Electric Charges and Fields – Review Flashcards

A comprehensive set of Q&A flashcards covering instructor info, study tips, key mechanics review, and core concepts of electric charge, field, dipoles, superposition, and continuous charge distributions as presented in the lecture notes.

Who is the instructor for Phys 2326 and what is his email?

Dr. Oomman K. Varghese, [e-mail]

What pre-class activity is recommended to earn high grades?

Read the textbook before coming to class and note your questions.

What is produced by the dot product of two vectors?

A scalar quantity.

What rule determines the direction of the vector from a cross product?

The right-hand-thumb rule.

State one condition that proves two vectors are equal.

Their x-, y-, and z-components are individually equal.

Give the kinematic equation relating displacement and constant acceleration in one dimension.

x = x₀ + v₀t + ½at²

What diagram shows all the forces acting on an object?

A free-body diagram.

Define a conservative force.

A force that allows conversion between kinetic and potential energy with no net energy loss.

What two quantities are conserved in an elastic collision?

Kinetic energy and linear momentum.

Write the definition of moment of inertia for discrete masses.

I = Σ mᵢ rᵢ²

Torque vector formula in terms of force and lever arm.

τ⃗ = r⃗ × F⃗

What is the first condition of static equilibrium?

ΣF = 0 in all directions.

What restoring-force relation defines simple harmonic motion?

F = −k x

General equation of a traveling sinusoidal wave.

y(x,t) = A cos(kx ± ωt)

What is the natural unit of electric charge?

The magnitude of the charge of one electron or proton, |e| = 1.602 × 10⁻¹⁹ C.

Define ionization.

Removing or adding electrons to an atom, producing a charged ion.

Difference between conductors and insulators regarding charge mobility.

Conductors allow easy movement of free electrons; insulators do not.

Charging a neutral body without contact is called what?

Charging by induction.

What is polarization in an insulator?

Slight displacement of bound charges when an external charge is nearby.

State Coulomb’s law qualitatively.

The magnitude of the electric force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.

SI unit of electric charge.

Coulomb (C).

Mathematical expression for Coulomb’s law in vacuum.

F = k |q₁q₂| / r² with k = 8.99 × 10⁹ N·m²/C²

What principle allows you to add electric forces from multiple charges?

Principle of superposition of forces.

Define electric field at a point.

E⃗ = F⃗ / q₀, the force per unit positive test charge placed at that point.

Direction of the electric field around a positive point charge.

Radially outward from the charge.

What is the electric field inside a conductor in electrostatic equilibrium?

Zero everywhere within the material.

Principle of superposition for electric fields.

Total field at a point is the vector sum of fields produced by each charge.

Define an electric dipole.

Two equal and opposite point charges separated by a distance d.

Formula for electric dipole moment magnitude.

p = q d

Direction of the dipole moment vector.

From negative charge toward positive charge along the dipole axis.

Torque on a dipole in a uniform field.

τ = pE sin φ

Potential-energy function of a dipole in a uniform field.

U = −pE cos φ

Symbols for charge densities: line, surface, volume.

λ (C/m), σ (C/m²), ρ (C/m³).

How does a charged ring behave at points far along its axis?

Its field approaches that of a point charge Q at the center.

Key property of electric field lines about intersection.

Field lines never cross because the field direction is unique at each point.

Appearance of field lines in a uniform electric field.

Straight, parallel, and equally spaced.

Electric field of an infinite charged sheet (magnitude).

E = σ / (2ε₀), independent of distance from the sheet.

Electric field between two oppositely charged infinite sheets.

Uniform and equal to σ/ε₀, independent of plate spacing.

Uniform field between parallel plates separated 1 cm with 100 V potential difference (value).

E ≈ 1.0 × 10⁴ N/C upward.

Acceleration of a charge q in a uniform field E (neglecting other forces).

a = qE / m

What is the work done by a torque during rotation through Δθ?

W = τ Δθ

First step to find the field of a continuous line charge.

Divide the line into infinitesimal charge elements dQ = λ dl and integrate their contributions.

Field of a very long uniformly charged line at distance r.

E = (2kλ)/r, directed radially outward (or inward for negative λ).

Define an electric field line.

A curve whose tangent at any point gives the direction of E⃗ at that point.

When does an electric dipole experience zero net force in an external field?

When the external field is uniform.

Condition for maximum torque on a dipole.

Dipole moment perpendicular to the electric field (φ = 90°).

Stable equilibrium orientation of a dipole in a uniform field.

Dipole moment aligned parallel to the field (φ = 0).

Unstable equilibrium orientation of a dipole in a uniform field.

Dipole moment antiparallel to the field (φ = π).

Expression for electric potential energy minimum of a dipole.

U_min = −pE

Kinematic equation for angular motion with constant angular acceleration.

ω = ω₀ + αt

Law of gravitation equation (review).

F_g = G m₁ m₂ / r²

First desirable study habit after class.

Prepare your own lecture notes each evening.

Grade boundary for an A in the course.

Above 90 %.

Relationship between work and change in kinetic energy.

Work done by net force equals the change in kinetic energy (Work–Energy Theorem).

Definition of impulse.

Change in momentum: J⃗ = ∫F⃗ dt = Δp⃗

State the superposition principle for potentials (optional extension).

Total electric potential equals the algebraic sum of potentials from individual charges.

What happens to induced charges on a metal sphere after the external inducer is removed while the sphere is grounded?

Electrons remain transferred to ground; the sphere retains a net positive charge.

Define polarization of a dielectric.

Alignment of molecular dipoles within an insulator under an external electric field.

What physical quantity does the spacing of electric field lines represent?

Relative magnitude of the electric field.

Why can’t field lines start and end on the same charge?

Because they originate on positive charges and terminate on negative charges or at infinity, reflecting charge conservation.

Equation for electric field magnitude due to a point charge q at distance r.

E = k |q| / r²

Specify the units of electric field.

Newtons per coulomb (N/C) or equivalently volts per meter (V/m).

Name the force that must balance qE for a charged oil drop at rest in Millikan’s experiment.

Gravitational weight, mg.

How does the net electric force on a test charge behave when placed exactly midway between equal like charges on a line?

Zero; forces cancel.

Electric field contribution from two equal opposite charges on the x-axis at a distant point on the y-axis (qualitative).

Predominantly in the negative y-direction because horizontal components cancel.

Condition for zero net force on a third charge placed along a line of two fixed charges of unequal magnitude.

Location where magnitudes of the individual forces are equal and directions opposite, found by solving k|q₁q₃|/r₁² = k|q₂q₃|/r₂².