4.1. Charge and Current

PMT resources for OCR A-Level Physics: Topic 4.1 - Charge and Current

Electric Current

• Definition: Electric current (I) is the rate of flow of charge.

• Formula: I = dQ/dt

• SI Unit: Amperes (A).

• Measurement: Current is measured with an ammeter in series, designed with low resistance to avoid altering the current.

Charge

• Definition: Charge (Q) can be positive or negative, measured in coulombs (C).

• Historical Context: Originates from early electricity studies by Benjamin Franklin.

• Relationship to Current: 1 C corresponds to a flow of charge in 1 second when current is 1 A.

• Interactions: Like charges repel; unlike charges attract.

Atomic Charge

• Basics: Protons have (+1 charge) and electrons have (-1 charge).

• Key Insight: Neutrons are neutral and contribute to atomic mass.

• Net Charge: Q = ±ne (n is the number of electrons added or removed).

Charge Carriers

• Conductors: In metals, current is carried by free-moving electrons in a lattice structure.

• Electrolytes: Ionic solutions conduct current by the movement of cations and anions.

Conventional Current

• Definition: Flows from positive to negative terminal, opposite to electron flow.

Kirchhoff’s First Law

• States that the sum of currents entering a junction equals the sum leaving, reflecting charge conservation.

Mean Drift Velocity

• Definition: Average velocity of electrons in a conductor.

• Number Density (n): Free electrons per unit volume.

• Current Equations: I = nAv and I = Q/t, with Q = n * e * V (where V is voltage).

This revised note keeps all the essential information while being more concise and clearer.