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Transcript

Maxwell’s Equations q S E dA εo B dA 0 Gauss's law electric Gauss's law in magnetism S dB E ds dt Faraday's law dE B ds μo I εo μo dt Ampere-Maxwell law •The two Gauss’s laws are symmetrical, apart from the absence of the term for magnetic monopoles in Gauss’s law for magnetism •Faraday’s law and the Ampere-Maxwell law are symmetrical in that the line integrals of E and B around a closed path are related to the rate of change of the respective fluxes • • • • • • • • Gauss’s law (electrical): The total electric flux through any closed surface equals the net charge inside that surface divided by eo This relates an electric field to the charge distribution that creates it Gauss’s law (magnetism): The total magnetic flux through any closed surface is zero This says the number of field lines that enter a closed volume must equal the number that leave that volume This implies the magnetic field lines cannot begin or end at any point Isolated magnetic monopoles have not been observed in nature q S E dA εo B dA 0 S • • • • Faraday’s law of Induction: This describes the creation of an electric field by a changing magnetic flux The law states that the emf, which is the line integral of the electric field around any closed path, equals the rate of change of the magnetic flux through any surface bounded by that path One consequence is the current induced in a conducting loop placed in a time-varying B • The Ampere-Maxwell law is a generalization of Ampere’s law • It describes the creation of a magnetic field by an electric field and electric currents The line integral of the magnetic field around any closed path is the given sum • dB E ds dt dE B ds μo I εo μo dt Maxwell’s Equation’s in integral form Q 1 A E dA eo eo A B dA 0 V dV Gauss’s Law Gauss’s Law for Magnetism d B d C E d dt dt A B dA Faraday’s Law d E dE C B d o Iencl oeo dt o A J eo dt dA Ampere’s Law Maxwell’s Equation’s in free space (no charge or current) A A E dA 0 Gauss’s Law B dA 0 Gauss’s Law for Magnetism d B d C E d dt dt A B dA d E d C B d oeo dt oeo dt A E dA Faraday’s Law Ampere’s Law Hertz’s Experiment • • • • • An induction coil is connected to a transmitter The transmitter consists of two spherical electrodes separated by a narrow gap The discharge between the electrodes exhibits an oscillatory behavior at a very high frequency Sparks were induced across the gap of the receiving electrodes when the frequency of the receiver was adjusted to match that of the transmitter In a series of other experiments, Hertz also showed that the radiation generated by this equipment exhibited wave properties – • Interference, diffraction, reflection, refraction and polarization He also measured the speed of the radiation Implication • A magnetic field will be produced in empty space if there is a changing electric field. (correction to Ampere) • This magnetic field will be changing. (originally there was none!) • The changing magnetic field will produce an electric field. (Faraday) • This changes the electric field. • This produces a new magnetic field. • This is a change in the magnetic field. An antenna Hook up an AC source We have changed the magnetic field near the antenna An electric field results! This is the start of a “radiation field.” Look at the cross section Accelerating electric charges give rise to electromagnetic waves Called: “Electromagnetic Waves” E and B are perpendicular (transverse) We say that the waves are “polarized.” E and B are in phase (peaks and zeros align) Angular Dependence of Intensity • This shows the angular dependence of the radiation intensity produced by a dipole antenna • The intensity and power radiated are a maximum in a plane that is perpendicular to the antenna and passing through its midpoint • The intensity varies as (sin2 θ / r2 Harmonic Plane Waves At t = 0 E l spatial period or wavelength l x l 2 l v fl T T 2 k At x = 0 E t T T temporal period Applying Faraday to radiation d B C E d dt C E d E dE y Ey dEy d B dB dxy dt dt dB dEy dxy dt dE dB dx dt Applying Ampere to radiation d E C B d oeo dt C B d Bz B dB z dBz d E dE dxz dt dt dE dBz o eo dxz dt dB dE o eo dx dt Fields are functions of both position (x) and time (t) dE dB dx dt Partial derivatives are appropriate B E o e o x t dB dE o eo dx dt E B 2 x x t 2 E B x t B 2E o eo 2 t x t 2E 2E o eo 2 2 x t This is a wave equation! The Trial Solution • The simplest solution to the partial differential equations is a sinusoidal wave: – E = Emax cos (kx – ωt) – B = Bmax cos (kx – ωt) • The angular wave number is k = 2π/λ – λ is the wavelength • The angular frequency is ω = 2πƒ – ƒ is the wave frequency The trial solution E E y Eo sin kx t 2E 2E o eo 2 2 x t 2E 2 k E o sin kx t 2 x 2E 2 E o sin kx t 2 t k 2Eo sin kx t oeo2Eo sin kx t 2 1 2 k oe o The speed of light (or any other electromagnetic radiation) l 2 l v fl T T 2 k 1 vc k o e o The electromagnetic spectrum l 2 l v fl T T 2 k Another look dE dB dx dt B Bz Bo sin kx t E E y Eo sin kx t d d E o sin kx t Bo sin kx t dx dt Eo k cos kx t Bo cos kx t Eo 1 c Bo k o e o Energy in Waves 1 1 2 2 u e0 E B 2 2 0 u e0 E 2 Eo 1 c Bo k o e o 1 2 u B 0 e0 u EB 0 Poynting Vector 1 S EB 0 EB E 2 c B 2 S μo μo c μo S cu • Poynting vector points in the direction the wave moves • Poynting vector gives the energy passing through a unit area in 1 sec. • Units are Watts/m2 Intensity • The wave intensity, I, is the time average of S (the Poynting vector) over one or more cycles • When the average is taken, the time average of cos2(kx - ωt) = ½ is involved 2 2 E max Bmax E max c Bmax I S av cu ave 2 μo 2 μo c 2 μo Radiation Pressure F 1 dp P A A dt Maxwell showed: U p c (Absorption of radiation by an object) 1 dU Save P Ac dt c What if the radiation reflects off an object? Pressure and Momentum • For a perfectly reflecting surface, p = 2U/c and P = 2S/c • For a surface with a reflectivity somewhere between a perfect reflector and a perfect absorber, the momentum delivered to the surface will be somewhere in between U/c and 2U/c • For direct sunlight, the radiation pressure is about 5 x 10-6 N/m2