Maxwell Equations: Electromagnetic Waves

Size: px

Start display at page:

Download "Maxwell Equations: Electromagnetic Waves"

Beverly McCoy
6 years ago
Views:

1 Maxwell Equations: Electromagnetic Waves Maxwell s Equations contain the wave equation The velocity of electromagnetic waves: c = x 10 8 m/s The relationship between E and B in an EM wave Energy in EM waves: the Poynting vector x z y 1

2 The equations so far... Gauss Law for E Fields Gauss Law for B Fields Faraday s Law Ampere s Law 2

3 A problem with Ampere s Law Consider a wire and a capacitor. C is a loop. Time dependent situation: current flows in the wire as the capacitor charges up or down. 3

4 Maxwell s Displacement Current, I d 4

5 Calculating Displacement Current Consider a parallel plate capacitor with circular plates of radius R. If charge is flowing onto one plate and off the other plate at a rate I = dq/dt what is I d? The displacement current is not a current. It represents magnetic fields generated by time varying electric fields. 5

6 Calculating the B field Example 6

7 Maxwell s Equations (1865) in Systeme International (SI or mks) units7

8 Maxwell s Equations (Free Space) Note the symmetry of Maxwell s Equations in free space, when no charges or currents are present We can predict the existence of electromagnetic waves. Why? Because the wave equation is contained in these equations. Remember the wave equation. h is the variable that is changing in space (x) and time (t). v is the velocity of the wave. 8

9 Review of Waves from Mechanics The one-dimensional wave equation: has a general solution of the form: A solution for waves traveling in the +x direction is: 9

10 Four Step Plane Wave Derivation Step 1 Assume we have a plane wave propagating in z (i.e. E, B not functions of x or y) Example: Step 2 Apply Faraday s Law to infinitesimal loop in x-z plane x E x E x x y B y z 1 z 2 Z z 10

11 Four Step Plane Wave Derivation Step 3 Apply Ampere s Law to an infinitesimal loop in the y-z plane: x E x Z y B y z 1 z 2 B y y z Step 4: Use results from steps 2 and 3 to eliminate B y 11

the measured values for µ 0 & ε 0, we get: This value is identical to

12 Velocity of Electromagnetic Waves We derived the wave equation for E x : The velocity of electromagnetic waves in free space is: Putting in the measured values for µ 0 & ε 0, we get: This value is identical to the measured speed of light! We identify light as an electromagnetic wave. 12

13 Electromagnetic Spectrum ~1850: infrared, visible, and ultraviolet light were the only forms of electromagnetic waves known. Visible light (human eye) 13

14 Electromagnetic Spectrum 14

Sunscreen Absorbs UV http://upload.wikimedia.

15 Sunscreen Absorbs UV _Vis_Sunscreen.jpg/800px-UV_and_Vis_Sunscreen.jpg 15

16 What a Bee Sees: 16

17 Wien s Displacement Law 17

18 How is B related to E? We derived the wave equation for E x : We could have derived for B y : How are E x and B y related in phase and magnitude? Consider the harmonic solution: where 18

19 E & B in Electromagnetic Waves Plane Wave: where: x z y The direction of propagation is given by the cross product where are the unit vectors in the (E,B) directions. Nothing special about (E x,b y ); eg could have (E y, -B x ) Note cyclical relation: 19

Energy in Electromagnetic Waves Electromagnetic

We know the energy density stored in E and B fields:

2c µ 0 2 The total energy density in an EM wave = u,

average power (P av =u av /Δt) transmitted per unit

20 Energy in Electromagnetic Waves Electromagnetic waves contain energy. We know the energy density stored in E and B fields: In an EM wave, B = E/c u B = 1 E = 1 ε E = u c µ 0 2 The total energy density in an EM wave = u, where 2 E The Intensity of a wave is defined as the average power (P av =u av /Δt) transmitted per unit area = average energy density times wave velocity: For ease in calculation define Z 0 as: 20

The Poynting Vector The direction of the

given by: This energy transport is defined

direction of propagation of the wave The

energy being transported by the wave The

21 The Poynting Vector The direction of the propagation of the electromagnetic wave is given by: This energy transport is defined by the Poynting vector S as: S has the direction of propagation of the wave The magnitude of S is directly related to the energy being transported by the wave The intensity for harmonic waves is then given by: 21

22 Characteristics z x S 22

23 Summary of Electromagnetic Radiation combined Faraday s Law and Ampere s Law time varying B-field induces E-field time varying E-field induces B-field E-field and B-field are perpendicular energy density Poynting Vector describes power flow E units: watts/m 2 B S 23

Maxwell s Equations & Electromagnetic Waves. The Equations So Far...

Maxwell s Equations & Electromagnetic Waves Maxwell s equations contain the wave equation Velocity of electromagnetic waves c = 2.99792458 x 1 8 m/s Relationship between E and B in an EM wave Energy in