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Electric Fields
SWBAT calculate the electric field at a
certain point in space.
Two electrostatic point charges of +60 μC and
+50 μC exert a force on each other of 175N.
a) What is the distance between the two
charges?
b) Is the force attractive or repulsive?
c) What would happen to the electrostatic force
if you doubled the distance between the
charges?
What direction would a positive test charge (q0) travel
if placed near a negatively charged sphere ?
What direction would a positive test charge (q0) travel
if placed near a positively charged sphere ?
1. Lines start on positive charges.
2. Lines end on negative charges.
3. The # of lines per unit area is proportional to
the strength of the electric field.
4. Lines can’t cross each other
Definition: A region where an electric force on a
positive test charge (q0) can be detected.
Units: N/C
Formula:
Fe
E=
q
Electric Field Strength= Electrostatic Force
charge
6
Let’s consider a positive test
charge q0 being brought
near a metal sphere with
charge Q.
Fe
E=
q
kQq0
2
r
If we plug in the formula for E =
Fe, we get:
q0
Rewritten:
kQq0
E= 2
r q0
If we plug in the formula for Fe, we get:
kQq0
E= 2
r q0
kQ
E= 2
r
Electric Field Strength= Electrostatic Constant x charge producing the field
(distance)2
Fe
E=
q
q
kQ
E= 2
r
q
Where is the formula for Electric Field Strength?
Fe
E
q
kq1 q2
Fe =
r2
q
kq
E= 2
r
Fe
E
r
kq1 q2
Fe =
r2
r
kq
E= 2
r
A +2.5x10-6 Coulomb test charge is placed near a
negatively charged metal sphere. The sphere
exerts an electrostatic force of magnitude
-5.5x10-5 Newtons on the test charge. What is
the magnitude and direction of the electric field
strength at this location?
A +2.5x10-6 Coulomb test charge is placed
2.5x10-3 m away from a metal sphere with a
charge of +1.5 C. What is the magnitude and
direction of the electric field strength at this
location?