Unit22b - Displacement Current.pdf - Physics 240 Fall 2017...

Physics 240 Fall 2017
Unit 22 – Displacement Current and
Electromagnetic Waves
Physics 240 Unit 22, Slide 1
• Prof. David Gerdes
• [email protected]
• Office hours: Tu 1-3pm in the
Physics Help Room

Countdown to end of semester
Physics 240 Unit 20, Slide 2
Quiz 6
Final Exam
7:30-9:30pm
Location TBA

d 


• Ampere-Maxwell Law: There are two
sources of magnetic field:
• Currents (moving charges)
• Changing electric fields
(displacement current)
• Both sources make B-field lines that
“curl” into closed loops.
• Note the appearance of
A big clue!
m
0
e
0
= 1/ c
2

dt
E d
J and E J note
A d J J I I l d B
D c
bounded
surf any
D c D c
path

  

 




 
 
     
 
1
:
) ( ) (
0 0
Displacement “Current”
(not actual charge flow)
E flux, Φ
E
is spread out over
the plate area. I
c
is
contained to the wire
Question: How does B inside the capacitor
depend on r? (r<R)
A. ~r B. ~r
2
C. ~1/r D. ~1/r
2

I
The diagram depicts a conduction current I charging a parallel plate capacitor.
Consider the contour (solid circle) drawn inside of the capacitor.
Two surfaces bounded by the contour are shown, the dashed line surface and
the shaded surface.
How is the displacement current
through the shaded surface related to
the conduction current in the wire?