Electric Charge and Field Review (Video Notes)

Vocabulary flashcards covering key terms and definitions from the lecture notes on electric charge, fields, and Gauss's law.

Elementary charge (e)

The smallest unit of electric charge; e = 1.6×10^−19 C.

Charge quantization

Total charge is an integer multiple of e: Q = n e.

Conservation of charge

The total charge of an isolated system remains constant.

Proton

Positively charged particle with charge +e.

Electron

Negatively charged particle with charge −e.

Coulomb's Law

F = k |q1 q2| / r^2; force along the line between charges.

Coulomb's constant (k)

k ≈ 9.0×10^9 N·m^2/C^2.

Inverse-square law

Coulomb force scales as 1/r^2; doubling distance makes force 1/4.

Electric field (E)

Force per unit positive test charge: E = F/q.

Electric field of a point charge

E = k Q / r^2.

Force on a charge in an electric field

F = q E.

Electric field lines

Point away from positive charges and toward negative charges.

Unit of electric field

Newtons per coulomb (N/C).

Electric potential difference (ΔV)

ΔV = E d for a uniform field.

Uniform electric field between plates

E = Q / (A ε0) (as given in the notes).

Gauss's Law

ΦE = Qenclosed / ε0; flux through a closed surface equals enclosed charge over ε0.

Electric flux (Φ_E)

Net outward electric flux through a surface.

Enclosed charge

Total charge inside a Gaussian surface.

Flux dependence on surface size

Flux depends on the net enclosed charge, not the surface size.

Proton in an electric field

Accelerates in the same direction as the field.

Electron in an electric field

Accelerates opposite to the field.

F = qE and F = ma

Two laws give acceleration: a = qE / m.

Excess charge and electrons

Number of excess electrons n = Q / e.

Coulomb force between two charges (example)

F = k|q1 q2| / r^2; for q1 = +2 μC, q2 = −3 μC at 1 m, F ≈ 5.4×10^−2 N (attractive).

Inverse-square scaling with distance (factor 9)

If distance is reduced to r/9, force increases by 81×.

Proton acceleration in a field (8.0×10^4 N/C)

F = qE ≈ 1.28×10^−14 N; a ≈ 7.7×10^12 m/s^2.

Electron acceleration in same field

Same magnitude of force as proton, opposite direction; a ≈ 1.4×10^16 m/s^2.

Parallel plates: ΔV

ΔV = E d; with E = 5.0×10^3 N/C and d = 0.020 m, ΔV = 100 V.

Electric flux for enclosed +3.5 μC

Φ = Q/ε0; Φ ≈ 3.95×10^5 N·m^2/C.

Net flux for enclosed +0.9 μC

Φ = Q/ε0; Φ ≈ 1.0×10^5 N·m^2/C.

Electron vs proton acceleration in same field

Electron accelerates more because mass is ~1836× smaller.

Flux unchanged when radius doubles

Flux depends on enclosed charge, not surface size.

Neutral atom concept

A neutral atom has protons and electrons; net charge is zero.

Movement of charges: electrons vs protons

Electrons move; protons are effectively fixed in the nucleus.

Charging by removing electrons (+3e)

Having +3e means three electrons removed; protons unchanged.

Electric field lines originate/end points

They originate on positive charges and terminate on negative charges.

Field line crossing rule

Electric field lines do not cross.

Negative charge effect on force direction

If q is negative, the force is opposite to the field direction.

Electric field outside a capacitor

Approximately zero outside large parallel plates; edge effects neglected.

Edge effects in introductory physics

Edge effects are ignored; treat field as uniform between plates.

Proton mobility in materials

Protons are not free movers in typical electrostatics; electrons are the mobile carriers.

Charge sign and force direction recap

Positive q yields force in the direction of E; negative q yields opposite.

Gauss’s Law sign convention

Flux sign follows the direction of the normal relative to enclosed charge.

Test charge concept

A small, positive charge used to probe the electric field.

Electric field units in experiments

Measured in N/C.

Potential difference vs electric field

ΔV and E are related by ΔV = ∫ E·dl in general; the uniform case gives ΔV = Ed.

Capacitance relation (implicit)

Between parallel plates, E ≈ σ/ε0 with σ = Q/A.

Charge neutrality in atoms

Atoms can be neutral while having internal separated charges.

Electric force on a charge

F = q E; direction follows q and E.

Mass and acceleration in electric fields

Acceleration depends on F and mass: a = F/m.

Electric field direction at a point

A single definite direction exists; field lines show this direction.

Charge distribution in a conductor

Charges rearrange themselves on the surface in electrostatic equilibrium.

Electric potential energy (implicit)

Related to ΔV; not explicit in the notes but connected to ΔV = qΔV.

Coulomb force magnitude

Depends on product of charges and inverse square of distance.

Electric field concept in one line

E is the force per unit positive charge due to the surrounding charges.