Capacitance, Dielectrics, and Electric Circuits (Video Notes)

Vocabulary flashcards covering key concepts from the lecture notes on capacitance, dielectrics, current, Kirchhoff’s laws, emf, batteries, and resistivity.

Capacitance

The ability of a system (e.g., a parallel-plate capacitor) to store charge per unit voltage; for a parallel-plate capacitor, C = ε0 A / d.

Parallel-plate capacitor

A capacitor made of two flat plates separated by a small gap; its capacitance depends on plate area A, separation d, and the dielectric between plates.

Plate area (A)

The surface area of the capacitor plates; larger A increases capacitance.

Plate spacing (d)

The distance between the plates; smaller d increases capacitance.

Permittivity of free space (ε0)

A constant (8.85 × 10^-12 F/m) used in C = ε0 A / d.

Dielectric

An insulating material placed between capacitor plates that polarizes in an electric field and increases capacitance.

Dielectric constant (k)

The factor by which a dielectric increases capacitance: C = k C0 (where C0 is the capacitance without the dielectric).

Capacitance without dielectric (C0)

Capacitance of a capacitor with air or vacuum between plates; C0 = ε0 A / d.

Capacitance with dielectric (C)

The capacitance when a dielectric is between plates: C = k C0.

Polarization

Alignment of dipoles in a dielectric due to an electric field, reducing the effective field.

Effect of dielectric on capacitor field

The dielectric’s polarization produces a field that opposes the capacitor’s original field, reducing the net field and increasing C.

Water as dielectric

Water has a high dielectric constant (≈80) because water molecules are permanent dipoles.

Insulator

A material that does not readily conduct electricity; in capacitors, the dielectric behaves as an insulator between plates.

Current

The rate of flow of electric charge; unit is the ampere (A), equal to 1 coulomb per second.

Conventional current

Flow of positive charge; by convention current is defined to move from high potential to low potential.

Electron drift velocity

The average steady speed of electrons moving under an applied electric field; it is typically very slow.

Charge carrier

Particles that carry charge: electrons in metals, ions in ionic solutions.

Conductor vs Insulator

Conductors have free charges that move; insulators lack free charges and do not conduct well.

Resistance (R)

Opposition to current flow; measured in ohms (Ω); for a wire, R depends on material, length, and cross-section.

Resistivity (ρ)

Intrinsic property of a material (Ω·m) that determines how strongly the material opposes current.

R = ρ L / A

Resistance as a function of resistivity, length, and cross-sectional area.

Junction (Kirchhoff’s current law)

A point where wires meet; Kirchhoff’s current law states that the sum of currents into a junction equals the sum out.

emf (electromotive force)

The potential difference created by a device that actively separates charges; symbol E, unit volts; not produced by a capacitor.

Battery

A device that provides emf via chemical reactions, lifting charges from the negative to the positive terminal; can be rechargeable.

Terminal voltage vs emf

Terminal voltage is the actual voltage across the battery when current flows; it is typically less than emf due to internal resistance.

Series batteries

When connected in series, their emfs add; e.g., three 1.5 V cells give 4.5 V.

Energy transformation in a battery

Chemical energy is converted to electrical potential energy as charges move; energy is stored in chemical form and released as current.

Nichrome resistivity

Resistivity of nichrome ≈ 1.5 × 10^-6 Ω·m; higher than copper, used in heating elements.

Salinity and water conductivity

Higher salinity increases the number of charge carriers in water, lowering resistivity and increasing current for a fixed voltage.