Charge and Current in A Level Physics AS

Charge and Current

This topic introduces fundamental electrical concepts, focusing on electric charge and current. Understanding these concepts is essential for further studies in physics and engineering.

Electric Charge

Electric charge is a property of subatomic particles, which determines their electromagnetic interactions. There are two types of charge: positive and negative. Like charges repel each other, while opposite charges attract.

Conservation of Charge

The principle of conservation of charge states that the total electric charge in an isolated system remains constant. This means that charge can neither be created nor destroyed, only transferred from one body to another.

Current

Current is defined as the rate of flow of electric charge. It is measured in amperes (A), where 1 ampere equals 1 coulomb of charge passing through a point in a circuit per second. Current can be classified into two types:

• Conventional Current: This is the flow of positive charge from the positive terminal to the negative terminal of a power source.
• Electron Flow: In reality, electrons, which carry a negative charge, flow from the negative terminal to the positive terminal.

Current as Rate of Charge Flow

The relationship between current (I), charge (Q), and time (t) is given by the formula:

I = Q/t

This equation shows that the current is directly proportional to the amount of charge passing through a point in a circuit over a specific time period.

Conductors and Insulators

Materials can be classified as conductors or insulators based on their ability to conduct electric current:

• Conductors: Materials like metals (e.g., copper and aluminum) allow electric charge to flow easily due to the presence of free electrons.
• Insulators: Materials such as rubber or glass do not allow electric charge to flow freely, as their electrons are tightly bound to their atoms.

Electrical Conduction in Metals

In metals, electrical conduction occurs due to the movement of free electrons. When a voltage is applied across a metal conductor, the free electrons gain energy and drift towards the positive terminal, creating an electric current.

Basic Circuit Symbols and Conventions

Understanding basic circuit symbols is crucial for interpreting circuit diagrams. Common symbols include:

• Battery: Represented by two parallel lines, one longer than the other.
• Resistor: Shown as a zigzag line.
• Switch: Depicted as a break in a line with a small circle.

These symbols help in constructing and analyzing electrical circuits effectively.

Worked Example

Problem: A circuit has a current of 3 A flowing for 5 seconds. How much charge has passed through a point in the circuit?

Solution:

• Given: I = 3 A, t = 5 s
• Using the formula Q = I × t, we find:
• Q = 3 A × 5 s = 15 C

Thus, 15 coulombs of charge have passed through the circuit.