Question 1

A charge of +6 μC is placed in an electric field of magnitude 500 N/C. What is the magnitude of the electric force experienced by the charge?

Accepted Answer

Force F = qE = 6 × 10⁻⁶ × 500 = 3 × 10⁻³ N.

Question 2

An electric charge of +2 C is located at the origin. What is the electric field magnitude at a distance of 3 m from this charge?

Accepted Answer

Electric field E = kq/r² = (9 × 10⁹ × 2) / 3² = 18 × 10⁹ / 9 = 2 × 10⁹ N/C.

Question 3

Two identical point charges repel each other with a force of 8 N when separated by 30 cm. What is the magnitude of each charge? (Take k = 9 × 10⁹ N·m²/C²)

Accepted Answer

From F = kq²/r², we get q² = Fr²/k = (8 × 0.09) / (9 × 10⁹) = 8 × 10⁻¹² , so q = 2√2 × 10⁻⁶ ≈ 4 × 10⁻⁶ C.

Question 4

Two point charges of +4 μC and -4 μC are separated by a distance of 2 m. What is the magnitude of the electric field at the midpoint between them?

Accepted Answer

At the midpoint, both charges are 1 m away; fields add in the same direction (opposite charges create fields pointing the same way at midpoint), giving E = 2 × (9 × 10⁹ × 4 × 10⁻⁶ / 1²) = 72 × 10³ N/C.

Question 5

According to Coulomb's Law, if the distance between two point charges is doubled, the electric force between them becomes:

Accepted Answer

Coulomb's force is inversely proportional to the square of distance (F ∝ 1/r²), so doubling distance makes force one-fourth of original.

Question 6

Three charges +q, +q, and -2q are arranged at the vertices of an equilateral triangle. The electric field at the centroid of the triangle is:

Accepted Answer

The two +q charges create equal fields that cancel at centroid; only the -2q charge contributes a net field directed toward it.

Question 7

What is the electric field at a point on the axis of a uniformly charged ring of radius R at a distance x from the center, where the ring has total charge Q?

Accepted Answer

By symmetry and integration, the axial field of a charged ring is E = kQx/(R² + x²)^(3/2), where components perpendicular to axis cancel.

Question 8

A test charge of +1 nC experiences a force of 4 × 10⁻⁵ N in an electric field. The magnitude of the electric field at that location is:

Accepted Answer

Electric field E = F/q = (4 × 10⁻⁵ N)/(1 × 10⁻⁹ C) = 4 × 10⁴ = 4000 N/C.

Question 9

Which of the following statements about electric field lines is INCORRECT?

Accepted Answer

Electric field lines cannot form closed loops because electric field is conservative; closed loops would violate the conservative nature of electrostatic field.

Question 10

A uniform electric field of magnitude 500 N/C points in the positive x-direction. A charge of -2 μC is placed in this field. What is the magnitude of force on the charge?

Accepted Answer

Force F = qE = (2 × 10⁻⁶ C)(500 N/C) = 10⁻³ N = 1 mN.

Question 11

The electric field due to an infinite uniformly charged plane with surface charge density σ = 2 × 10⁻⁸ C/m² is (ε₀ = 8.85 × 10⁻¹² C²/N·m²)

Accepted Answer

Electric field from infinite plane E = σ/(2ε₀) = (2 × 10⁻⁸)/(2 × 8.85 × 10⁻¹²) ≈ 1.13 N/C.

Question 12

Two identical charges of +3 μC each are placed 6 m apart. What is the magnitude of electric field at the midpoint between them?

Accepted Answer

At the midpoint, both charges are equidistant (3 m) and produce equal magnitude fields in opposite directions, resulting in net field = 0.

Question 13

Two point charges of +2 μC and +8 μC are separated by 10 cm. At what distance from the +2 μC charge will the electric field be zero?

Accepted Answer

At the zero-field point between charges, E₁ = E₂, so k(2)/r² = k(8)/(10-r)², solving gives r ≈ 3.33 cm from +2 μC charge.

Question 14

A charge of +5 μC is placed at the origin. What is the magnitude of electric field at a distance of 2 m from the charge? (k = 9 × 10⁹ N·m²/C²)

Accepted Answer

Electric field E = kQ/r² = (9 × 10⁹ × 5 × 10⁻⁶)/(2²) = 11.25 × 10³ N/C.

Question 15

Two charges of +6 μC and -6 μC are separated by 10 cm. What is the net electric field at the midpoint between them?

Accepted Answer

At the midpoint, fields due to both charges point in the same direction (from +6 μC toward -6 μC), so E_total = 2 × kQ/r² = 2 × (9 × 10⁹ × 6 × 10⁻⁶)/(0.05)² = 43.2 × 10⁶ N/C.

Question 16

A point charge Q creates an electric field of 400 N/C at a distance of 3 m. What is the magnitude of charge Q? (k = 9 × 10⁹ N·m²/C²)

Accepted Answer

Electric field E = kQ/r², so Q = Er²/k = (400 × 9)/(9 × 10⁹) = 4 × 10⁻⁷ C.

Question 17

A uniform electric field of magnitude 200 N/C exists between two parallel plates separated by 4 cm. What is the potential difference between the plates?

Accepted Answer

Potential difference V = E × d = 200 × 0.04 = 8 V, where E is field strength and d is distance between plates.

Question 18

Two point charges of +3 μC and +8 μC are separated by 5 cm in vacuum. What is the magnitude of the electric force between them? (k = 9 × 10⁹ N·m²/C²)

Accepted Answer

Using Coulomb's law: F = k(q₁q₂)/r² = (9 × 10⁹ × 3 × 10⁻⁶ × 8 × 10⁻⁶)/(0.05)² = 86.4 N.

CAPF AC Electric Charges & Fields

Electric Charges & Fields — Practice Questions