Physics Class 12 – Chapter 1: Electric Charges and Fields

25 question-and-answer flashcards covering definitions, laws, formulas, and applications from the lecture on Electric Charges and Fields.

What is an electric charge?

A fundamental property of matter that causes it to experience a force when placed in an electric field.

Name the two types of electric charge.

Positive (+) charge and Negative (−) charge.

State three fundamental properties of electric charge.

Additivity, Quantization, and Conservation.

Write the quantization-of-charge equation and the value of the elementary charge.

q = n e, where n is an integer and e = 1.6 × 10⁻¹⁹ C.

How do conductors differ from insulators with respect to electric charge?

In conductors, charges move freely; in insulators, charges are bound and cannot move freely.

List three methods of charging a body.

Charging by friction, by conduction, and by induction.

State Coulomb’s law in scalar form for two point charges.

F = (1 / 4πϵ₀) · (q₁q₂ / r²).

Give the numerical value of (1 / 4πϵ₀) in vacuum.

9 × 10⁹ N m² C⁻² (approximately).

What does the principle of superposition say about electric forces?

The net force on a charge equals the vector sum of individual forces exerted by all other charges.

Define electric field (E) in terms of force.

E = F / q₀, i.e., the force experienced per unit positive test charge.

Write the expression for the electric field due to a point charge.

E = (1 / 4πϵ₀) · (q / r²).

In what directions do electric field lines originate and terminate?

They start on positive charges and end on negative charges.

Give three key properties of electric field lines.

They never cross, are denser where the field is stronger, and emerge perpendicular from conductive surfaces.

What is an electric dipole?

A pair of equal and opposite charges separated by a small distance 2a.

Provide the formula for electric dipole moment.

p = q · 2a (directed from −q to +q).

What are the electric fields of a dipole on (a) its axial line and (b) its equatorial line?

(a) Eaxial = (1 / 4πϵ₀) · (2p / r³) ; (b) Eequatorial = (1 / 4πϵ₀) · (p / r³).

Write the expression for torque on a dipole in a uniform electric field.

τ = p × E or τ = pE sinθ.

Define linear, surface, and volume charge densities.

Linear: λ = dq/dl ; Surface: σ = dq/dS ; Volume: ρ = dq/dV.

Define electric flux and give its SI unit.

Φ_E = E · A · cosθ; it is a scalar whose SI unit is N m² C⁻¹.

State Gauss’s law in electrostatics.

The net electric flux through a closed surface equals the total charge enclosed divided by ϵ₀ (ΦE = qinside/ϵ₀).

Using Gauss’s law, what is the electric field at distance r from an infinitely long line of charge with density λ?

E = λ / (2πϵ₀ r), directed radially outward (for λ > 0).

Using Gauss’s law, what is the electric field near an infinite uniformly charged plane sheet of surface charge density σ?

E = σ / (2ϵ₀), normal to the sheet and independent of distance.

What is the electric field (a) outside and (b) inside a uniformly charged spherical shell?

(a) Outside (r > R): E = (1 / 4πϵ₀) · (q / r²); (b) Inside (r < R): E = 0.

Under what condition is Gauss’s law most useful for calculating electric fields?

When the charge distribution possesses high symmetry (spherical, cylindrical, or planar).

Why must vector nature be considered in electric field and force problems?

Because both magnitude and direction determine the resultant; incorrect vector handling leads to wrong results.