Concept of Static Electricity

Static electricity

Definition: Static electricity is the build-up of electric charges on the surface of a material. These charges stay in one place until discharged, unlike current electricity where charges flow continuously.

The origin of charges

Electrification by rubbing: When two different materials (like a plastic pen and a cloth) are rubbed together:

1. One material gains electrons → becomes negatively charged.
2. The other loses electrons → becomes positively charged.

This process is called triboelectric charging (or electrification by rubbing). It can make a charged object attract light materials like paper.

Ancient Greeks first observed this with amber. Later, materials were classified as:

1. Electrics: easily become charged (e.g., plastic, rubber).
2. Non-electrics: do not easily become charged (e.g., metals).

Types of electric charges

1. Positive Charge (+): When electrons are lost.
2. Negative Charge (–): When electrons are gained.

Like charges repel, unlike charges attract.

How to identify the type of charge

Simple Experiment:

1. Suspend a polythene rod rubbed with fur (Rod A).
2. Bring another polythene rod rubbed the same way (Rod B) near it.

Observations:

1. Rod A repels Rod B → both have same charge (negative).
2. A polythene rod (negative) attracts an acetate rod rubbed with silk (positive) → opposite charges attract.

Fundamental law of electrostatics

Statement: "Like charges repel each other, and unlike charges attract each other."

This is the basic rule governing how electric charges interact:

Positive ↔ Positive → Repel

Negative ↔ Negative → Repel

Positive ↔ Negative → Attract

What is charging?

Definition: Charging is the process of giving or removing electric charges (usually electrons) to or from an object.

1. Positively charged object → has lost electrons
2. Negatively charged object → has gained electrons

Methods of charging

There are three main methods used to charge objects:

Charging by rubbing (friction)

1. When two materials are rubbed together, electrons can move from one material to the other.
2. One object gains electrons (becomes negative), and the other loses electrons (becomes positive).

Example:

Rubbing a polythene rod with fur:

1. Electrons transfer from the fur to the rod.
2. The rod becomes negatively charged.
3. The fur becomes positively charged.

Only electrons are transferred during this process.

Charging by induction

Definition: Charging by induction is the process of charging an object without direct contact. It involves bringing a charged object close to a neutral object, causing a redistribution of charges inside the neutral object.

How it works (example with a negatively charged polythene rod)

Bring the charged object near: A negatively charged polythene rod is brought close to (but does not touch) a neutral metal rod like copper.

Separation of charges in the metal:

1. Negative electrons in the metal are repelled by the negatively charged rod.
2. Electrons move to the far end of the metal rod.
3. The near end of the metal becomes positively charged (due to lack of electrons).

Grounding (optional for permanent charging):

1. If the far end of the metal rod is connected to the ground, excess electrons flow away.
2. After removing the ground and the charged rod, the metal rod remains positively charged.

Key points

1. No contact is needed.
2. Useful for charging conductors like metals.
3. The final charge on the object is opposite to that of the inducing object.

Positively charged, and the far end becomes negatively charged.

Earthing (Grounding) the metal rod: If you touch the metal rod with your finger while the charged rod is still nearby, you provide a path for electrons to flow.

The excess negative electrons from the metal rod flow through your body to the ground (this process is called earthing or grounding).

Removing the finger and charged rod:

1. After earthing, the negative charge has been removed from the metal rod.
2. When you remove your finger (disconnecting the path to the ground), and then remove the charged rod, the metal rod is left with a net positive charge. This happens because there is now an imbalance of charge after the electrons have been removed.

Charging by Contact: In this method, a charged object is brought into direct contact with an uncharged object.

1. Example: A positively charged metal can touches an uncharged conductor.
2. Charges flow from the charged object to the uncharged one until both have the same type of charge.