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VI. Electromagnetic Waves

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VI. Electromagnetic Waves

All the important physics in electromagnetism can be expressed in four Maxwell’s Equations, the Lorentz force and conservation of charge with interesting consequences.

VI–1 Maxwell’s Equations

- Generalized Faraday’s Law.
- Generalized Ampère’s Law.
- Maxwell’s Equations.
- Production of Electromagnetic Waves.
- Electromagnetic Waves Qualitatively.

- From electrostatics we know that:
- From Faraday’s law we know that if flux in a circuit changes the induced EMF is equal to work done by electric field to transport unit charge one closed loop around the circuit.

- We can obtain this fact from:
- When we substitute for the EMF from the Faraday’s law we get general formula:
- The integration must be performed in the positive counterclockwise direction!

- The line integration can be performed along anyclosed path in anymaterialor of course even in vacuum.
- We consider change of flux through a surfacesurroundedby our integration path.
- When fluxchanges the electricfield is notconservative any more. Otherwise the integral along a closed path would be zero.

- Let’s have a magnetic field (with a non-zero component) perpendicular to the plane of drawing and flux through some fixed loop increases. Then:
- This means that the electric intensity must be clockwise, which is in accord with Lenz law.
- Note that we have information only about the change of the magnetic induction not about the direction of the original field!

- We have learned that lineintegral of magneticinduction over any closed path depends on the total current which is enclosed by this path.
- But is it valid generally?
- If we are charging a capacitor, experiment shows that magnetic field can be measured near to the capacitor as if a current went through it. But charge doesn’t pass through the capacitor!

- If a theory is in a contradictionwith the experiment it must be improved or changed!
- We have to accept that whateverhappens in the capacitor when it is being charged or discharged behaveslike a current because it producesmagneticfield. It must be a newtype of current, since it is definitely not movement of charges.

- It is, of course, the electricfield what changes in the charged (or discharged) capacitor. So we define a new type of current which we call a displacement or shiftcurrent, which we attribute to the change-in-time of the electric field, more accurately of the electric flux.
- Enclosingmeans the same as in the previous case of Faraday’s law:

- Up to now, when we used Ampere’s law we usually used a circular closed path and have taken into account the total current which passed the circular surface bounded by it.
- Generally, we can consider surface of any shape, bounded by the enclosed path. The currents, which would enter the surface but would not pass through our path would have to leave the surface somewhere so they would not be counted in.

- The fact that some of these surfaces can also pass between the plates of the capacitor means that what passes there through them must be equivalent to the electric current. And now since we are interested in an electric field passing through surfaces, we are, in fact interested in electric flux.
- Again we integrate in the positive sense.

- The existence of displacement current actually means an important symmetry between electric and magnetic fields since not only changes of magnetic fields produceelectric fields but also the changes of electric fields produce magnetic fields!
- Thanks to this symmetry electromagnetic waves and also WE do exist!

- The displacement current for a parallel plate capacitor can be calculated simply from the definition of its capacity and the current:
Q = CV = (0A/d)(Ed) = 0AE

I = dQ/dt = d(0AE)/dt = 0 de/dt

- This conclusion is valid generally so the Ampere’s law has a new term:

- If we consider the relation:
- We can finally write:

- For instance if we are charging a (parallel plates) capacitor from a power source V0 , through a resistor R. The current decreases exponentially from an initial value I0 = V0/R and :

- The magnetic field B outside the capacitor can be obtained from the Amperes law:
Which is exactly the induction we get close to a straight wire feeding the capacitor.

- Now we are ready to write the Maxwell’s equations.
- There are several types of them and several levels of generality. So exactly we shall deal with Maxwell’s equations in integral form valid for vacuum.

- The first equation is the Gauss law of electrostatics. It means that:
- Sources of electric field – chargesexist.
- If charges are present, the electric field lines begin in positive charges (or infinity) andend in negative charges (or infinity).
- The field of a single charge decreases as 1/r2.

- The second equation is the Faraday’s law of electromagnetic induction. It means that:
- The electric field can be produced also by time changes of magnetic field. This field is not conservative and its field lines are closed.
- In the absence of time-varying magnetic field the magnetic field is conservative and a scalarpotential exists.

- The third equation is the Gauss law of magnetism. It means that:
- Separate sources of magnetic field – magnetic monopolesdo not exist.
- The magnetic field lines are closed.
- The field of an elementary current decreases as 1/r2.

- The fourth ME is the generalized Ampères law. It means that:
- The magnetic field is produced either by electric currents or equivalently by time changes of electric field.
- The field lines are closed lines.

- ME plus the Lorentz force carry all the information on all electromagnetism.
- They have many important consequences.
- We are actually dealing with one electro-magnetic field but when electric and magnetic fields don’tchange in time the first two equations are not connected with the other two and electricity and magnetism can be treated separately.
- The most important outcome is the existence of electromagneticwaves.

- Important solution of ME is a planar wave. If it moves in positive x direction with a speed c, the field can be described as:
E = Ey =E0sin(kx - t)

B = Ez =B0sin(kx - t)

- E and B are in phase
- Vectors, , form right (hand) turning system
- wave number: k = 2/
- angle frequency: = 2f
- speed of the wave: c = f = /k

- Since changes of electric field produces magnetic field and vice versa these fields oncegenerated can continue to exist and spread into the space.
- This can be illustrated using a simple dipoleantenna and an AC generator.
- Planar waves will exist only far from the antenna where the dipole field disappears.

- No homework assigned today!

- This lecture covers
Chapter 29 – 7; 32 – 1, 2, 3, 4

- Advance reading
Chapter 32 – 5, 6, 7, 8, 9

- Try to understand the physical background and ideas. Physics is not just inserting numbers into formulas!