Positive and Negative charge
ELECTRIC CHARGE
Electric charge is a fundamental property of matter responsible for all electrical phenomena such as attraction, repulsion, lightning, current flow, static electricity, etc.
All matter is made of atoms, and every atom contains:
Protons → positively charged
Electrons → negatively charged
Neutrons → no charge (neutral)
Normally, an atom has equal numbers of protons and electrons, so it is electrically neutral.
TYPES OF ELECTRIC CHARGE
There are only two types of electric charge:
1. Positive Charge
2. Negative Charge
There is no third type of charge.
POSITIVE ELECTRIC CHARGE
A body becomes positively charged when it loses electrons.
Why electrons move, not protons
Protons are tightly bound inside the nucleus.
Electrons are loosely bound and can move from one object to another.
How a body gets positive charge
When electrons are removed from it.
Number of protons > number of electrons.
Examples
Glass rod rubbed with silk
Human body after losing electrons
Clouds that have lost electrons
Properties of Positive Charge
Repels another positive charge
Attracts a negative charge
Represented by +
Measured in Coulomb (C)
NEGATIVE ELECTRIC CHARGE
A body becomes negatively charged when it gains extra electrons.
How a body gets negative charge
When electrons are added to it.
Number of electrons > number of protons.
Examples
Ebonite rod rubbed with wool
Plastic comb rubbed with dry hair
Balloon rubbed on woolen cloth
Properties of Negative Charge
Repels another negative charge
Attracts a positive charge
Represented by −
Measured in Coulomb (C)
INTERACTION BETWEEN CHARGES
Basic Law of Charges
Like charges repel each other
Unlike charges attract each other
ELECTRIC NEUTRALITY
A body is neutral when:
Number of protons = number of electrons
Net charge = 0
Neutral objects can still be attracted by charged objects due to induced charges.
FINDING THE SIGN OF AN ELECTRIC CHARGE
Sometimes we know a body is charged, but we don’t know whether it is positive or negative. There are several methods to determine the sign of charge.
1. Using a Known Charged Body
Procedure
Take a body whose charge is known (positive or negative).
Bring it near the unknown charged body.
Observation and Conclusion
Repulsion → same type of charge
Attraction → opposite type of charge
This method works because repulsion is the sure test of charge.
2. Using an Electroscope
An electroscope is a device used to:
Detect the presence of electric charge
Determine the type (sign) of charge
Parts of an Electroscope
Metal knob
Metal rod
Thin gold or aluminum leaves
Glass enclosure
Working Principle
Like charges repel each other.
Procedure
Bring the charged object near the metal knob.
If leaves diverge → object is charged.
Compare divergence with known charges to find sign.
3. By Charging Effects (Induction Test)
Bring a charged body near a neutral conductor.
Observe redistribution of charges.
The nature of induced charge helps identify the original charge.
CHARGING OF A BODY
Charging means making a neutral body electrically charged.
There are three main methods of charging:
Charging by friction
Charging by conduction
Charging by induction
(Your focus asked especially for friction, but others are mentioned for completeness.)
CHARGING BY FRICTION
Definition
Charging by friction is the process of charging a body by rubbing it with another body, causing transfer of electrons.
Mechanism
When two different materials are rubbed:
Electrons move from one material to another.
One body loses electrons → becomes positively charged
Other body gains electrons → becomes negatively charged
Important Facts about Charging by Friction
Only electrons are transferred
Protons never move
Both objects get equal and opposite charges
Total charge remains conserved
Works best for insulators
CONSERVATION OF CHARGE
Charge can neither be created nor destroyed. It can only be transferred from one body to another.
REAL-LIFE EXAMPLES
Lightning
Static shocks
Clothes sticking together
Dust sticking to TV screens
Balloon sticking to walls