Download Electric Fields and Electric Potential

Electric Fields
and Electric Potential
SPH4U
Fields
Field: a region surrounding an object in which a
force exists that will affect certain objects in
that region
Objects with mass generate gravitational fields.
Objects with charge generate electric fields.
Electric Field Lines
Electric field lines indicate the direction and
strength of the field at any point in the region.
Electric Field Lines
The direction is the
direction of the force
vector on a positive
charge.
The arrows therefore
always point away
from positive charges
and toward negative
charges.
Electric Field Lines
The direction is the
direction of the force
vector on a positive
charge.
The arrows therefore
always point away
from positive charges
and toward negative
charges.
Electric Field Lines
The strength is indicated by the density of the
lines.
The field is
stronger in here
than out here
e
Electric Field Strength (or Intensity) is the
electric force per +1 C unit charge:

 Fe
e
q
It has units of N/C (or V/m).
e
Electric Field Strength (or Intensity) is the
electric force per +1 C unit charge:

 Fe
e
q

qe  Fe

It has units of N/C (or V/m).
e for a Point Charge
So if a point charge of q2 is placed in the field
generated by a point charge of q1:
kq1q2
2
Fe
kq1
r
e 
 2
q2
q2
r
e for a Point Charge
So if a point charge of q2 is placed in the field
generated by a point charge of q1:
kq1q2
2
Fe
kq1
r
e 
 2
q2
q2
r
e for a Point Charge
So if a point charge of q2 is placed in the field
generated by a point charge of q1:
kq1q2
2
Fe
kq1
r
e 
 2
q2
q2
r
Parallel Plates
Between two charged
parallel plates, the field
between the plates is
constant, with field lines
equally spaced and
perpendicular to the
plates.
Parallel Plates
Between two charged
parallel plates, the field
between the plates is
constant, with field lines
equally spaced and
perpendicular to the
plates.
The strength of the field
depends only on the
charge/unit area on the
plates.
Parallel Plates
(The field outside the plates is zero, other than a
slight “bulging” distortion on the end.)
Parallel Plates
(The field outside the plates is zero, other than a
slight “bulging” distortion on the end.)
constant e
So force is also constant:


Fe  qe
Electric Potential Energy
Recall that
Gm1m2
Gm1m2
FG 
 EG 
2
r
r
Electric Potential Energy
Recall that
Gm1m2
Gm1m2
FG 
 EG 
2
r
r
So
kq1q2
kq1q2
Fe  2  Ee 
r
r
Electric Potential Energy
Recall that
Gm1m2
Gm1m2
FG 
 EG 
2
r
r
So
kq1q2
kq1q2
Fe  2  Ee 
r
r
Where’s the negative?
In the signs of the charges.
Electric Potential Energy
kq1q2
Ee 
r
Note that Ee is negative
for opposite charges
that attract and
positive for like charges
that repel.
Electric Potential
Electric Potential is the is the energy per +1 C
unit charge:
Ee
V
q
It has units of J/C or Volts.
Electric Potential
Electric Potential is the is the energy per +1 C
unit charge:
Ee
V
q
qV  Ee
It has units of J/C or Volts.
Electric Potential Difference
Consider a positive point charge q being moved
from position A to position B in a uniform
electric field:
Electric Potential Difference
Consider a positive point charge q being moved
from position A to position B in a uniform
electric field:
W  Ee  qVB  qVA
 qVB  VA 
 qV
V is the
potential
difference.
Electric Potential Difference
Also:
W  Fd  qed
where d is the straight-line distance
parallel to the field
Electric Potential Difference
Also:
W  Fd  qed
where d is the straight-line distance
parallel to the field
(If we are moving the charge
from one plate to another,
d is the plate separation.)
Electric Potential Difference
q V  q ed
 V  e d
V
e
d
Electric Potential Difference
voltage across
the plates
separation
between the plates
q V  q ed
 V  e d
V
e
d
Electric Potential Difference
q V  q ed
 V  e d
V
e
d
Note the units:
1 V/m = 1 N/C
More Practice
“pHet Electric Field Simulation Activity”
Textbook Questions
• p. 343 #1, 2, 3, 4
• p. 354 #1, 2, 3
• p. 356 Practice #6