Purcell & Morin Ch. 7–11 Conceptual Vocabulary Flashcards

Vocabulary terms and core concepts from Purcell & Morin Chapters 7-11, covering electric and magnetic fields in matter, induction, Maxwell's equations, and electromagnetic waves.

Polarization

The tiny separation of positive and negative charge inside matter when an electric field is applied, which creates bound charge that modifies the field.

Bound charges

Charges that arise from the response of matter rather than from free charges, calculated using the formulas $\rho_b = -\nabla \cdot \mathbf{P}$ and $\sigma_b = \mathbf{P} \cdot \hat{n}$.

$\mathbf{D}$ field

A field used to make Gauss's law cleaner in matter by separating free charge ("what you did") from polarization ("what the material did").

Magnetization

The alignment of tiny internal current loops in matter, which produces bound currents as the magnetic analogue of bound charge.

Bound currents

Currents arising from a material's internal structure rather than from wires, defined by the formulas $\mathbf{J}_b = \nabla \times \mathbf{M}$ and $\mathbf{K}_b = \mathbf{M} \times \hat{n}$.

$\mathbf{H}$ field

A field that separates the applied magnetic field from the material’s magnetization, making Amp re’s law cleaner in matter.

Faraday’s law

The conceptual principle that a changing magnetic field creates a non-conservative, swirling electric field that can drive currents around loops.

Motional EMF

A voltage created when charges move through a magnetic field and feel a magnetic force pushing them sideways; it is a result of geometry and motion rather than changing flux.

Non-conservative induced electric field

An electric field whose line integral around a loop is nonzero because it originates from changing magnetic flux rather than static charges.

Displacement current

The term $\epsilon_0 \frac{\partial \mathbf{E}}{\partial t}$ added by Maxwell to Amp re’s law to maintain charge conservation and allow changing electric fields to create magnetic fields.

Maxwell’s Unification

The big idea that electric and magnetic fields are not separate but are one unified field where each can generate the other when changing in time.

Electromagnetic (EM) waves

A self-sustaining cycle where a changing $E$ field creates a $B$ field, which in turn creates an $E$ field, allowing the cycle to propagate through space.

Speed of light ($c$)

The wave speed predicted directly from vacuum constants in Maxwell's equations: $c = \frac{1}{\sqrt{\mu_0 \epsilon_0}}$.

Poynting vector ($\mathbf{S}$)

A vector representing the flow of electromagnetic energy, pointing in the direction of $\mathbf{E} \times \mathbf{B}$.

Dielectric

Electrical insulator that becomes polarized when exposed to an external electric field, allowing it to store electrical energy