Fundamentals of Electric Charge and Current

Introduction to Electric Charge

Electric charge is a fundamental property of matter that governs the behavior of particles and their interactions. Charge comes in two types: positive and negative. Like charges repel each other, while opposite charges attract.

The principle of conservation of charge states that the total charge in an isolated system remains constant. In other words, charge cannot be created or destroyed, only transferred from one object to another.

Conductors and Insulators

Materials can be classified as conductors or insulators based on their ability to allow the flow of electric charge:

• Conductors (e.g., metals) have loosely bound valence electrons that can move freely, allowing charge to flow through them.
• Insulators (e.g., plastics, rubber) have tightly bound electrons that cannot move freely, preventing the flow of charge.

Electric Current and Electron Flow

Electric current is the rate of flow of electric charge through a conductor. It can be caused by the movement of free electrons (negatively charged particles) or positive ions (positively charged atoms).

By convention, current (I) is defined as the flow of positive charge. This is known as conventional current. However, in metals, it is the negatively charged electrons that actually move, which is called electron flow.

Worked Example: Calculating Current

Problem: If 2.5 × 10¹⁸ electrons flow through a wire in 10 seconds, what is the current?

Given: Charge on one electron = 1.6 × 10⁻¹⁹ C

Solution:

1. Calculate the total charge that flowed: Q = (2.5 × 10¹⁸) × (1.6 × 10⁻¹⁹ C) = 4.0 × 10⁻¹ C
2. Current (I) = Charge (Q) / Time (t) = (4.0 × 10⁻¹ C) / 10 s = 4.0 × 10⁻² A

Circuit Symbols and Conventions

In electrical circuits, standard symbols are used to represent different components. Some common symbols include:

• Battery or power source:
• Resistor:
• Switch:

Circuits are typically drawn with the positive terminal of the power source on the left, and the conventional current flow indicated by arrows.

This topic lays the foundation for understanding more advanced electrical concepts and circuit analysis in A Level Physics.