Transcript Lecture 2 - UCF Physics

The Electric Field
Chapter 23
The Electric Field
A group of fixed charges exerts a force F, given by
Coulomb’s law, on a “test charge” q
at position r.
q
F
r
• The electric field E (at a given point in space) is the
force per unit charge that would be experienced by a
test charge at that point.
E=F/q
This is a vector function of position;
i.e., a vector field.
Electric Field of a Point Charge Q
F
q
r
kQq
F  2 rˆ
r
Q
• Dividing out q gives the electric field at r:

kQ
E(r)  2 rˆ
r
Radially outward,
falling off as 1/r2
Electric Field Lines (Point Charge)
Electric Field
(vector)
Outward from + charges
Inwards towards - charges
Field Lines
(Lines of
force)
Electric field lines (lines of force) are continuous lines
whose direction is everywhere that of the electric field
Force Due to an Electric Field
F=qE
Just turn the definition of E around.
If E(r) is known, the force F on a
charge q, at point r is:
q
+
F = q E(r)
The electric field at r
points in the direction
that a positive charge
placed at r would be
pushed.
Electric field lines are
bunched closer where
the field is stronger.
The Electric Dipole
+q
d
-q
An electric dipole consists of two equal and opposite
charges (q and -q ) separated a distance d.
The Electric Dipole
+q
d
p
-q
We define the dipole moment p
magnitude = qd,
p
direction = from -q to +q
The Electric Dipole
E
+q
d
q
-q
Suppose the dipole is placed in a uniform electric
field (i.e., E is the same everywhere in space).
Will the dipole move ??
The Electric Dipole
E
+q
d
q
-q
What is the total force acting on the dipole?
The Electric Dipole
F+
E
+q
d
Fq
-q
What is the total force acting on the dipole?
The Electric Dipole
F+
E
+q
d
Fq
-q
What is the total force acting on the dipole?
Zero, because the force on the two charges cancel:
both have magnitude qE, and are opposite. The center of
mass does not accelerate.
The Electric Dipole
F+
E
+q
d
Fq
-q
What is the total force acting on the dipole?
Zero, because the force on the two charges cancel:
both have magnitude qE, and are opposite. The center of
mass does not accelerate.
But the charges start to move. Why?
The Electric Dipole
F+
E
+q
d
Fq
-q
What is the total force acting on the dipole?
Zero, because the force on the two charges cancel:
both have magnitude qE, and are opposite. The center of
mass does not accelerate.
But the charges start to move. Why?
There’s a torque because the forces aren’t collinear.
F+
+q
d
d sin q
F-
q
q
The torque is:
t(magnitude of force) (moment arm)
t  (qE)(d sin q)
and the direction of t is (in this case)
into the page
q
+
pq
d
q
E
-q
but we have defined : p = q d
and the direction of p is from -q to +q
Then the torque can be written as:
tpxE
t = p E sin q
with an associated potential energy
U= -p.E
Electric fields due to various
charge distributions
The electric field is a vector which
obeys the superposition principle.
Begin with a simple example with
discrete charges: the dipole
Field Due to an Electric Dipole
at a point x straight out from its midpoint
Y
Electric dipole moment
p = qd
+q
l
q
d
X
x
E+
E-q
E
Calculate E as a function of p, x,and d
Y
+q
d
l
q
X
x
E-
E+
-q
E
Electric Fields From Continuous
Distributions of Charge
Up to now we have only considered the electric field of point charges.
Now let’s look at continuous distributions of charge
lines - surfaces - volumes of charge
and determine the resulting electric fields.
Sphere
Ring
Sheet
Electric Fields From Continuous Distributions
For discrete point charges, we can use the superposition
principle and sum the fields due to each point charge:
q2
q3
q1
E(r ) 
E
i=1,2,3
q4

i
Electric Fields From Continuous Distributions
For discrete point charges, we can use the superposition
principle and sum the fields due to each point charge:
q2
q3
q1
E(r ) 
E
i
i=1,2,3
q4
What if we now have a continuous charge distribution?

q
E(r)
Electric Field Produced by a
Continuous Distribution of Charge
In the case of a continuous distribution of charge
we first divide the distribution up into small pieces,
and then we sum the contribution, to the field,
from each piece:
dEi
r
The small piece of charge dqi
produces a small field dEi at
the point r

Note: dqi and dEi are differentials
dqi
In the limit of very small pieces, the sum is an integral
Electric Field Produced by a
Continuous Distribution of Charge
In the case of a continuous distribution of charge we first
divide the distribution up into small pieces, and then we
sum the contribution, to the field, from each piece:
In the limit of very small pieces, the sum is an integral
dEi


dqi
Each dq 
 
dq
dE(r)  k 2 r
r
Then  E =  dEi
 

For very small pieces  E ( r )   dE
 
kdq
E( r )   2 r
r
Continuous Charge Distributions
charge density
units
differential
LINE
AREA
VOLUME
=Q/L
=Q/A
=Q/V
C/m
C / m2
C / m3
dq =  dL dq =  dA dq =  dV
Charge differential dq
to be used when finding
the electric field of a
continuous charge distribution
 
kdq
E( r )   2 r
r
Example: An infinite thin line of charge.
Y
P
Find the electric
field E at point P
y

Charge per unit length
is 
i.e., the charge in a length dx
is dq=dx
X

Find the electric
field E at point P
P
y
dq

x

Example: An infinite thin line of charge.
dE+
y
P
r
dq


dq
dE   k 2 r
r
x 2  y2

x
• Consider small element
of charge, dq, at position x.
• dq is distance r from P.
Example: An infinite thin line of charge.
dE+
P
y
dq


dq
dE   k 2 r
r

x
• Consider small element
of charge, dq, at position x.
• dq is distance r from P.
• For each dq at +x, there
is a dq at -x.
dE
dE+


E  yE
dE-
q

dq

dq
dE   k 2 r
r
dq
dE y  2k 2 cosq
r
dq
-x

x
• Consider small element
of charge, dq, at position x.
• dq is distance r from P.
• For each dq at +x, there
is a dq at -x.
dE
dE+
dE-
q

dq

dq
dE   k 2 r
r
dq
dE y  2k 2 cosq
r
dq= dx, cosq=y/r
dq
-x
x

dE
dE+
dE-
q

dq

dq
dE   k 2 r
r
dq
dE y  2k 2 cosq
r
dq= dx, cosq=y/r
dq
-x
x
2kdx y
dE y  2
2 
(x  y ) r
2k y
E y   2 2 3/2 dx
(x  y )
x 0
x

dE
dE+
dE-
q

dq

dq
dE   k 2 r
r
dq
dE y  2k 2 cosq
r
dq= dx, cosq=y/r
dq
-x

x
2kdx y
dE y  2
2 
(x  y ) r
2k y
2k
E y   2 2 3/2 dx 
(x

y
)
y
x 0
x
Example of Continuous Distribution:
Ring of Charge
Find the electric field at a point along the axis.
Hint: be sure to use the symmetry of the problem!
dE
z
r
dq
Thin ring with total charge q
charge per unit length is
q/2pR
R
Break the charge up
into little bits, and find
the field due to each
bit at the observation
point. Then integrate.