Lecture 5
Lecture 5
R
2
R
Lecture 5
•
Yesterday we introduced electric field and
electric field lines
•
Today we will cover some more topics on
Electric Fields
–
Dipoles
–
Motion of
a charge in an electric field
–
Continuous charge distributions
Question 1
•
Consider a circular ring with total charge
+
Q
.
The charge is spread uniformly around the
ring, as shown, so there is
λ
= Q/2
π
R
charge
per unit length.
•
The electric field at the origin is
R
x
y
+
+
+
+
+
+
+
+
+
+
+
+
+ + +
+
+
+
+
+
+
+
(a)
zero
0
1
2
4
R
πλ
πε
(b)
(c)
2
0
1
4
R
R
π
λ
πε
•
The key thing to remember here is that the total field at the origin is
given by the VECTOR SUM of the contributions from all bits of charge.
•
If the total field were given by the ALGEBRAIC SUM, then (b) would be
correct. (exercise for the student).
•
Note that the electric field at the origin produced by one bit of charge
is exactly cancelled by that produced by the bit of charge diametrically
opposite!!
•
Therefore, the VECTOR SUM of all these contributions is ZERO!!
•Lines leave (
+
) charges and return to (

) charges
•Number of lines leaving/entering charge
∝
amount
of charge
•Tangent of line = direction of
E
•Local density of field lines
∝
local magnitude of
E
•Field lines cannot cross
•Lines from isolated charges go to infinity
Rules for Field Lines
+

Field Lines From Two Opposite Charges
Electric Dipoles
•
Dipoles consist of a
positive and negative
charge
•
Field lines leave the
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 Fall '07
 Ndili
 Charge, Electric Fields

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