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III. ____________ Fields
How does a ____________ (or a collection of ____________)
exert a force on another _____________ from a distance?
charged
object B
charged
object A
Answer: By means of their electric ________________ .
(Note: The letter E can also represent ______________ .)
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 ________________.
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
units of E:
E = Fe / q
[E] =[
]/[ ]
=
/
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
Determine the strength and direction of the
electric field at the point shown.
strength:
E
=
=
=
direction:
(__________ direction as ____)
Ex 2: What electrical force will a proton experience
when placed in the electric field at point P shown below?
E = ____________
P
magnitude:
=
=
=
direction:
______________ direction as _____
Ex 3: What electrical force will an electron experience
when placed in the same point as the previous example?
magnitude:
direction:
_________________
__________________
Ex 4: The E field around a ______________ point charge qp
qp
+
Note:
1. The arrows represent “_________________” that act
on a ______________ test charge q.
2. E is directed _____________________ away from “____”
3. The closeness of the lines show the field ___________.
•
The arrows show ________________ because
they point ______________________.
5. Field lines _________________, because if that were
true, the field would have ______________________________
6. E is _________________________. It is NOT constant.
For a point charge:
Let q1 = the __________ charge qp,
and q2 = the __________ charge q,
Then, _________________ Law:
becomes
and
E = Fe/q
becomes
E=
For a
point
charge:
Fe = kq1q2/r2
Fe =
E
E
qp
r
Ex 5: The
E field
around a
____________
point charge.
-
Note:
1. The arrows represent
“__________________________ .”
2. E is directed radially __________ towards the ___ .
3. E is __________________________. It is NOT constant.
4. The _________________ of the lines represents the field
strength.
5. The arrows show ________________because they point
__________________.
6. This E resembles Earth’s _____________________ field as it
would be seen _____________________
Ex 6: The E field around 2 __________________ charges.
Start by placing a ____ test charge q at one point.
Find resultant of the forces from both charges on q.
q
-
+
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…
Note:
1. Near each charge, E is like that of a ___________ charge.
2. The direction of the force at any point is ______________
to the field lines.
3. Lines are always:
____________ the “+
and ___________ the “-”
4. E lines ______________ cross b/c the net force at any
point ____________________________________.
Ex. 7:
For two _________________ charges, E will
look like:
Ex. 8: The E look like around two ______________
charges will look _______________________ but with
______________________________________
Ex. 8: The electric field around two charged
___________________________
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 ___________________
Suppose ___________________ is placed between the
plates and released from rest. The strength of the
electric field is __________________.
What direction will the
electrostatic force on it be?
Compare the electrostatic force to the gravitational
force.
Fg = w = mg =
Fe = qE =
After released, it will fall _____________ . Its acceleration
will ____________________ because the _________________
__________________________ in the ______________ E field.
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 _________________ .
Ex. 9: Two interesting facts:
1. Any charge placed on a conductor ____________________
___________________________.
2. The E field inside any conductor is ____________ .
This is called ______________________________ .
You are a __________________ . E field lines "end on"
the surface of a conductor and___________________ .
Painting rooms with a _______________ paint prevents
(electromagnetic) radio signals from entering.