Electricity & Optics

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1 Physics Electricity & Optics Lecture 21 Chapter 30 sec. 1-4 Fall 2012 Semester Matthew Jones

2 Question An LC circuit has =100 and =100. If it oscillates with an amplitude of 100 mv, what is the amplitude of the current? (a) 100 (b) 100 (c) 10 (d) 10

3 Question The energy bounces back and forth between the capacitor and the inductor. = 1 2 () = 1 2 = = = 10 =10 =

4 Maxwell s Equations (so far)!" $ % &'= ( )!*)&+, - (Gauss s law)!". % $ &l &' =- = & & 1!".&' % (Gauss s law for magnetism) (Faraday s law). &l =2 - (Ampere s law)

5 The Problem with Ampere s Law Current through 3 is Current through 3 is 0! The current through 3 is zero, but the electric flux is not zero. The electric flux changes as charge flows onto the capacitor.

6 Maxwell s Displacement Current We can think of the changing electric flux through 3 as if it were a current: 67 4 =5 68 = : ;< 6 = >?. &l =2 - + & =2 - A+2 -, - & & 9 $!" % B &'

7 Calculating Displacement Current Parallel plate capacitor Gauss s law: (Lecture 5) : = C 5 = D 5 Electric flux: 7 =:= D 5 Change in electric flux: = 1 6D 5 68 = 5 Displacement current: 4 = =

8 In Lecture 8 we calculated the electric field between the inner and outer conductors using Gauss s law: Coaxial Capacitor? ;< : = 6= D EFGE4 5 We choose 3to be a cylindrical Gaussian surface of length and radius Hwith I <H<I. But the left-hand-side is just 7 so the displacement current is 67 4 =5 68 =6D EFGE4 68 The real current flows into the ends of the Gaussian surface, the displacement current flows out the sides.

9 Clicker Question Suppose charge flows between two spherical conductors. Which surface will have the largest displacement current flowing through it? (a) (b) (d) (c)

10 Clicker Question Suppose charge flows between two spherical conductors. Which surface will have the largest displacement current flowing through it? (a) (b) (d) 67 4 =5 68 =6D EFGE4 68 (c)

11 Maxwell s Equations (1864)?!" $ % &'= ( )!*)&+, -?!". %? $ &l?. &l &'=- = &K L & = M - &K + &

12 Maxwell s Equations in Free Space In free space where there are no electric charges or sources of current, Maxwell s equations are quite symmetric:?!" $ %?!". %? $ &l &'=- &'=- = &K L &?. &l =2 - M - &K + &

13 Maxwell s Equations in Free Space? $ &l?. &l = &K L & =2 - M - &K + & A changing magnetic flux induces an electric field. A changing electric flux induces a magnetic field. Will this process continue indefinitely?

14 Light is an Electromagnetic Wave Maxwell showed that these equations contain the wave equation: N O NP = 1 N O Q N8 where O P,8 is a function of position and time. General solution: O P,8 =S P Q8 + S (P+Q8) Harmonic solutions: O P,8 =O sin WP±Y8 Wavelength, Z=2[/W and frequency S =Y/2[. Velocity, Q =Z S =Y W.

15 Light is an Electromagnetic Wave Faraday s Law: : 6l ^ = 4_` 4a = bc d ba P f? : 6l ^ =: g f ΔP : g (f )ΔP x bj k bl f P E x E x ΔP z 1 z 2 z y B y Δf

16 Light is an Electromagnetic Wave Ampere s law: m 6l ^ = 5 4_ n 4a = 5 bj k ba O f? m 6l ^ =m o f ΔO m o (f )ΔO bc d bl f O y B y x E x f z 1 z 2 B y O z

17 Putting these together N: g Nf = Nm o N8 Nm o Nf = N: g 5 N8 Differentiate the first with respect to f: N : g Nf = N m o NfN8 Differentiate the second with respect to 8: N m o NfN8 = N : g 5 N8 p B $ q pr B =2 p B $ q -, - p B

18 Velocity of Electromagnetic Waves N O NP = 1 N O Q N8 p B $ q pr B =2 p B $ q -, - p B Speed of wave propagation is Q = 1 5 = 1 4[ 10 u v/ /v =B.yyz {- z /} (speed of light)

19 The electromagnetic spectrum In 1850, the only known forms electromagnetic waves were ultraviolet, visible and infrared. The human eye is only sensitive to a very narrow range of wavelengths:

20 Discovery of Radio Waves

21 The Electromagnetic Spectrum

22 Electromagnetic Waves p B $ q pr B =2 p B $ q -, - p B A solution is : g f,8 =: sin Wf Y8 where Y =W~ =2[~/Z What is the magnetic field? Nm o N8 = N: g Nf = W: cos Wf Y8 m o P,8 = W Y : sin WP Y8

23 Electromagnetic Waves Px z f y O :, mand Q are mutually perpendicular. In general, the direction is =: m

24 Energy in Electromagnetic Waves Energy stored in electric and magnetic fields (lecture 17): = : = 1 m 2 For an electromagnetic wave, m =: ~ =: 5 = 1 m = : = The total energy density is = + =5 :

25 Intensity of Electromagnetic Waves Intensity is defined as the average power transmitted per unit area. Intensity = Energy density wave velocity =5 ~ : = : ~ ~ =377 Ω Š (Impedance of free space)

26 PoyntingVector We can construct a vector from the intensity and the direction =: m : 3= : m 3 = : = Š This represents the flow of power in the direction Average electric field: : G =: / 2 3 = : Units: Watts/m 2 2Š

27 Question The magnetic field component of a radio wave is expressed: m o =m sin Wf Y8 In which directions do the Poyntingvector and the :-field point? (a) (b) (c) (d) (e) +z,+x +z,-x -z,+x -z,-x +y,+x

Maxwell Equations: Electromagnetic Waves

Maxwell Equations: Electromagnetic Waves Maxwell s Equations contain the wave equation The velocity of electromagnetic waves: c = 2.99792458 x 10 8 m/s The relationship between E and B in an EM wave Energy