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Chapter 17
Electrostatics
Review
1. What is the
basic law of
electrostatics?
Like charges
repel, unlike
charges attract.
2. What is an
electroscope?
What is a proof
plane?
Electroscope – a device
for detecting charge.
Proof plane – a metal
plate on an insulating
handle, used to transfer
charge.
3. What particles
are moved when
charging an
object?
Only the
negatively
charged
electrons.
4. Describe three
methods of
charging an
object.
1. Stripping electrons
by friction.
2. Charging by
conduction.
3. Charging by
induction.
5. Describe how
to positively
charge an object
by induction.
1. Hold a negative object
close to the object you want
to charge.
2. Ground the object you
want to charge.
3. Remove the ground.
4. Remove the negative
object.
6. What property
of an electrical
conductor makes
it a good
conductor?
Loosely held
electrons. The
electrons are not
held tightly in a
crystal lattice.
7. What is the rule for
the residual charge
of an object charged
by conduction? By
induction?
The residual charge produced
by conduction is always the
same charge as the original
charge.
The residual charge produced
by induction is always the
opposite charge as the original
charge.
8. What is the
charge of an
electron?
-19
10
-1.6 x
coulombs
9. List two
differences in
electrostatic and
gravitational
forces.
Electrostatic forces
are much stronger
than gravitational
forces, and they
can repel.
10. Two electrostatic
charges of +30.0 mC and
-20.0 mC are 5 m apart.
Calculate the force
between the two charges.
Is this force attractive or
repulsive?
F = k q1 q2 /
2
d
F = (9 x 109)(30 x 10-6)(20 x 10-6)/52
F = 0.216 N
The charges are opposite so the
force is attractive.
11. Two electrostatic
charges of -60.0 mC and
-40.0 mC are 55 cm apart.
Calculate the force
between the two charges.
Is this force attractive or
repulsive?
F = k q1 q2 /
2
d
F = (9 x 109)(60 x 10-6)(40 x 10-6)/0.552
F = 71.4 N
The charges are the same so
the force is repulsive.
12. Two charges are
separated by 20 cm.
If they are moved to a
distance of 60 cm apart,
how does the electrostatic
force between them
change?
20 cm to 60 cm
2
F = k q1 q2 / d
Triple the distance
th
means 1/9 the
force.
13. Draw the lines of force around
single positive charge. Around a
single negative charge.
14. Draw the lines of force between
two equally charged negative
charges. Between two equally
charged positive charges.
15. Draw the lines of force between
one positive and one negative
charge that are equal in
magnitude.
16. A charge of 20 mC
experiences a force of
10 Newtons. What is
the strength of the
electric field?
E = F/q
E = 10 N/ 20 mC
E = 10 N/(20 x 10
E=5x
5
10
-6
C)
N/C
17. What is the force on
a +12 mC charge if
placed in an electric
field of 4000 N/C?
What is the direction
of the force?
E = F/q
4000 N/C = F/ +12 mC
4000 N/C = F/(12 x 10
-6
C)
F = 0.048 N
In the direction of the field.
18. What is the electric
field strength at a point
halfway between a
+10 mC charge and a
+30 mC charge that are
separated by a distance
of 4 meters?
E10 =
2
kq/d
E30 =
2
kq/d
E10 = (9 x 109)(+10 x 10-6)/22
E30 = (9 x 109)(+30 x 10-6)/22
E10 = 22500 N/C
E30 = 67500 N/C
The net field is 67500 – 22500 =
45000 N/C directed away from
the +30 mC charge.
19. What force is
applied to a +20 mC
charge when placed
halfway between the
two charges listed in
problem 18?
F = qE
-6
F = (20 x 10 )45000
F = 0.9 N
Directed away from the
+30 mC charge.
20. How would the
answer to problem
19 be different if it
were a -20 mC
charge?
Same magnitude
force, but the
opposite direction.
21. What is the strength of the
electric field at a distance of 2 m
from a +40 mC charge? What is
the direction of the field? In
what direction would the field
push a -20 mC charge? What is
the magnitude of this force?
E = kq/d2
E = (9 x 109)(+40 x 10-6)/22
E = 90000 N/C
The direction is away from the positive
charge.
A -20 mC charge would be pushed in
the opposite direction of the field.
F = qE
F = (20 x 10-6)90000
F = 1.8 N