OCR A Physics A-level Topic 4.1: Charge and Current

Study Notes

Charge and Current

Electric current (I) is defined as the rate of flow of charge, expressed mathematically as I = Q/t, where Q is charge and t is time.
The SI base unit for current is Amperes (A). An ammeter measures current and must be connected in series within a circuit.

Charge

Charge (Q) is a physical quantity that can be positive or negative, measured in coulombs (C).
1 coulomb (C) represents the flow of 1 ampere over 1 second (1 C = 1 A·s).
Charge values are quantized: a proton has a charge of +1 (1.6 x 10^-19 C) and an electron has a charge of -1 (1.6 x 10^-19 C).
In neutral atoms, the number of protons equals electrons, resulting in no net charge. Adding electrons creates negative ions, while removing them creates positive ions.
The net charge formula is Q = ±ne, where n is the number of electrons added or removed.

Charge Carriers

Electric current involves charge carriers that vary by material type; electrons typically carry current in metals.
Metals have a lattice of positive ions surrounded by free-moving electrons. When an electric field is applied, electrons move toward the positive electrode, creating current.
Electrolytes, such as ionic solutions, allow charge conduction via positive and negative ions. For instance, saltwater (NaCl in H₂O) dissociates into Na+ cations and Cl- anions, which move towards their respective electrodes, generating current.

Conventional Current

Conventional current was established before electron discovery and is defined as the flow of charge from positive to negative terminals, irrespective of the actual movement of electrons.
In metals, electrons flow from negative to positive, resulting in electron flow opposite to conventional current direction.

Kirchhoff's First Law

Kirchhoff’s first law dictates that at any junction in an electrical circuit, the sum of currents entering equals the sum of currents leaving the junction, reflecting the conservation of charge.
This reinforces that charge cannot be created or destroyed, reinforcing its conservation as a fundamental property.

Mean Drift Velocity

Mean drift velocity (v) defines the average velocity of electrons moving through a conductor, which varies due to collisions with positive metal ions.
When connected to a power supply, electrons are attracted to the positive terminal but still experience these collisions, which affect their overall motion.
Number density (n) indicates the number of free electrons per unit volume. Conductors like metals possess high number densities (around 10²⁸ m⁻³), while insulators have lower densities, requiring faster electron speeds to maintain the same current level.

Description

This quiz focuses on Topic 4.1 of the OCR A-level Physics curriculum, covering the concepts of electric charge and current. Explore the definitions, units, and measurements related to current, including its rate of flow and the significance of Amperes.