Current Electricity Practice Flashcards

A set of vocabulary flashcards covering the fundamental concepts of current electricity, including Ohm's law, drift velocity, resistivity, and circuit rules as described in the lecture notes.

Electric Current

The net amount of charge flowing across a cross-sectional area per unit time, defined as $I = \frac{q}{t}$ for steady flow or $I = \frac{dQ}{dt}$ for non-steady flow.

Ampere (A)

The SI unit of electric current; typical magnitudes range from microamperes in human nerves to tens of thousands of amperes in lightning.

Conductors

Materials, notably metals, where some electrons are practically free to move within the bulk material and develop electric currents when an electric field is applied.

Ohm’s Law

A basic law stating that the potential difference $V$ across the ends of a conductor is proportional to the current $I$ flowing through it, expressed as $V = RI$.

Resistance ($R$)

The constant of proportionality in Ohm's law, measured in ohms ($\Omega$); it depends on the material's resistivity and the physical dimensions of the conductor.

Resistivity ($\rho$)

A property of a material that defines its resistance per unit length and cross-sectional area, given by the relation $R = \rho \frac{l}{A}$. For metals, it typically ranges from $10^{-8}$ to $10^{-6} Ω m$.

Current Density ($j$)

The amount of current per unit area taken normal to the direction of flow, denoted as $j = \frac{I}{A}$, with SI units of $A/m^2$.

Conductivity ($σ$)

The reciprocal of resistivity ($σ = \frac{1}{\rho}$), often used to express Ohm's law in the vector form \mathbf{j} = σ ̀ \mathbf{E}.

Drift Velocity ($v_d$)

The average velocity that electrons acquire in a conductor due to an applied electric field, calculated as v_d = \frac{-e E ̀ τ}{m}, where $\u03C4$ is the relaxation time.

Relaxation Time ($\u03C4$)

The average time elapsed between successive collisions of an electron with the fixed ions in a conductor.

Mobility ($μ$)

The magnitude of the drift velocity per unit electric field, defined as $\u03BC = \frac{|v_d|}{E} = \frac{eτ}{m}$, measured in $m^2/Vs$.

Temperature Co-efficient of Resistivity ($α$)

The fractional increase in resistivity per unit increase in temperature, used in the approximate relation $\rho_T = \rho_0 [1 + α (T-T_0)]$.

Nichrome

An alloy of nickel, iron, and chromium that exhibits a very weak dependence of resistivity on temperature, making it suitable for heating elements and standard resistors.

GaAs (Gallium Arsenide)

A material that does not obey Ohm’s law and exhibits a non-unique relationship between current and voltage.

Ohmic Loss

The power dissipated as heat in a conductor of resistance $R$ carrying a current $I$, calculated as $P = I^2 R$ or $P = \frac{V^2}{R}$.

Electromotive Force (emf, $\varepsilon$)

The potential difference between the positive and negative electrodes of a cell in an open circuit (when no current is flowing).

Internal Resistance ($r$)

The finite resistance of the electrolyte and electrodes inside a cell, which causes the terminal voltage $V$ to drop to $\varepsilon - Ir$ when a current $I$ flows.

Kirchhoff’s Junction Rule

A rule based on the conservation of charge stating that at any junction, the sum of currents entering must equal the sum of currents leaving.

Kirchhoff’s Loop Rule

A rule based on the conservation of energy stating that the algebraic sum of changes in potential around any closed loop in a circuit is zero.

Wheatstone Bridge

An arrangement of four resistors used to determine an unknown resistance by achieving a balanced state where the galvanometer current is zero ($\frac{R_1}{R_2} = \frac{R_3}{R_4}$).

Meter Bridge

A practical device based on the principle of the Wheatstone bridge used to measure unknown resistances.