ch.7 electric charges & electric fields - science rules! · pdf...
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Ch.7 Electric Charges &
Electric Fields
7.3 Electric Fields
“You must feel the Force Luke...”
Introduction to Electric Fields
An x-ray machine uses an
electric field to accelerate
electrons which then
interact with matter to
produce x-rays
Cell phones, LCD screens,
telephones, photocopiers,
and computers all rely on
the principles of
electric fields
Force at a Distance
Force - a push or pull on an object.
Like masses that experience a gravitational
attractive force, charged objects do not need
to be in contact with each other in order to
experience forces of attraction/repulsion.
“action-at-a-distance”
Field of Force
“A field of force exists in a region of space
when an appropriate object placed at any
point in the field experiences a force.”
Properties of Electric Fields
A charge generates an electric field
An electric field (𝜀) causes an electric force
and is the spatial region in which a force is
exerted on any electric charge.
Electric field = the electric force per unit
positive charge (units = N/C).
An electric field exerts attractive/repulsive
forces on other charged objects.
𝜀 is a vector (has both magnitude and
direction); 𝜀 and FE act in the same direction
Electric Fields
Electric field lines ALWAYS start on +ve
charges and end on –ve charges.
Electric field lines NEVER CROSS.
refer to diagrams on pg.339 & 340
http://www.youtube.com/watch?v=laGSICm_agM
Electric Fields
Electric dipole
Field Theory
This diagram illustrates the
generation of an electric field, 𝜺,
by a charge, +q1 . The density of the
shading designates the strength of
the field. If a + test charge, q,
is introduced, it is the field that
interacts with q to produce the
electric force FE (repulsion).
The field strength ↓ as r ↑ FE ↓also;
At infinity 𝜺 and FE become zero
Note: If q1 were negative the electric field and electric
force would be in the opposite direction.
Field Theory
q
+
+
+
+
+
+
+
+
+
+
+ +
positively charged sphere
(which creates electric field)
positive test charge
(which experiences electric field)
field strength
e = FE
q
where e = electric field strength or intensity (N/C)
FE = electric force (N)
q = +ve test charge (C)
NOTE: e and FE
have the same
direction
Comparing Laws
Force exerted
by field (N)
Quantity
affected by field
Field strength
N/C or N/kg
Fg = mg
FE = qe
More Field Theory
FE = qe e = kq1
r2
q1 q
positively charged sphere
(which creates electric field)
positive test charge
(which experiences electric field)
FE
FE = kqq1
r2
Equating both formulas for FE gives an equation dependent upon q1 and r only.
Two “Points” of View
Requires FE and a + test charge q
experiencing the force in the field
(works for charge distributions)
Electric Field
Strength
Requires a master point charge q1,
that is creating the field, and the
distance from this charge (does
NOT work for charge distributions;
only for single point charges)
FE = e = kq1
q r2
Comparison of Gravitational and
Electric Fields and Forces
-
q
FE
FE = kq1q2
r2
FE = qe
e = kq1
r2
m
Fg
Fg = GMEm
r2
Fg = mg
g = GME
r2
Uniform Electric Fields
Instead of point charges, suppose you have two large conducting
plates charged by dry cells. As with the dipole, one plate has a
positive charge and the other plate has a negative charge. In both
cases, the charge spreads uniformly along each plate. The electric
field between the plates of charge extends from the positive plate
to the negative plate and is uniform. These field lines are straight,
parallel to each other, and perpendicular to the plates.
Uniform Electric Fields
In parallel plates because the electric field
lines run straight from 1 plate to the other,
𝜀 does not depend on the separation of
the plates
The magnitude of the electric field b/w 2
plates is directly proportional to the charge
per unit area on the plates. 𝜀 ∝ 𝑞
𝜀 is uniform everywhere in the space b/w
the plates
Example 1a)
Two point charges, q1 = 3.6x10-6 C and
q2 = -2.7x10-6 C, are arranged as shown. If
a test charge is placed at point A, find the
net electric field strength at point A due to
the combined electric fields of both charges.
q1 q2
30 cm 20 cm
A
Example 1b)
What force is exerted on a charge of
4.5x10-6C placed at point A?
q1 q2
30 cm 20 cm
q
A
Example 2
In the diagram, A and C are situated as
shown. What is the magnitude and direction
of the electric field intensity at point B?
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