Download r 2 - Wando High School

Electric Forces
and
Electric Fields
Coulomb’s Law
r
F = kC q1q2
______
r2
q1 = -4.0 μC
q2 = = -1.5 μC
r = 0.03 m
A coulomb (C) is a lot of charge. Most objects can hold about a
microCoulomb of charge.
Suppose the circles above represent balloons. Find the force F12,
the force from charge 1 on charge 2.
 Use only the magnitude of the charge on q, not the sign.
 Determine the direction of the force based on the picture.
Coulomb’s Law: Ex. 1
r
F12 is the force from q1
on q2
q1 = -4.0 μC
q2 = = -1.5 μC
r = 0.03 m
F12 = kC q1q2 =
______
r2
 Use only the magnitude of the charge on q, not the sign.
 Determine the direction of the force based on the picture.
Coulomb’s Law: Ex 2
r
What is F21?
q1 = -4.0 μC
q2 = = -1.5 μC
r = 0.03 m
F12 = kC q1q2 =
______
r2
 Use only the magnitude of the charge on q, not the sign.
 Determine the direction of the force based on the picture.
Coulomb’s Law: Ex 3
r
What is F12?
+
+
q1 = 3.2 x 10-19 C
q2 = = -1.6 x 10-19 C
r = 0.001 m
F12 =
 Use only the magnitude of the charge on q, not the sign.
 Determine the direction of the force based on the picture.
Electric Field, E
The electric field is the space
around an electrical charge
just like
a gravitational field is the space
around a mass.
Electric Field
 Arrows point in the direction a positive “test
charge” (charge with small mass) would move.
Electric Field Lines
Lines indicate the strength and direction of the electric
field.
The more dense the lines, the stronger the field.
Electric Field Lines of two Positive
Charges
Electric Field Lines of three
charges
Coulomb’s Law, revisited
F=kqQ
______
r2
q = test charge, Q = central charge
What if we want to know how strong the
force field is, unrelated to test charge q?
Electric Field Vector, E
 E = F/ q (Electric field = force per charge)
E = k Q
______
r2
 Unit: N/C
 E is analogous to gravitational field,
g. Remember g = GM/r2 (units: N/kg)
Example 1
 A charge of 3µC (q) is used to test the electric
field of a central charge of +6C (Q) that
causes a force of 800N. What is the
magnitude of the electric field? the direction?
 Answer: 2.7 x108 N/C, away from Q
Example 2
 A test charge of +3µC is located 5m to the
east of a -4C charge. (draw picture)
 A) Find the electric force felt by the test
charge.
 B) Find the electric field at that location.
 Answer: 4.32x103 N west, 1.44 x 109 N/C
west.
Example 3
 If a test charge is moved to a location three
times as far as its original location, how does
the magnitude of the electric field change?
 If a satellite is moved to a location three times
as far as its original location, how does the
magnitude of the gravitational field change?
Electric Field inside a conductor
 The electric field is zero inside a charged
conductor.
 Excess charge on an isolated conductor
resides on the surface.
 Excess charge accumulates on sharp
points. Practical application:

Electric field lines meet the conductor
perpendicular to the surface of the
conductor.
Thought questions
1. Where are you safer during a
thunderstorm and why?
A) In a car
or
B) Outdoors
2. Why can you not get radio reception in a
tunnel or in a steel bridge?