ELECTROSTATICS
A BRIEF INTRODUCTION
WHAT IS ELECTROSTATICS?
• Electrostatics is the study of electric charges at rest.
• Electrostatics focuses on:
• Electric charges (positive and negative)
• Forces between charges
• Electric fields created by charges
• Electric potential and energy
• Charge distribution on conductors and insulators
• A key law of electrostatics is Coulomb’s Law, which describes the
force between two stationary charges.
ELECTRIC CHARGE
• Electric charge is a physical property of particles such as
electrons and protons.
• Electrons carry negative charge (–)
• Protons carry positive charge (+)
• Neutrons have no charge
• Electric charge is symbol: q
SI unit of charge: Coulomb (C)
BASIC PROPERTIES OF ELECTRIC CHARGE
1. Like Charges Repel
• Two positive charges repel
Two negative charges repel
• 2. Unlike Charges Attract
• A positive and a negative charge attract each other.
3. Charge is Quantized
• Charge exists in smallest unit:
• 𝒆 = 𝟏. 𝟔 × 𝟏𝟎−𝟏𝟗 C
• All charges are multiples of this value.
• Charge is Conserved
• Charge cannot be created or destroyed.
It only transfers from one object to another.
EXAMPLES OF ELECTRIC CHARGE IN DAILY
LIFE
✨ 1. Balloon Sticks to Wall
• When a balloon is rubbed with hair, it gains electrons → becomes negatively
charged.
The wall has induced positive charge, so balloon sticks.
✨ 2. Lightning
• Huge buildup of charge in clouds breaks down air and discharges to ground.
✨ 3. Shocks from Door Handle
• Walking on carpet builds up charge on your body.
Touching metal discharges it → shock.
✨ 4. Paper Attracted to Comb
• Combing hair transfers charges to the comb.
Charged comb attracts small paper pieces.
COULOMB’S LAW
• Statement of Coulomb’s Law
• The electrostatic force between two point charges is:
• Directly proportional to the product of the charges
• Inversely proportional to the square of the distance between
them
• Acts along the straight line joining the two charges
• In equation form:
𝑞 𝑞
𝑟
• 𝐹 = 𝑘 12 2
Symbol
Meaning
F
Electrostatic force (N)
q₁, q₂
Magnitude of charges (C)
r
k
Distance between charges
(m)
Coulomb’s constant = 𝟗
𝑵𝒎
𝟗
× 𝟏𝟎
𝑪𝟐
𝟐
MEANING OF COULOMB’S LAW
• ✔ If q₁ and q₂ have the same sign (+/+ or –/–)
• → Force is repulsive
(Charges push each other away)
• ✔ If q₁ and q₂ have opposite signs (+/–)
• → Force is attractive
(Charges pull toward each other)
• ✔ If distance r increases
1
𝑟
• Force becomes weaker, because it depends on 2
VECTOR FORM OF COULOMB’S LAW
• When direction is considered:
𝒒𝟏 𝒒𝟐
•𝑭=𝒌 𝟐 𝒓
𝒓
• 𝒓is a unit vector pointing from one charge
to the other.
• This tells us the exact direction of the
force.
CONDITIONS FOR COULOMB’S LAW
•
•
•
•
•
•
•
•
Coulomb’s law works accurately only when:
1. Charges are stationary (electrostatic)
No moving charges.
2. Charges behave like point charges
Their size is very small compared to distance between them.
3. Medium is uniform
The constant 𝑘 changes if medium is air, vacuum, water, oil, etc.
For any medium:
• 𝑘=
1
4𝜋𝜖
COULOMB’S FORCE IN DIFFERENT MEDIUMS
• In vacuum/air → Strong force
• In water → Much weaker force (because water
has high permittivity)
• They are inversely related:
• High permittivity → weaker interaction
• Low permittivity → stronger interaction
• Vacuum has the lowest permittivity → strongest electric force.
EXAMPLE TO UNDERSTAND COULOMB’S LAW
• Example 1
• Two charges:
𝑞1 = 2 C
𝑞2 = 3 C
Distance 𝑟 = 1 m
• 𝐹 = 9 × 109
2 3
12
• 𝐹 = 54 × 109 N
• This is a huge force because real charges are usually very small
(microcoulombs or nanocoulombs).
SIMPLE REAL-LIFE EXAMPLES
1. Rubbing a balloon
• The balloon becomes negatively charged and attracts hair due to Coulomb
force.
2. TV or computer glass attracting dust
• The dust particles are charged → get attracted by opposite charge on the
screen.
3. Lightning
• Large charges in clouds attract opposite charges on the ground → strong
Coulomb force, leading to discharge.
IN SIMPLE WORDS
• Coulomb's law tells us how strongly two charges pull or
push each other, and this depends on:
• How big the charges are
• How far apart they are
• What medium is between them
ELECTRIC FIELD
What Is an Electric Field?
An electric field is a region around a charged object in which
another charge experiences an electric force.
In simple words:
Electric field tells us how a charge affects the space around
it.
It shows how a test charge will be pushed or pulled if placed
nearby.
• Field around a charge
• Direction: away from + charge, toward – charge
ELECTRIC FIELD LINES
• Represent field direction
• Never cross
• Denser lines = stronger field
REAL-LIFE APPLICATIONS
• Photocopiers & laser printers
• Electrostatic precipitators
• Capacitors in circuits
ELECTRIC POTENTIAL ENERGY (U)
• Definition:
• Electric potential energy is the energy stored in a charge due to its
position in an electric field.
• 𝑈 = 𝑞𝑉
• Where:
• 𝑈= Electric potential energy (Joule)
• 𝑞= Charge (Coulomb)
• 𝑉= Electric potential (Volt)
• A charge of 𝟐 𝑪is placed at a point having potential 𝟓 𝑽:
•
𝑼 = 𝒒𝑽 = 𝟐 × 𝟓 = 𝟏𝟎 𝑱
• So the charge possesses 10 joules of electric potential energy.
ACTIVITY
MARKS 5
Problem 1
• Two charges 𝒒𝟏 = 𝟒 𝝁𝑪and 𝒒𝟐 = 𝟔 𝝁𝑪are placed
2 m apart in air.
Find the electrostatic force between them.
Problem 2
• Two point charges are located 1 m apart in
vacuum.
The force between them is 𝟑. 𝟔 N.
If one charge is 𝟒 𝝁𝑪, find the magnitude of the
other charge.