Download Chapter 21, Electric Charge, and electric Field

Chapter 21, Electric Charge, and electric Field
Charles Allison © 2000
21-6 The Electric Field
• Gravitational Field
Gm1m2
Fg =
=
m
g
2
2
r
•Electric Field
g=
FE =
Units N/C
k q1 q2
E =
+
+
r2
Fg
m
= qE
FE
+
q
-e
+
Charles Allison © 2000
21-6 Electric Field
https://www.youtube.com/watch?v=xg0Lg-uSMSQ
Charles Allison © 2000
21-6 Electric Field: point charge
If we have a charge (or a system of
charges) we can ask ourselves what
the electric field is at any point in
space, P, a distance r from the
charge.

E P
-e
FE = qE
E=
FE
q
=
kq
r2
Units: N/C
r

P E
+e
r
Charles Allison © 2000
21-6 The Electric Field
The electric field is defined as the force
on a small charge, divided by the magnitude
of the charge:
The electric field is a vector
Charles Allison © 2000
21-6 The Electric Field
For a point charge:
ε0 is the permittivity of the free space=8.85x10-12C2/N.m2
Charles Allison © 2000
21-6 The Electric Field
Force on a point charge in an
electric field, the direction
of the force depends on the
charge sign
Charles Allison © 2000
Electric Field:
system of charges
If we have a system of charges we can ask ourselves what
the electric field is at any point in space, P?
In this case we find the Electric field due to each charge
in the system at that point, P, and we do a vector sum of
these fields.
E = E 1 + E 2 + .... + E n
Q2
r2
r1
Q1
P
r3
Q3
EP = E1 + E2 + E3
Charles Allison © 2000
Problem 26
26.(I) What is the electric field at a point
when the force on a 1.25µC charge placed at
that point is 𝐹 = (3.0𝑖 −3.9𝑗)× 10−3 𝑁
Charles Allison © 2000
Problem 27
• 27.(II) Determine the magnitude of the
acceleration experienced by an electron in an
electric field of 576N/C. How does the
direction of the acceleration depend on the
direction of the field at that point?
(me=9.11× 10−31 kg)
Charles Allison © 2000
21-6 The Electric Field
Example 21-7: E at a point between two charges.
Two point charges are separated by a distance of 10.0 cm.
One has a charge of -25 μC and the other +50 μC. (a)
Determine the direction and magnitude of the electric field at
a point P between the two charges that is 2.0 cm from the
negative charge. (b) If an electron (mass = 9.11 x 10-31 kg) is
placed at rest at P and then released, what will be its initial
acceleration (direction and magnitude)?
Charles Allison © 2000
Electric Field 2-D
Problem: Two charges are positioned as seen below.
Q1= 1 is +2.0 µC and Q2= +8.0µC.
What is the magnitude and the direction of the
Electric Field at point P?
y
q1
E=
r1=0.090m
P
kq
r
2
q2
x
r2=0.12m
k = 8.99x10+9 Nm2/C2 = Coulomb’s constant
Charles Allison © 2000
Problem 34
34.(II) Calculate the electric field at the
center of a square 52.5 cm on a side if one
corner is occupied by a -38.6μC charge and the
other three are occupied by -27 μC charges.
Charles Allison © 2000