Electric Charge

Nature of Electric Charge

Electric charge is a fundamental property of matter. There are two types of electric charge:

• Positive charge
• Negative charge

Objects can have an excess of either positive or negative charge.

Charge is quantised, meaning it exists in discrete amounts. The smallest unit of charge is the charge on a single electron or proton, approximately $1.6 \times 10^{-19}$ coulombs (C). All charges are multiples of this fundamental charge.

The forces between charged objects follow these rules:

• Like charges repel each other (positive repels positive, negative repels negative).
• Unlike charges attract each other (positive attracts negative).

This attraction or repulsion is due to the electric force, which acts over a distance without contact.

For example, if two balloons are rubbed on a jumper and brought close together, they repel because they both gain the same type of charge (negative).

Charge is measured in coulombs (C), and the total charge on an object is the sum of all the excess positive and negative charges.

The fact that charge is quantised means you cannot have a charge smaller than the charge of one electron or proton.

For instance, if an object has a charge of $-3.2 \times 10^{-19} \text{ C}$, it means it has an excess of 2 electrons (since $2 \times 1.6 \times 10^{-19} = 3.2 \times 10^{-19}$).

Example: If an object has a charge of $-1.6 \times 10^{-19} \text{ C}$, how many electrons does it have in excess? The answer is 1 electron.

Charging by Friction

Charging by friction occurs when two different materials are rubbed together. Electrons can be transferred from one material to the other, causing both objects to become electrically charged.

Only electrons move; protons remain fixed in the atoms. The object that loses electrons becomes positively charged, and the object that gains electrons becomes negatively charged.

Common examples include:

• Rubbing a plastic rod with a cloth
• Rubbing a balloon on hair
• Rubbing a polythene rod with a duster

The materials involved affect which way electrons move. For example, when a polythene rod is rubbed with a duster, electrons move from the duster to the rod, making the rod negatively charged.

This transfer of electrons creates an imbalance of charge, so the objects attract or repel other charged objects.

For example, rubbing a balloon on a woolly jumper transfers electrons to the balloon, making it negatively charged. The balloon can then attract small pieces of paper or stick to a wall due to electrostatic forces.

The amount of charge depends on the materials, the force of rubbing, and the time spent rubbing.

The charge remains on the surface of the object until it is discharged, for example, by touching a conductor connected to the Earth (earthing).

For instance, if a plastic rod is rubbed with a cloth and gains a charge of $-4.8 \times 10^{-7} \text{ C}$, this means it has gained electrons. The number of electrons transferred is:

$$\text{Number of electrons} = \frac{\text{Charge}}{\text{Charge per electron}} = \frac{4.8 \times 10^{-7}}{1.6 \times 10^{-19}} = 3 \times 10^{12}$$

Conductors and Insulators

Materials can be classified based on how easily electric charge can move through them:

• Conductors: Allow electric charge (electrons) to flow freely through them.
• Insulators: Do not allow electric charge to flow freely; electrons are not free to move.

Conductors include:

• Metals such as copper, aluminium, and iron
• Graphite (a form of carbon)

Insulators include:

• Plastic
• Rubber
• Glass
• Dry wood

When an insulator is charged by friction, the charge stays on the surface because it cannot move through the material.

In conductors, any excess charge quickly spreads out over the surface or moves away if the conductor is connected to the Earth (earthing).

For example, when a metal sphere is charged, the charge distributes evenly over its surface because the electrons can move freely.

This difference explains why static electricity is commonly observed with insulators like plastic rods and balloons, rather than metals.

Conservation of Charge

Electric charge is always conserved. This means:

• The total charge in an isolated system remains constant.
• Charge cannot be created or destroyed, only transferred from one object to another.

When two objects are rubbed together, electrons move from one to the other, but the total charge before and after remains the same.

For example, if one object gains a charge of $-5 \times 10^{-6} \text{ C}$, the other must lose the same amount of negative charge (or gain an equal positive charge).

This principle is important in understanding static electricity and charge interactions.

If an object is charged negatively, it has gained electrons; if positively, it has lost electrons. The total number of electrons and protons in the system remains constant.