电磁振荡和电磁波 Chapter 16 Electromagnetic Oscillation and Waves

§16-4 Energy in Electromagnetic Waves 电磁波的能量 §16-3 The Properties of Electromagnetic Waves Electromagnetic 电磁波的基本性质 §16-2 Generation & Radiation of an Electromagnetic Wave 电磁波的产生和辐射 §16-1 LC Circuit & LC Oscillations 振荡电路 电磁振荡 §16-5 Electromagnetic Spectrum ※ 电磁波譜 ※

§16-1 LC Circuit & LC Oscillations 1. LC circuit consists of an inductor L and a capacitor C, which can produce an electric field and magnetic field varying with the time. L C K

2. LC oscillations L C C and L: charge, electric field, electric current, magnetic field,….. All are varying with the time. --electromagnetic oscillations Assume the initial conditions: I=0, q=q 0 at t=0. oscillator 2. LC oscillating quation

L C Similar to simple harmonic motion ： q  x. At time t,

 The charge on the C oscillates with t: The current in the circuit too: The electric energy in C and the magnetic energy in L:

Frequency and period L C Can LC circuit oscillate forever? No. there is the resistance in which the thermal energy will produce.

3. Damped oscillation and forced oscillation in a RLC circuit L C RLC circuit Forced oscillation L C ~

L ~ Electromagnetic resonance

Magnetic field Electric field Can the above circuit produce the electromagnetic wave? L ~

§16-2 Generation & Radiation of the Electromagnetic Wave I L C +q -q The E-field and M-field produced by LC oscillator are radiated in the space to form electromagnetic wave.

The source of EM-wave is an oscillating electric dipole. Electriv dipole antenna + q - q P l

                       

Varying E-field induces a vaying M-field, and varying M-field induces a varying E-field… They propagate through the space from one region to another to form electromagnetic wave. Electric field: magnetic field: Oscillating quantities EM Wave

+ q - q P complicated §16-3 The Properties of Plane electromagnetic waves 1. EM wave produced by Electric dipole simple Plane Waves Far region In the far region: E0E0 H0H0

characteristics (2) (3) (4) and are perpendicular to each other. Their directions satisfy “right hand rule”. + q - q P (1) They can be seen as plane waves when r>>

2. The basic properties of EM waves: (1) EM wave is transverse wave (2) E and H have same phase at any point of the space.

（ 3 ） E and H are not independent. Their relation is （ 4 ） the speed of EM wave vacuum

§16-4 Energy in EM Waves 1. Energy in EM waves Energy density ( 能量密度） :  V=1

In the vector form Energy flux density( 能流密度 )S ： (magnitude) Direction:propagating Poynting vector

The intensity of EM wave: 2. The radiation average power( 功率 ) of electric dipole:

§16-5 The Electromagnetic spectrum