, and the induction coil is adjusted to generate the transmitting vibratorelectric spark。When the two cylinder faces directly, the receiving vibrator will also send sparks;It is proved that electromagnetic waves have the same focusing and straightforwardness as light waves.Hertz also uses a 1.5m high, 500kg blockasphaltmakeprismLet the electromagnetic wave pass through. Like light, the electromagnetic wave also refracts.He measuredMinimum deflection angleIs 22 °, and the top angle of the prism is 30 °, from which the asphalt'sRefractive indexIt is 1.69.He also used "metal grid" to showelectromagnetic wavePolarization.
On December 13, 1888, he gave a report entitled "On Electric Radiation" to the Berlin Academy of Sciences. He fully confirmed the identity of electromagnetic waves and light waves with sufficient experimental evidence.He wrote: "I think these experiments effectively eliminate the lightRadiant heatAnd electromagnetic waves. "
