Electric Charges and Fields

Vocabulary practice flashcards covering fundamental concepts, definitions, and laws of electric charges, fields, dipoles, and Gauss's law from Chapter 1.

Electrostatics

The branch of physics that deals with the study of forces, fields, and potentials arising from static (non-moving) charges.

Polarity of Charge

The property that differentiates the two kinds of electric charges: positive and negative.

Gold-leaf Electroscope

A simple apparatus used to detect charge on a body, consisting of a vertical metal rod housed in a box with two thin gold leaves attached to its bottom end that diverge when charged.

Conductors

Substances that readily allow the passage of electricity through them because they possess electric charges (electrons) that are comparatively free to move inside the material, such as metals, earth, and human or animal bodies.

Insulators

Substances (such as glass, porcelain, plastic, nylon, and wood) that offer high resistance to the passage of electricity through them and keep acquired electric charges localized.

Point Charges

Charged bodies whose linear physical sizes are very small compared to the distance separating them, allowing their entire charge content to be treated as concentrated at a single point in space.

Additivity of Charges

The property stating that the total electric charge of a system is the algebraic scalar sum of all the individual point charges in the system: $q_{\text{total}} = q_1 + q_2 + \dots + q_n$.

Conservation of Charge

The principle stating that the total electric charge of an isolated system remains unchanged with time; net charge cannot be created or destroyed, even if charge-carrying particles are created or destroyed.

Quantisation of Charge

The fact that all observable free electric charges are integral multiples of a basic quantum unit of charge $e$, expressed by the relation $q = n\,e$, where $n$ is any integer and $e \approx 1.602 \times 10^{-19}\,\text{C}$.

Coulomb

The SI unit of electric charge (symbol $\text{C}$), defined as the charge flowing through a wire in $1\,\text{s}$ when the electric current is $1\,\text{A}$.

Coulomb's Law

A quantitative statement that the electrostatic force between two point charges varies inversely as the square of the distance separating them, is directly proportional to the product of their magnitudes, and acts along the line joining them: $F = \frac{1}{4\pi\varepsilon_0}\frac{|q_1 q_2|}{r^2}$.

Permittivity of Free Space

A physical constant denoted by $\varepsilon_0$, having the value $\varepsilon_0 = 8.854 \times 10^{-12}\,\text{C}^2\,\text{N}^{-1}\,\text{m}^{-2}$ in SI units.

Principle of Superposition (Electrostatics)

The principle stating that the force acting on any single charge due to multiple surrounding charges is the vector sum of all the individual Coulomb forces exerted on it by each charge, unaffected by the presence of other charges.

Electric Field

A vector field defined at every point in space as the electrostatic force experienced by a unit positive test charge placed at that location without disturbing the source charge configuration: $\mathbf{E} = \lim_{q \to 0} \frac{\mathbf{F}}{q}$.

Electric Field Line

A continuous curve drawn such that the tangent to it at each point specifies the direction of the net electric field at that point, with the spatial density of lines indicating the relative strength of the field.

Electric Flux

A scalar measure of the number of electric field lines passing through a given surface area element $\Delta S$, defined by $\Delta \phi = \mathbf{E} \cdot \Delta\mathbf{S} = E\,\Delta S\,\cos(\theta)$, with SI unit $\text{N}\,\text{C}^{-1}\,\text{m}^2$.

Area Element Vector

A vector associated with a small surface area element, defined as $\Delta\mathbf{S} = \Delta S\,\mathbf{\hat{n}}$, where $\Delta S$ is its area magnitude and $\mathbf{\hat{n}}$ is a unit vector directed along the outward normal to the surface.

Electric Dipole

A system composed of a pair of equal and opposite point charges, $q$ and $-q$, separated by a distance $2a$.

Electric Dipole Moment

A vector quantity defined by $\mathbf{p} = 2qa\,\mathbf{\hat{p}}$, with magnitude equal to the charge $q$ times the separation $2a$ and direction pointing along the dipole axis from $-q$ to $+q$.

Point Dipole

An idealized electric dipole considered in the limit where the separation distance $2a \to 0$ and the charge $q \to \infty$ such that the dipole moment product $p = 2qa$ remains finite.

Polar Molecules

Molecules in which the centres of positive and negative charges do not coincide, thereby possessing a permanent electric dipole moment even in the absence of an external electric field (e.g., $\text{H}_2\text{O}$).

Linear Charge Density

The electric charge per unit length along a wire on a macroscopic scale, denoted by $\lambda = \frac{\Delta Q}{\Delta l}$ and measured in $\text{C}\,\text{m}^{-1}$.

Surface Charge Density

The electric charge per unit surface area on a conductor or sheet on a macroscopic scale, denoted by $\sigma = \frac{\Delta Q}{\Delta S}$ and measured in $\text{C}\,\text{m}^{-2}$.

Volume Charge Density

The electric charge per unit volume within a continuous charge distribution, denoted by $\rho = \frac{\Delta Q}{\Delta V}$ and measured in $\text{C}\,\text{m}^{-3}$.

Gauss's Law

A law stating that the total electric flux through any closed surface $S$ is equal to $\frac{1}{\varepsilon_0}$ times the total net electric charge $q$ enclosed by that surface: $\phi = \frac{q}{\varepsilon_0}$.

Gaussian Surface

Any closed imaginary surface chosen to apply Gauss's law for calculating the electric field generated by a symmetric charge distribution.