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GENERAL PHYSICS 2 LEARNING MODULE
ü State that there are positive and negative charges and that a charge is
measured in coulombs.
ü Predict charge distribution and the resulting attraction or repulsion.
ü Calculate the net electric force on a point charge by a system of point
charges.
: Introductory Concepts in Electricity
Electric Charge is an electrical property of matter that exists because of access or a
deficiency of electrons. The SI unit of charge is coulomb (C), named after French physicist
Charles Agustin Coulomb, who made important discoveries on electricity. Electrical
charges are usually represented by q.
Properties of Electric Charge
- The charge is a basic property of a material body which
attracts or repels another object.
- Friction produces two different types of charge on
different materials (such as glass or plastic).
- Like charges always repel each other.
- Unlike charges always attract each other.
Two Types of Electric Charge
- POSITIVE CHARGE – When a material losses electron, the
number of protons increase in the material, making it
positively charged.
- NEGATIVE CHARGE – When a material gains electron, the
number of electrons increase in the material, making it
negatively charged.
GENERAL PHYSICS 2 LEARNING MODULE
Table 1: Location, mass, and charge of sub-atomic particles
Subatomic Particle Location Mass Charge
Proton inside nucleus 1.673 x 10
kg 1.602 x 10
C
Neutron inside nucleus 1.675 x 10
kg 0
Electron around nucleus 9.109 x 10
kg - 1.602 x 10
C
When rubbing two materials, electrons may be transferred from one material to the
other. When this occurs, one material ends up with an excess of electrons and becomes
negatively charged, while the other ends up with a deficiency of electrons and becomes
positively charged. This imbalance of charges on objects results in the phenomena we
commonly refer to as static electricity.
All charge in nature is carried by electrons and protons. Electrons carry the charge
we have named negative. Protons carry an equal-magnitude charge that we call positive.
Conductors and Insulators
Substances such as metals and salty water allow charges to move through them
with relative ease. The electrons in some metals and similar conductors are not bound to
individual atoms or sites in the material. These free electrons are free to move away from
its atomic orbit and can move through the material as much as air moves through loose
sand. Any substance that has free electrons and allows charge to move freely through it is
called a conductor.
Superconductors allow the movement of charge without any loss of energy. Salty
water and other similar conducting materials contain free ions that can move through
them. An ion is an atom or molecule having a positive or negative (nonzero) total charge.
In other words, the total number of electrons is not equal to the total number of protons.
On the other hand, some substances, such as glass, do not allow charges to move
through them. These materials that can hold electrons securely within their atomic orbits
are called insulators. Electrons and ions in insulators are bound in the structure and
cannot move easily—as much as 10
23
times more slowly than in conductors. Pure water
and dry table salt are examples of insulators, whereas molten salt and salty water are
examples of conductors.
- Conductors are materials that allow the flow of charges. Metals are good
conductors, the outermost or valence electrons are free to move around the entire
material.
- Insulators are materials that resist the flow of charges. The electrons are more
tightly bound to the atom that they do not easily move around. Example of
insulators are rubber, plastic, paper and glass.
GENERAL PHYSICS 2 LEARNING MODULE
Solution: Substitute the values into the equation.
𝐹
"
= k
,
.
3
(
(
<'=
<@
(
3
(
(
<'A
(
<B
(
3
<'D
Answer: = 3.6 x 10
- 5
N
The electrostatic force is repulsive because both charges are
negative.
Superposition Principle
States that each charge will exert a force on other charges as if no other charges are
present. The total force that a particular charge experiences due to a collection of charges
is the vector sum of all the individual forces.
Sample Problem 1 :
Three-point charges are located along the x-axis. Point charge q 1
= 3.5 x 10
is at x
= 0, point charge q 2
= 8.5 x 10
is at x = 2m, and point charge q 3
= - 5.0 x 10
is at x = 3m.
Find the resultant force acting on q 1
Given: q 1
= 3.5 x 10
C
q 2
= 8.5 x 10
C
q 3
= - 5.0 x 10
C
k = 9 x 10
9
N⋅m
2
/C
2
r 2on
= 2.0 m
r 3on
= 3.0 m
Solution: Solve first the individual forces F 2on
(force exerted by q 2
on q 1
) and F 3on
(force exerted by q 3
on q 1
𝐹
(EF'
= k
𝑞
'
𝑞
(
(𝑟
(EF'
)
(
= 9 𝑥 10
9
𝑁 •
𝑚
2
𝐶
2
G 3. 5 𝑥 10
− 6
𝐶J G 8. 5 𝑥 10
− 6
𝐶J
2 𝑚
2
GENERAL PHYSICS 2 LEARNING MODULE
= 0.0669 N
Point charge q 2
repels q 1
. Thus, F 2on
is directed to the left
𝐹
BEF'
= k
𝑞
'
𝑞
(
(𝑟
BEF'
)
(
= 9 𝑥 10
9
𝑁 •
𝑚
2
𝐶
2
G 3. 5 𝑥 10
− 6
𝐶J G− 5 𝑥 10
− 6
𝐶J
( 3 𝑚)
2
= 0. 0175 N
Point charge q 3
attracts q 1
. Thus, F 3on
is directed to the right.
Add now the two forces. F2on1 will have a negative sign because its direction is going
to the left while F 3on
is positive because it is going to the right.
𝐹
F"L
= 𝐹
(EF'
BEF'
= − 0. 0669 𝑁 + 0. 0175 𝑁
Answer:
= − 0.0494 N
The magnitude of F acting upon q 1
is 0.0494N and is directed to the left
(because of the negative sign)
Sample Problem 2 :
Three identical point charges with a charge q = 3.0 x 10
C are placed at each
vertex of an equilateral triangle. If the sides of the equilateral triangle are 0.01m, find the
resultant electric force acting on q 1
Given: q 1
= 3.0 x 10
C
q 2
= 3.0 x 10
C
q 3
= - 3.0 x 10
C
k = 9x
9
N⋅m
2
/C
2
r 2on
= 0.01 m
r 3on
= 0.01 m
Solution: Solve first the individual forces F 2on
(force exerted by q 2
on q 1
) and F 3on
(force exerted by q 3
on q 1
𝐹
(EF'
= k
𝑞
'
𝑞
(
(𝑟
(EF'
)
(
GENERAL PHYSICS 2 LEARNING MODULE
Direction. Solve the following problems. Show your complete solution. ( 20 points)
- Two small conducting and identical spheres A and B have charges – 25 x 10
C and
15 x 10
C. They are separated by a distance of 0.02 m. What is the magnitude of
the electric force between the two spheres, is the force attractive or repulsive?
Name: ______________________________________ Score: _______
Put a ✓ mark:
____I worked alone on this activity._
____I received assistance from _________________________ on this activity._
GENERAL PHYSICS 2 LEARNING MODULE
- Two charged spheres q 1
= – 2.00 x 10
C and q 2
= – 1.50 x 10
C are 10.0 cm apart.
Calculate the force that each charge exerts on a third charge q 3
= 5.00 x 10
C
which is 6 cm apart from q 2
. (Note: Convert r to meters)
