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Electric. III. ____________ Fields. charge. charges. How does a ____________ (or a collection of ____________) exert a force on another _____________ from a distance ?. charge. charged object B. F e. charged object A. fields, E.
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Electric
III. ____________ Fields
charge
charges
How does a ____________ (or a collection of ____________)
exert a force on another _____________ from a distance?
charge
charged
object B
Fe
charged
object A
fields, E
Answer: By means of their electric ________________ .
(Note: The letter E can also represent ______________ .)
energy
every charge
An electric field E exists around ___________________.
Its strength is found by placing a positive
"__________________" (any charge) q in the field and
measuring the electric force Fe that ________________.
test charge
acts on q
test
Fe/q,
E =
where q is the _______ charge
E is a _______________ because it has ______________ .
Its direction is given by the direction of the _____________
____________ that acts on the __________________________
that is placed in the field
direction
vector
electrical
force
positive test charge
units of E: E = Fe / q
[ E ] = [ ]/[ ]
= /
Fe
q
C
N
Page 3: top
Ex 1: A positive charge q = 5.0 mC experiences
an electrical force of 10.0 x 103 N when placed at the
position shown.
q
Fe
Determine the strength and direction of the
electric field at the point shown.
strength: E =
=
=
Fe / q
10.0 x 103 N / 5.0 x 103 C
2.0 x 106 N/C
Fe
same
(__________ direction as ____)
direction:
Ex 2: What electrical force will a proton experience
when placed in the electric field at point P shown below?
3.7 x 106 N/C
E = ____________
P
Fe / q
magnitude: =
=
=
E
Fe / 1.6 x 1019C
3.7x106 N/C
5.9 x 1013 N
Fe
E
same
direction: ______________ direction as _____
Ex 3: What electrical force will an electron experience
when placed in the same point as the previous example?
same
_________________
magnitude:
opposite
__________________
direction:
positive
Ex 4: The E field around a ______________ point charge qp
pos. test q
qp
+
Fe
1 force
line weaker
2 force
lines stronger
lines of force
positive
radially outward
+
strength
repulsion
outward
never cross
2 directions at one place
inverse square in r
For a point charge:
Let q1 = the __________ charge qp,
and q2 = the __________ charge q,
Then, _________________ Law: Fe = kq1q2/r2
becomes Fe =
and E = Fe/q
becomes E =
point
test
Coulomb's
kqpq/r2
kqpq/r2
= kqp/r2
q
E
E
For a
point
charge:
qp
r
Ex 5: The
E field
around a
____________
point charge.
pos.
test q
Fe
negative

lines of force
""
inward
inverse square in r
closeness
attraction
inward
gravitational
from far away
Ex 6: The E field around 2 __________________ charges.
opposite
pos.
Start by placing a ____ test charge q at one point.
Find resultant of the forces from both charges on q.
Fe+
R
q
Fe

+
+
where Fe+ = Fe due to the ___charge
and Fe = Fe due to the ___charge

Then repeat for every other possible point.
The result will be…
R
single
tangent
out of
into
never
cannot have two directions
positive
Ex. 7: For two _________________ charges, E will look like:
R
q
negative
Ex. 8: The E look like around two ______________
charges will look _______________________ but with
______________________________________
the same
the arrows pointing in.
Ex. 8: The electric field around two charged
___________________________
oppositely charged plates
positive
negative
Notes:
1. Lines are ____________ “+" and _________________ “”
2. Within the plates, E is _______________.
_______________ spaced lines ____________ field
3. Same idea as the constant ___________________ field
near Earth’s ________________.
4. In reality, the lines near the edges ___________________
out of the
into the
constant
Equally
constant
gravitational
surface
bend a little
an electron
Suppose ___________________ is placed between the
plates and released from rest. The strength of the
electric field is __________________.
7.6 x102 N/C
What direction will the
electrostatic force on it be?
up
Compare the electrostatic force to the gravitational
force.
(9.8 m/s2)
(9.11 x 1031 kg)
Fg = w = mg =
(1.6 x 1019 C)
Fe = qE =
(7.6 x102 N/C)
upwards
After released, it will fall _____________ . Its acceleration
will ____________________ because the _________________
__________________________ in the ______________ E field.
F acting on it
remain constant
constant
remains constant
horizontally
Suppose an e is fired ___________________ between
the plates. Draw its trajectory. Ignore gravity.
vi
initial
direction
A
B
In region A, it follows a ____________________
because the net force acting on it is _________________ .
Within the plates, it follows a ____________________
because the net force acting on it is _________________
In region B, it follows a ____________________
because the net force acting on it is _________________ .
straight line
zero
parabola
constant
straight line
zero
remains on the
surface of the conductor
zero
electrical shielding
Why are you safe within your car during a lightning
storm?
Any charge resulting from a strike remains on the
outside, and the E field within the car will be zero.
conductor
You are a __________________ . E field lines "end on"
the surface of a conductor and___________________ .
cannot get in
conducting
Painting rooms with a _______________ paint prevents
(electromagnetic) radio signals from entering.
Tesla invented:
modern generators and
motors (Niagara Falls)
radio
electromagnetic induction
tesla coils
xray tubes
arc lighting
etc…and died penniless…