Transcript Electric Field Lines

There is a region near the origin of a coordinate system that you are told
contains charges. You are not able to see them but have been asked to
learn as much about them as possible. In your experimentation you place a
test charge at three positions a distance d from the origin and determine the
direction of the electric field. Along the positive x axis the field points in
the +x direction. Along the negative x axis it also points in the +x
direction. On the y axis however it points in the negative
x direction. Consider this result and suggest what you might be able to say
about the charges near the origin.
●It would seem from this information that there are at least two charges near
the origin, one positive and one negative.
●It may be said that there is a positive and a negative charge present in the
cluster. The positive charge would be closer to the +x direction than the
negative charge. This would explain the direction of the field staying
consistent in the +x direction on either side of the axis. Along the y axis the
field would be moving in the -x direction because the electric field line from
the positive charge on the right would be moving towards the negative
charge on the left.
●It could be two charges right next to each other. There would be a negative
charge on the left side, and a positive charge on the right. This would cause
a test charge on the right to move right, away from the positive charge, and
the test charge on the left to be attracted towards the positive charge. The
test charge placed on the y axis would be pushed left, because it is slightly
attracted by the negative charge, and slightly repelled by the positive one.
The y-coordinate's would cancel out.
●I believe that the only way for this to be possible is for there to be equal
numbers of positive and negative charges ….
Electric Field Lines
• The direction of the Electric field is the direction
the force on a positive test charge would be at a
given point if a charge were present.
• The direction of the Electric field lines show the
direction of the field.
• The density of the field lines tells how strong the
field is (how much force a positive test charge
would feel)
• Field lines always start and end on a charge.
Which is a valid representation of field lines when no charges
are present in the viewed area?:
e) More than one are valid
You have an insulating rod with small spheres at each end. On one sphere
you place a charge Q and on the other a charge -Q. This rod is placed into
a uniform electric field in the +x direction. It is placed such that the midpoint
of the rod is fixed and cannot move but the rod can rotate about that
point. Briefly describe how the rod might respond if positioned at some
random orientation and released.
● “assuming it is a positively charged field”
● The rod should rotate clockwise until the rod is aligned along the y-axis.
● The rod would rotate because the sphere with the -Q charge would be pulled
towards the +x direction and the +Q sphere would be repelled from the +x
direction.
● The positive Q end would point towards the direction of the field lines. That is,
if the -Q end were the back of an arrow, and the +Q was the head, the arrow
would point the same way as the field lines.
This configuration is called an electric dipole
An electrically neutral dipole is placed in an external field. In which situation(s) is
the net force on the dipole zero?
E) More than one of the above is correct
Electric dipole moment

p  2aq
Where 2a is the separation between the –q and the +q.
The direction of p is from the negative charge to the
positive charge.
An electric dipole will feel a torque if there is a
component of p perpendicular to the electric field.
a
z

r
• What is the Electric field at a distance z
along the z axis due to a disc with a
surface charge Q which is evenly
distributed.
Individual question: The hemisphere shown is charged
uniformly with a positive surface charge density. At the
origin, what direction is the electric field?
A.
B.
C.
D.
E.
+x direction
+y direction
+z direction
Something else not along a x,y, or z axis
Zero magnitude – thus no direction
In the packet –What does it mean
to integrate?
How do I find the surface area?
Flux
Bucket 1 will
catch more
water than
bucket 2
Area vector
1
2
Define an Area vector perpendicular to the surface.
The more that vector is lined up with the rain, the more
rain gets into the bucket.
If this vector is perpendicular to the rain, no rain gets into
the bucket.
Electric Flux
   
  E  A  E A cos 
 
   E  dA
surface
The net flux through a surface is the number
of lines leaving the surface minus the
number of lines entering the surface.
A cylindrical piece of insulating material is placed in an external electric
field, as shown. The net electric flux passing through the surface of the
cylinder is
A. positive.
B. negative.
C. zero.