Electric charge is a fundamental property of matter that causes objects to experience electric forces.

• • There are two types of electric charge: positive (+) and negative (−).

  • Like charges repel each other, and opposite charges attract.

  • Electric charge is carried by protons (+) and electrons (−); neutrons have no charge.

  • The SI unit of electric charge is the coulomb (C).

In short: electric charge determines how matter interacts through electric forces.

A coulomb (C) is the SI unit of electric charge.

• • 1 coulomb is the amount of charge carried by about 6.24 × 10¹⁸ electrons.

  • The charge of a single electron is −1.60 × 10⁻¹⁹ C.

  • The charge of a single proton is +1.60 × 10⁻¹⁹ C.

So, a coulomb measures how much electric charge an object has.

1.60 × 10⁻¹⁹ C is the fundamental (elementary) charge of nature.

• • Experiments showed that electric charge always comes in fixed, smallest-sized packets.

  • The smallest charge that appears on a free particle is the charge of the electron (and proton).

  • This value was measured experimentally (not chosen arbitrarily), most notably in Millikan’s oil-drop experiment.

  • All observable charges are whole-number multiples of this value.

So 1.60 × 10⁻¹⁹ C is the basic “unit size” of electric charge, and larger charges are built from it.

https://www.thephysicsaviary.com/Physics/Programs/Labs/MillikanOilDropLab/ 

Charging & Charge Distribution: 

Charging by Friction: Transferring electrons by rubbing two different neutral insulators. The material with higher affinity steals electrons.

The charging by friction process involves rubbing of one particle on another resulting in electrons moving from one surface to another. This method is useful for charging insulators.

Insulators are materials that do not allow electric charges to move freely through them.

• • Examples: rubber, glass, plastic, wood.

  • Because charges can’t move easily, insulators block or slow the flow of electricity.

Insulator Polarization

• • Charges are not free to move.

  • Electrons shift slightly within atoms or molecules.

  • No charges travel across the object.

  • Often called polarization or dipole alignment.

Example: a charged balloon attracting paper or bending a water stream.

https://phet.colorado.edu/sims/html/balloons-and-static-electricity/latest/balloons-and-static-electricity_all.html 

Examples:

Balloon and paper bits

1. 1. Rub a balloon on your hair (balloon becomes negatively charged)

   2. Hold it near tiny pieces of paper.

   3. The paper is neutral, but its charges shift slightly, so the side near the balloon becomes positive and is attracted.

Dust sticking to a charged object

1. 1. A charged rod can attract neutral dust.

   2. The dust’s charges shift, creating a temporary attraction.

Conductor Polarization (often called Induction)

• • Electrons are free to move through the material.

  • Charges redistribute over large distances.

  • Creates distinct positive and negative regions.

  • Commonly called induction, not polarization.

Example: a charged rod near a metal sphere causing charge separation.

Clean distinction

• Insulators → polarization (small, local shifts)

• Conductors → induction (large, mobile charge rearrangement)

Both are caused by the same idea — electric forces at a distance — but the mobility of charges makes the behavior different.

Induction is the rearrangement of electric charges within a conductor caused by the presence of a nearby electric field, without direct contact.

Electroscope demonstration

• • Bring a charged rod close to the top of a neutral electroscope.

  • The leaves separate as charges redistribute within the electroscope.

Key point: induction creates a net separation of charge in a conductor without direct contact.

Induction: Nearby charges push or pull electrons in a conductor, creating regions of positive and negative charge without touching.

Charging by Conduction: Charging by direct contact. Electrons flow physically to neutralize the potential difference, leaving both objects with the same sign.

The charging by conduction process involves touching of a charged particle to a conductive material. This way, the charges are transferred from the charged material to the conductor. This method is useful for charging conductors.

Conductors and insulators

In a conductor, electric current can flow freely, in an insulator it cannot. 

Demo - Soda can races

Grounding is the process of connecting an object to the Earth (infinite reservoir) so that excess electric charge can flow to or from the ground, making the object electrically neutral.

• Example: touching a charged metal rod to a wire connected to the Earth allows electrons to move, removing or supplying charge safely.

Induction charging is a charging method that charges an object without actually touching the object to any other charged object. 

• The charging by induction process is where the charged particle is held near an uncharged conductive material that  is grounded on a neutrally charged material. The charge flows between two objects and the uncharged conductive material develop a charge with opposite polarity. These were the different methods of charge transfer. 

Charging by Induction Using a Negatively Charged Object

Charging by Induction Using a Positively Charged Object

Charging by Induction Using a Ground

Charging by induction is a method of giving an object a net electric charge without direct contact.

How it works:

1. 1. Bring a charged object near a neutral conductor.

   2. Charges in the conductor rearrange: opposite charges move toward the nearby object, like charges move away.

   3. Ground the conductor (optional) to let some charges leave or enter.

   4. Remove the ground and then the nearby charged object.

   5. The conductor now has a net charge opposite to the nearby object.

Charging by Induction: Charging without contact. Steps: 1. Polarize, 2. Ground, 3. Isolate. Result: Object gains the opposite sign.

AP Physics only