1. Applied Electricity and Magnetism
ECE 3318
Applied Electricity and Magnetism
Spring 2017
Prof. David R. Jackson
ECE Dept.
Notes 9

2. Flux Density Define: “flux density vector” From Coulomb’s law:
Charge in free space
We then have

3. Flux Through Surface Define flux through a surface:
In this picture, flux is the flux crossing the surface in the upward sense.
The electric flux through a surface is analogous to the current flowing through a surface.
Cross-sectional view

4. Current Analogy Analogy with electric current Cross-sectional view
Conducting medium
Analogy with electric current
A small electrode in a conducting medium spews out current equally in all directions.
Cross-sectional view

5. Current Analogy (cont.)
Free-space
Conducting medium

6. Each electric flux line is like a stream of water.
Water Analogy
Water nozzle
N streamlines
Water streams
Each electric flux line is like a stream of water.
Each stream of water carries
F / N [m3/s].
F = flow rate of water [m3/s]
Note that the total flow rate through the surface is F.

7. Water Analogy (cont.)
Here is a real “flux fountain” (Wortham fountain, on Allen Parkway).

8. Example S
Find the flux from a point charge going out through a spherical surface.
(We want the flux going out.)

9. Example (cont.)
We will see later that his has to be true for any surface that surrounds the charge, from Gauss’s law.

10. Flux through a Surface using Flux Lines
The flux through the surface is proportional to the number of flux lines Ns that cross the surface.

11. Flux in 2D Problems
The flux is now the flux per meter in the z direction.
We can also think of the flux through a surface S that is the contour C extruded h in the z direction.

12. Flux Plot (2D)
Rules:
Flux plot for a line charge

13. Please see the Appendix for a proof.
Flux Property
The flux (per meter) l through a contour is proportional to the number of flux lines that cross the contour.
NC is the number of flux lines through C.
Please see the Appendix for a proof.

14. Example Graphically evaluate C Note:
The answer will be more accurate if we use a plot with more flux lines!

15. Equipotential Contours
The equipotential contour CV is a contour on which the potential is constant.
Equipotential contours CV
Flux lines
Line charge example

16. Equipotential Contours (cont.)
Property:
( = constant )
The flux line are always perpendicular to the equipotential contours.
(proof on next slide)

17. Equipotential Contours (cont.)
Proof of perpendicular property:
Proof:
Two nearby points on an equipotential contour are considered.
On CV :
The r vector is tangent (parallel) to the contour CV.

18. Method of Curvilinear Squares
2D Flux Plot
“Curvilinear square”
Assume a constant voltage difference V between adjacent equipotential lines in a 2D flux plot.
Note: Along a flux line, the voltage always decreases as we go in the direction of the flux line.
Note:
It is called a curvilinear “square” even though the shape may be rectangular.

19. Method of Curvilinear Squares (cont.)
Theorem: The shape (aspect ratio) of the “curvilinear squares” is preserved throughout the plot.
Assumption: V is constant throughout plot.

20. Method of Curvilinear Squares (cont.)
Proof of constant aspect ratio property
If we integrate along the flux line, E is in the same direction as dr .
Hence, so
Therefore

21. Method of Curvilinear Squares (cont.)+ -
Hence,
Also, so
Reminder:
In a flux plot the magnitude of the electric field is inversely proportional to the spacing between the flux lines.
Hence,
(proof complete)

22. Summary of Flux Plot Rules
Rule 1: We have a fixed V between equipotential contours.
Rule 2: Flux lines are perpendicular to the equipotential contours.
Rule 3: L / W is kept constant throughout the plot.
If all of these rules are followed, then we have the following properties:
Flux lines are in direction of E.
The magnitude of the electric field is inversely proportional to
the spacing between the flux lines.

23. Example Line charge D l0 yW
Note how the flux lines get closer as we approach the line charge: there is a stronger electric field there.
The aspect ratio L/W has been chosen to be unity in this plot.
In this example L and W are both proportional to the radius .

24. Example A parallel-plate capacitor Note: L / W  0.5

25. Example Coaxial cable with a square inner conductor
Figure 6-12 in the Hayt and Buck book.

26. In the class project, you will be drawing in flux lines.
Example
In the class project, you will be drawing in flux lines.
Parallel-plate capacitor region

27. In the class project, you will be drawing in flux lines.
Example (cont.)
In the class project, you will be drawing in flux lines.
Parallel-plate capacitor region

28. Flux Plot with Conductors
Some observations:
Flux lines are closer together where the field is stronger.
The field is strong near a sharp conducting corner.
Flux lines begin on positive charges and end on negative charges.
Flux lines enter a conductor perpendicular to it.

29. Example of Electric Flux Plot
Note:
In this example, the aspect ratio L/W is not held constant.
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30. Example of Magnetic Flux Plot
Solenoid wrapped around a ferrite core (cross sectional view)
Flux plots are often used to display the results of a numerical simulation, for either the electric field or the magnetic field.
Magnetic flux lines
Solenoid windings
Ferrite core

31. Appendix:
Proof of Flux Property

32. One small piece of the contour
Proof of Flux Property
NC : flux lines
Through C
One small piece of the contour
(the length is L) so or

33. Flux Property Proof (cont.)
Also,
(from the property of a flux plot)
Hence, substituting into the above equation, we have
Therefore,
(proof complete)