Chapter 25
Electric
Charges and Forces
Physics 133
Workbook 25.4
25.4 Two lightweight balls hang straight down when both are
neutral. They are close enough together to interact, but not
close enough to touch. Draw picture showing how the balls
hang if:
a)
Both are touched with a plastic rod that was rubbed with
wool.
b)
The two balls of part a are moved farther apart.
c)
Ball A is touched by a plastic rod that was rubbed with wool
and ball B is touched by a glass rod that was rubbed with
silk.
d)
Both are charged by a plastic rod, but ball A is charged
more than ball B.
e)
Ball A is charged by a plastic rod. Ball B is paper (neutral).
f)
Ball A is charged by a glass rod. Ball B is paper (neutral).
Physics 133
Hanging Charged Masses
• Construct a free-body diagram for the hanging
charged object (it is not moving).
• Apply Newton's second law to your diagram.
• Rank the forces (descending):T , F (Q on q), w
• What happens to orientation of rope if Q  Q/2?
• What happens to orientation of rope if w  w/2?
Physics 133
Workbook 25.5
25.5 Four lightweight balls 1, 2, 3, 4 are suspended by threads. Ball 1 has
been touched by a plastic rod that was rubbed with wool. When the balls are
brought close together, without touching, the following observations are made:
•Balls 2, 3, and 4 are attracted to ball 1.
•Balls 2 and 4 have no effect on each other.
•Ball 2 is attracted to ball 3.
What are the charge states (glass, plastic, neutral) of balls 1, 2, 3, and 4?
Explain.
Answer:
Charge 1
2
3
4
A
plastic
plastic
plastic
neutral
B
plastic
neutral
neutral
glass
C
plastic
neutral
glass
neutral
D
plastic
neutral
neutral
neutral
Physics 133
Workbook 25.6
25.6 Charged plastic and glass rods hang by
threads.
a) An object repels the plastic rod. Can you
predict what it will do to the glass rod? If
so, what? If not, why not? Explain.
b) A different object attracts the plastic rod.
Can you predict what it will do to the glass
rod? If so, what? If not, why not? Explain.
Physics 133
Coulomb’s law
• Point particles
• Along the line between
q1 q2
F =F =K 2
r
Physics 133
Atom
• Positive core
• Electron cloud
Physics 133
Triboelectric series
•
•
•
•
•
•
•
•
•
•
•
•
•
•
+++ (positive)
Human Hands (too moist)
Rabbit Fur
Glass
Human Hair
Nylon
Wool
Fur
Lead
Silk
Aluminum
Paper
Cotton
Steel (neutral)
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Wood
Amber
Hard Rubber
Nickel, Copper
Brass, Silver
Gold, Platinum
Polyester
Styrene (Styrofoam)
Polyurethane
Polyethylene (like Scotch tape)
Polypropylene
Vinyl (PVC)
Silicon
Teflon
- - - (negative)
(List is adapted from Nature’s Electricity, by C. K.
Adams)
Physics 133
Insulators and Conductors
Physics 133
Electroscope
• Predict what will
happen if a charged
rod is brought near
an electroscope.
Physics 133
Electroscope
• Predict what will
happen if a charged
rod is brought near
an electroscope with
an insulator below
the top ball.
Physics 133
Workbook 25.12
25.12 A negatively charged electroscope has separated leaves.
a) Suppose you bring a negatively charged rod close to the top
of the electroscope, but not touching. How will the leaves
respond? Use both charge diagrams and words to explain.
b) How will the leaves respond if you bring a positively charge
close to the top of the electroscope, but not touching? Use
both charge diagrams and words to explain.
Answer:
A Leaves separate more
B Leaves separate less
C No change
Physics 133
Can.ans
Physics 133
Workbook 25.19
25.19
A positively charged rod is held near, but not touching, a neutral metal sphere.
a)
Add plusses and minuses to the figure to show the charge distribution on
the sphere.
A
-
B
-
-
-
-
-
C
+
-
+
-
+
+
D
-
+
+
no
charge
-
b) Does the sphere experience a net force? If so, in which direction?
A
B
C
D
no force
Physics 133
Board.ans
Physics 133
Board2.ans
Physics 133
Workbook 25.21
25.21
The figure shows an atom with four protons in the
nucleus and four electrons in the electron
cloud.
a) Draw a picture showing how this atom will look
if a positive charge is held just above the atom.
b) Is there a net force on the atom? If so, in which
direction? Explain.
Physics 133
Coulomb’s law
• Point particles
• Along the line between
q1 q2
F =F =K 2
r
Physics 133
Workbook 25.22
25.22 For each pair of charges, draw a force vector on
each charge to show the electric force acting on that
charge. The length of each vector should be proportional
to the magnitude of the force. Each + and – symbol
represents the same quantity of charge.
--
--
++
--
++
++
--
++
++
++
Physics 133
Workbook 25.22 (Ans)
25.22 For each pair of charges, draw a force vector on
each charge to show the electric force acting on that
charge. The length of each vector should be proportional
to the magnitude of the force. Each + and – symbol
represents the same quantity of charge.
--
--
++
--
++
++
--
++
++
++
Physics 133
Force magnitude: Consider two small
balls (1: q1=10 nC, m1=20 mg; 2: q2=5
nC, m2=10 mg) separated by 3 cm.
What is the magnitude of the electric force
between them?
q1q2
9
2
2
Felec = K 2 ,K = 8.99 ´10 Nm /C
r
What is the magnitude of the gravitational
force between them?
m1m2
Fgrav = G 2 ,G = 6.67 ´10-11 Nm2 /kg 2
r Physics 133
Electric Force – Superposition
(graphical addition)
II
I
III
IV
Into which quadrant does the vector point that
represents the total force on charge #3?
Physics 133
Electric Force – Superposition
(adding components)
Find the total force on charge #3.
(Hint: Use superposition.)
K = 8.99 x 109 Nm2/C2
Physics 133
Electric Force – Superposition
(answer)
F3 = -4.11´10-4 N iˆ - 3.24 ´10-4 N ˆj
= 5.23 ´10-4 N, -142
Physics 133
Unit vector
• Rectangular: always same direction
• Polar: different directions for each spot
Pick a source charge location. (ball)
Students make x, y vector components.
Students make r unit vector.
Physics 133
Physics 133
Problem 25.62: What are the electric fields at points 1 and 2?
Give your answer as a magnitude and direction.
Physics 133
ANSWER Problem 25.62: are the electric fields at points 1
and 2? Give your answer as a magnitude and direction.
E=1.8x105 N/C, 60o north of west
Ex=-9.0x104 N/C, Ey=1.56x105 N/C
Physics 133