Download Chapter 22 Electric Field

Electric Fields
*slides adapted from Peggy Bertrand
Questions
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If nothing existed to experience the
gravitational field of Earth, would the field still
exist?
How do we describe the gravitational field of
earth?
If we introduce an object to the gravitational
field of Earth, how can we determine what
force it will experience?
The Electric Field
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The presence of + or – charge modifies empty space. This
enables the electrical force to act on charged particles without
actually touching them.
We say that an “electric field” is created in the space around a
charged particle or a configuration of charges.
If a charged particle is placed in an electric field created by other
charges, it will experience a force as a result of the field.
Sometimes we know about the electric field without knowing
much about the charge configuration that created it.
We can easily calculate the electric force from the electric field.
Why use fields?
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Forces exist only when two or more particles
are present.
Fields exist even if no force is present.
The field of one particle only can be
calculated.
The Electric Field
We bring in a positive
charge q0 as a test
charge, which is carefully
selected with a very small
magnitude, so that it does
not alter the locations of
the other charges
How does particle q0 “know” of the presence of other charge?
THE ELECTRIC FIELD
The electric field E that
exists at a point is the
electrostatic force F
experienced by a small test
charge q0 placed at that point
divided by the charge itself:
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 F
E
q0
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or
kqo
E 2
r
The electric field is a vector, and its direction is the
same as the direction of the force F on a positive test
charge
(in other words, E-field lines point away from positive
source charges, and toward negative source charges)
SI Unit of Electric Field: newton per coulomb
(N/C)
Important about electric field
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It is the surrounding charges that create an electric field at a
given point.
Any charge q placed at the point with the electric field E will
experiences a force, F=qE. For a positive charge, the force
points in the same direction as the electric field; for a
negative charge, the force points in the opposite direction as
the electric field.
At a particular point in space, each of the surrounding charges
contributes to the net electric field that exists there
Point charge: usually the source of the
e-field, fixed at a “point”
Test charge: the charge brought into
the E-field, is influenced by the E-field
Spherical Electric Fields
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The Electric Field surrounding a point charge or a
spherical charge can be calculated by:
kQq0
Fe  2  q0 E
r
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E: Electric Field (N/C)
k: 8.99 x 109 N m2/C2
q: Charge (C)
r: distance from center of charge q (m)
Remember that k = 1/4πε0
kQ
E 2
r
Sample Problem
There is an isolated point charge of q=+15 μC in a vacuum. Using a
test charge of q0=+0.80 μC, determine the electric field at point P,
which is 0.20 m away.
Superposition
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When more than one charge contributes to
the electric field, the resultant electric field is
the vector sum of the electric fields produced
by the various charges.
Again, as with force vectors, this is referred to
as superposition.
Sample Problem -2-D Superposition
Calculate the total E-field at point A due to both charges,
Q1 and Q2
A
60 cm
30 cm
52 cm
Q2 = +50 µC
Q1 = -50 µC
Electric Field Lines & Their Properties
1.
2.
3.
The electric charges create an
electric field in the space
surrounding them. It is useful to
have a kind of “map” that gives
the direction and indicates the
strength of the field at various
places. This can be done by
drawing the electric field lines.
Electric fields lines are not
vectors, they are “lines of force.”
The indicate the direction of
force on a positive test charge
placed in the field.
Field Vectors from Field Lines
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The electric field at a given point is not the
field line itself, but can be determined from
the field line.
The electric field vectors is always tangent to
the line of force at that point.
Vectors of any kind are never curvy!
The properties of the electric field lines
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At any point, the tangent direction of the electric line is the
direction of electric field.
The density of the electric field lines provides information about
the magnitude of the field. The lines are closer together where
the electric field is stronger, the lines are closer together. The
lines are more spread out where the electric field is weaker.
The electric field lines always begin on a positive charge and
end on a negative charge and do not start or stop in
midspace.
Electric Dipole
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Two charged particles of
magnitude q but of opposite sign,
separated by a distance d. We call
this configuration an electric
dipole
Two Identical Charges
The Electric Field Inside a Conductor:
Shielding
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At equilibrium under
electrostatic conditions, any
excess charge resides on the
surface of a conductor.
At equilibrium under
electrostatic conditions, the
electric field is zero at any
point within a conducting
material.
The Electric Field Inside a Conductor:
Shielding
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The conductor shields any charge within it from
electric fields created outside the conductor.
The electric field just outside the surface of a
conductor is perpendicular to the surface at
equilibrium under electrostatic conditions.