8.3 and 8.4 - Kirchoffs 1st Law and Mean Drift Velocity

Vocabulary flashcards covering key definitions, principles, and equations from OCR A Physics A-level Topic 4.1: Charge and Current.

Electric current ($I$)

The rate of flow of charge, having the SI base unit of amperes ($\text{A}$) and measured in series using an ammeter.

Charge ($Q$)

A physical quantity that can be positive or negative, where like charges repel and opposite charges attract; its net value on a particle is given by $Q = \pm ne$.

Coulomb ($\text{C}$)

The unit of electric charge, defined as the flow of charge in $1\,\text{s}$ when the current is $1\,\text{A}$, with SI base units of $\text{A}\,\text{s}$.

Elementary charge ($e$)

The fundamental unit of electric charge, equal to $1.6 \times 10^{-19}\,\text{C}$, of which all ion and atomic charges are multiples.

Charge carriers

Charged particles that move through a material to carry electric current, such as free electrons in metals or ions in electrolytes.

Electrolytes

Conducting liquids, commonly ionic solutions containing positive cations and negative anions, that conduct charge via ion movement.

Anode

The positive electrode placed in an electrolyte solution, which attracts anions.

Cathode

The negative electrode placed in an electrolyte solution, which attracts cations.

Conventional current

The rate of flow of charge defined as travelling from the positive terminal to the negative terminal, which is opposite to the flow of electrons in metals.

Kirchhoff’s first law

A law stating that for any point in an electrical circuit, the sum of the currents into that point is equal to the sum of the currents coming out of that point.

Conservation of charge

The law that states electric charge cannot be created or destroyed, which serves as the basis for Kirchhoff’s first law.

Mean drift velocity ($v$)

The average velocity of charge carriers (such as electrons) as they travel down a wire while continually colliding with positive metal ions.

Number density ($n$)

The number of free charge carriers (electrons) per unit volume in a material, typically around $10^{28}\,\text{m}^{-3}$ for metal conductors.

Current transport equation ($I = A n e v$)

The relationship expressing electric current ($I$) as the product of cross-sectional area ($A$), number density ($n$), elementary charge ($e$), and mean drift velocity ($v$).