UP II – Spring 2022
Lecture 2 for 62NK Class
Chapter 3: Basic Electric Processes
Instructor : Dang Thi Minh Hue
 Number of the Day : Number of Electrons in Universe:
1 × 1079
 From Chapter 2
• Charge is Conserved:
• Net Charge Moves in a Conductor not an Insulator:
 From Lab (Coulomb’s Law)
• Opposites Attract / Likes Repel
• Electric Force Weakens with Increasing Distance:
• Electric Force Acts Along the Line Between Charges:
2.1 Golf Tube and Soda Can
 Problem: Why does a charged golf tube attract an uncharged
soda can?
 Solution
• (a) Charge Separates: Charge can move in a conductor. Like
charges repel, opposites attract. Therefore, some + charge
moves nearer the GT and some − charge farther away. Since
charge is conserved, the amount of + and − charge is the
same.
• (b) Force on the Separated Charge: The separated charge still
feels the electric force, but cannot get out of the conductor. The
+ charge feels attractive force, F+, and − charge a repulsive
force F−.
• (c) Net Force: Electric force decreases with increasing distance.
Therefore F+ > F− and the net force is attractive.
• (d) Diagram: A diagram is an important part of any physics
problem.

•
•

Problems
Why did the plastic bottle roll?
If we separated charge, why didn’t the universe blow up?
Problem: Why does charge separation stop if all electrons
in the metal feel the electric force?
 Solution
• (a) Force On First Electron: The first electron separated
feels a force FGT .
• Force on Second Electron: The second electron separated




feels a force Fq 2  FGT  F 2  F2

which is smaller than FGT .
 Shielding: A conductor or dielectric in the presence of an
external charge produces a separated surface charge that
reduces the electric force on the interior of the object.
 Net Electric Force is Zero in a Conductor: In a conductor
the shielding is perfect, reducing the NET electric force on
charges in the conductor to zero.
 Why We’re Still Here?
The universe didn’t blow up
because ….
 A conductor shields charge
in its interior from external
electric forces.
 Why did the Plastic of an Insulator:
A dielectric or insulator
polarizes in the presence
of an external charge
producing a surface charge.
 Why is the Plastic Bottle Attracted?
Same reasoning as the can except charge separation
become polarization.
2.2 Charge on a Conductor
• Conducting Marbles: We can understand many electric
phenomena by considering conducting marbles of different
sizes.
• Problem 1: A patch of + charge is placed on a conducting
marble. Where is the charge at a later time?
Solution
• Charge Spreads Out on a Conductor: Like charges repel
and charge can move in a conductor, so the charges
spread apart over the surface of the conductor.
 Problem 2: Two conducting sphere’s of the same size in
contact. Where is the charge? The net charge of the
system is positive.
Solution
• Charge is shared evenly Between
identical conductor:
Like charges repel, so the charges
get as far apart as possible.
Since the sphere’s are identical Q1 = Q2.
• Definition Charge Sharing:
• Charge spreading out on a
system of connected conductors
is called Charge Sharing.
 Problem 3: Two conducting
sphere’s of different size in
contact.
Solution
Charge is shared unevenly: Like charges still repel, so the
charges get as far apart as possible, leaving Q1 < Q2 .
 Capacity of a Sphere: For spherical objects,
Q1
R1

Q2
R2
where R1 and R2 are the radii.
 Grounding: Connecting a conductor to a very large
conductor (usually the earth) to remove net charge is
called grounding.
•
Symbol for Ground
•
Problem: Use the golf tube to
charge a conducting marble.
•
Solution
(a) Bring Charged Object
Near Uncharged Object:
Charge separates on the
conductor, but the conductor
remains neutral.
• (b) Ground the Conductor:
Opposite + charge held in
place by GT. Unlike − charge
free to spread out, charge
moves to much larger conductor
ground. (Likes repel, charge
moves in conductor)
(c) Disconnect Ground:
Charged conductor remains
• Charging by induction:
Using a charged object to
draw charge from ground,
charging a conductor without
directly transferring charge from
the charged object to the conductor.
2.3 Coulomb’s Law
• Strength of the Electric Force: The strength of the electric
force is inversely proportional to distance
F12
kq1 q 2

d2
(2.1)
where d is the distance between the charges.
• Coulomb’s Law: The electric force, F12, charge 1 exerts on
charge 2 is

kq1 q 2
(2.2)
F12 
r̂12
2
r12
Note the order of subscripts, the charge 1 is providing the
force. The charge 2 is feeling the force.
• Features of Coulomb’s Law
• Constant k = 8.99 × 109 Nm2/C2 is a new fundamental
constant.
• Displacement Vector: r12
• Vector Modulus or Vector Length: r12 and Unit Vector:
• Problem: A point charge, q1 = −10μC, is located at
5cmxˆ  10cmyˆ  3cmzˆ
Compute the force that q1 exerts on q0 = 5μC, which is
located at
3cmxˆ
• Solution……
. Assume two significant figures.