Download Concept Questions with Answers W13D1: Displacement Current

W13D1:
Displacement Current,
Maxwell s Equations,
Wave Equations
Concept Questions with
Answers
8.01
W13D2
Today s Reading Course Notes: Sections 13.1-13.4
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Concept Q. Answer: Capacitor
Concept Question: Capacitor
If instead of integrating the magnetic field around the pictured
Amperian circular loop of radius r we were to integrate around an
Amperian loop of the same radius R as the plates (b) then the
integral of the magnetic field around the closed path would be
1. 
2. 
3. 
the same.
larger.
smaller.
Answer 2. The line integral of B is larger for larger r
As we increase the radius of our Amperian loop we
enclose more flux and hence the magnitude of the
integral will increase.
! !
!
d
B ! d s = µ0# 0 "" E ! n̂ da
dt
circle
disk
"
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Concept Question: Capacitor
Concept Q. Answer: Capacitor
Answer 2. The line integral of B is positive.
Consider a circular capacitor, with an Amperian circular loop
(radius r) in the plane midway between the plates. When the
capacitor is charging, the line integral of the magnetic field
around the circle (in direction shown) is
1. 
2. 
3. 
4. 
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There is no enclosed current through the disk. When
integrating in the direction shown, the electric flux is positive.
Because the plates are charging, the electric flux is increasing.
Therefore the line line integral is positive.
Zero (No current through loop)
Positive
Negative
Can t tell (need to know direction of E)
! !
!
d
B ! d s = µ0# 0 "" E ! n̂ da
dt disk
circle
"
5
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1
Concept Question: Capacitor
Concept Q. Answer: Capacitor
Answer 1. The charge Q is increasing in time
The figures above show a side and top view of a
capacitor with charge Q and electric and magnetic
fields E and B at time t. At this time the charge Q is:
The B field is counterclockwise, which means that the
if we choose counterclockwise circulation direction,
the electric flux must be increasing in time. So positive
charge is increasing on the bottom plate.
1.  Increasing in time
2.  Constant in time.
3.  Decreasing in time.
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