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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
Fe
charged
object B
charged
object A
fields, E
Answer: By means of their electric ________________
.
energy
(Note: The letter E can also represent ______________
.)
every charge
An electric field E exists around ___________________.
Its strength is found by placing a positive
test charge
"__________________"
(any charge) q in the field and
acts on q
measuring the electric force Fe that ________________.
E = Fe/q,
test charge
where q is the _______
direction .
E is a _______________
because it has ______________
vector
Its direction is given by the direction of the _____________
electrical
force
positive test charge
____________
that acts on the __________________________
that is placed in the field
units of E:
E = Fe / q
[ E ] = [ Fe ]/[ q ]
= N /C
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 10-3 C
= 2.0 x 106 N/C
direction:
Fe
same direction as ____)
(__________
Ex 2: What electrical force will a proton experience
when placed in the electric field at point P shown below?
6 N/C
3.7
x
10
E = ____________
P
magnitude:
E
3.7x106 N/C
= Fe / q
= Fe / 1.6 x 10-19 C
5.9 x 10-13 N =
direction:
Fe
E
same
______________
direction as _____
Ex 3: What electrical force will an electron experience
when placed in the same point as the previous example?
magnitude:
direction:
same
_________________
opposite
__________________
positive
Ex 4: The E field around a ______________
point charge qp
qp
2 force
lines stronger
+
pos. test q
Fe
1 force
line  weaker
Note:
lines of force
1. The arrows represent “_________________”
that act
on a ______________
test charge q.
positive
radially outward away from “____”
2. E is directed _____________________
+
strength
3. The closeness of the lines show the field ___________.
•
repulsion
The arrows show ________________
because
outward
they point ______________________.
never cross
5. Field lines _________________,
because if that were
2 directions at one place
true, the field would have ______________________________
inverse square in r
6. E is _________________________.
It is NOT constant.
For a point charge:
point
Let q1 = the __________
charge qp,
and q2 = the __________
charge q,
test
Then, _________________
Law:
Fe = kq1q2/r2
Coulomb's
becomes
and
E = Fe/q
becomes
E = kqpq/r2
q
For a
point
charge:
Fe = kqpq/r2
= kqp/r2
E
E
qp
r
Ex 5: The
E field
around a
negative
____________
point charge.
Fe
pos.
test q
-
Note:
1. The arrows represent
lines of force
“__________________________
.”
"-" .
inward
2. E is directed radially __________
towards the ___
inverse square in r
3. E is __________________________.
It is NOT constant.
closeness
4. The _________________
of the lines represents the field
strength.
5. The arrows show ________________because
they point
attraction
__________________.
inward
gravitational
6. This E resembles Earth’s _____________________
field as it
from far away
would be seen _____________________
Ex 6: The E field around 2 __________________
charges.
opposite
pos.test charge q at one point.
Start by placing a ____
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
Note:
single
1. Near each charge, E is like that of a ___________
charge.
tangent
2. The direction of the force at any point is ______________
to the field lines.
out of
3. Lines are always:
____________
the “+
into
and ___________
the “-”
never
4. E lines ______________
cross b/c the net force at any
cannot have two directions
point ____________________________________.
positive
Ex. 7:
For two _________________
charges, E will
look like:
R
q
negative
Ex. 8: The E look like around two ______________
the same
charges will look _______________________
but with
______________________________________
the arrows pointing in.
Ex. 8: The electric field around two charged
oppositely charged plates
___________________________
positive
negative
Notes:
out of the “+" and _________________
into the
1. Lines are ____________
“-”
constant
2. Within the plates, E is _______________.
Equally
_______________
spaced lines  ____________
constant field
3.
Same idea as the constant ___________________
field
gravitational
surface
near Earth’s ________________.
bend a little
4. In reality, the lines near the edges ___________________
an electron
Suppose ___________________
is placed between the
plates and released from rest. The strength of the
7.6 x102 N/C
electric field is __________________.
What direction will the
electrostatic force on it be? up
Compare the electrostatic force to the gravitational
force.
Fg = w = mg = (9.11 x 10-31 kg) (9.8 m/s2)
Fe = qE =
(1.6 x 10-19 C) (7.6 x102 N/C)
upwards
After released, it will fall _____________
. Its acceleration
F acting on it
remain constant
will ____________________
because the _________________
constant
remains constant
__________________________
in the ______________
E field.
horizontally
Suppose an e- is fired ___________________
between
the plates. Draw its trajectory. Ignore gravity.
vi
initial
direction
A
B
straight line
In region A, it follows a ____________________
because the net force acting on it is _________________
.
zero
parabola
Within the plates, it follows a ____________________
constant
because the net force acting on it is _________________
straight line
In region B, it follows a ____________________
zero
because the net force acting on it is _________________
.
Ex. 9: Two interesting facts:
remains on the
1. Any charge placed on a conductor ____________________
surface of the conductor
___________________________.
zero
2. The E field inside any conductor is ____________
.
electrical shielding
This is called ______________________________
.
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
x-ray tubes
arc lighting
etc…and died penniless…