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Electric Field Direction
Forces and Fields 7
Electric Field Direction

The direction of the electric force
that a positive charge and a
negative charge experience within
an electric field are opposite
E
+
+ q1
Fe
attract
-
Fe
repel

The direction of the vector, E is defined
as the same direction as the electrical
force exerted onto a positive test charge
placed within an electrical field
E
E
- q1
Fe
E
E
q2 test charge
+
The direction of the
electrical force on the
positive test charge is
used to indicate the
direction of the
electrical field
Examples
E
q2 test charge
E
+ q1
+
Fe
E
E
E
E
-q2
q2 test charge
Fe
E
E
+
Field Direction

The direction of E about a negative
charge is toward its center

The direction of E about a positive
charge is away from its center
+
Irregular Shaped Object
Electric field
is
stronger at
the tip as
charge is
more
concentrated
there.
Hollow Conducting Object
The electric
field inside a
hollow
conductor is
zero.
Field Direction Between Charges

The direction of E is from positive to
negative
+
-
Comparison
of Fields
Gravitational
Electrical
Force is . . .
Attractive
Attractive or
repulsive
Field
Direction
Direction of F Direction of F
on mass
on + charge
Field direction Toward
source
Field varies
1
 2
r
Toward or
away from
source
1
 2
r
Net Field Example:
5.00 cm
+2.00uC

P
+3.00uC
Find the net electric field at point P,
midway between the two charges.
Free Body Diagram
EB
ER
P
Enet  EB  ER
Blue
kq
EB  2
r
8.99  109 Nm2 / C 2 (3.00  106 C )

(0.025m)2
 4.3152  107 N / C [ L]
Red
kq
EB  2
r
9
2
2
6
8.99  10 Nm / C (2.00  10 C )

(0.025m) 2
 2.8768  10 N / C [ R]
7
NET FIELD
Enet  EB  ER
 4.3152  10 N / C  2.8768  10 N / C
7
 1.4384  10 N / C
7
 1.4  10 N / C [ L]
7
7
Example 2: Determine the net E at a point P midway between
the two charges shown below.
10.0 cm
E1
q1
P
E2
q2
E Net
+9.00 uC
+4.00 uC
E1 (q1 to P )
E2 (q2 to P )
kq1
E1  2
r
8.99  109 (9.00  106 C )
E1 
2
0.0500
m


kq2
E2  2
r
8.99 109 (4.00 106 C )
E2 
2
0.0500
m


E1  3.24  107 N / C
E1  1.44 107 N / C
Enet  E1  E2
 3.24  107 N / C  1.44  107 N / C
 1.80  107 N / C
• midway along a line joining the centers of
two unlike charges, the electric field E is
uniform in strength
E  constant
q1
+1.0 C
•
q2
-1.0 C
midway between two identical charges, E = 0
q1
+1.00 C
q2
+1.00 C
Example 3:
Two charges of +3.00 C and +5.00 C are
10.0 cm apart. At what distance from
the 3.0 C charge will the field be zero?
E1  E2 where Enet  0
10.0 cm
q2
q1
+3.00
C
+5.00 C
x
10 - x
10.0 cm
Solution:
q2
q1
+3.00
C
+5.00 C
x
kq1
kq2
E1  2 and E2  2
r
r
E1  E2
kq1 kq2
 2
2
r
r
q1 q2
 2
2
r
r
3.00C
5.00C

2
x
(10  x) 2
10 - x
1.7320508 2.236068

x
10cm  x
cross multiply
17.3205  1.73205 x  2.236068 x
17.3205  3.968 x
x  4.36cm
Assignment:


Handout
Reading from textbook