Electricity — Set 2

Correct Answer: C. N/C

• **N/C** = Electric field is force per unit charge, E = F/q, giving units of N/C. • **N/C = V/m — both forms equivalent** — N/C used in force contexts; V/m in potential gradient contexts. • 💡 Wrong-option analysis: C/N: inverted, would be inverse electric field; J/C: unit of electric potential (volt); N·m: unit of torque or energy.

Correct Answer: A. 1/r^2

• **1/r^2** = Coulomb's law: F = kq₁q₂/r², so force is inversely proportional to the square of distance. • **F ∝ 1/r² — doubling distance reduces force to one-quarter** — Same inverse-square behaviour as gravity. • 💡 Wrong-option analysis: 1/r: first-power inverse, describes different geometries like infinite cylinders; r²: direct square means force increases with distance, opposite to observation; r: direct proportion, also incorrect.

Correct Answer: D. 0.216 N

• **0.216 N** = Using F = kq₁q₂/r² = 9×10⁹ × 2×10⁻⁶ × 3×10⁻⁶ / 0.25 = 54×10⁻³/0.25 = 0.216 N. • **F = 9×10⁹ × 6×10⁻¹²/0.25 = 0.216 N** — Both charges are positive so the force is repulsive. • 💡 Wrong-option analysis: 0.108 N: uses r = 0.5 instead of r² = 0.25; 0.432 N: arithmetic error in numerator; 2.16 N: forgets to square r, uses r = 0.5 in denominator.

Correct Answer: A. Work done per unit charge

• **Work done per unit charge** = Potential difference V = W/Q is the work done per unit positive charge moved between two points. • **V = W/Q (volt = J/C)** — A 1 V difference means 1 J of work per coulomb of charge. • 💡 Wrong-option analysis: Work done per unit time: that is power (watt); Force per unit charge: that is electric field (N/C); Charge per unit work: inverse of potential, has no standard name.

Correct Answer: A. 4 V

• **4 V** = Using V = W/Q = 20/5 = 4 V. • **V = W/Q = 20/5 = 4 V** — Moving more charge through the same potential requires proportionally more work. • 💡 Wrong-option analysis: 25 V: inverts Q/W instead of W/Q; 2 V: arithmetic error, perhaps 10/5; 100 V: multiplies W×Q instead of dividing.

Correct Answer: D. Energy supplied per unit charge by the source

• **Energy supplied per unit charge by the source** = EMF is the energy a source provides per coulomb of charge, measured in volts. • **EMF = Energy/Charge (V = J/C)** — When no current flows, terminal voltage equals the emf exactly. • 💡 Wrong-option analysis: Force on unit charge outside: that is electric field (N/C); Resistance of source: that is internal resistance r; Current through source: current depends on the external circuit, not a fixed property of emf.

Correct Answer: B. Opposite to electron drift

• **Opposite to electron drift** = Conventional current is defined as the flow of positive charge, which is opposite to the direction electrons actually move. • **Conventional current: + → − (external); electron drift: − → +** — Electrons drift toward the positive terminal; conventional current flows from + to − externally. • 💡 Wrong-option analysis: Always toward the negative terminal: electrons go to positive, conventional current away from positive; Always upward: direction is set by the circuit, not gravity; Same as electron drift: exactly opposite.

Correct Answer: A. 20 W

• **20 W** = Using P = I²R = (2)² × 5 = 4 × 5 = 20 W. • **P = I²R = 4 × 5 = 20 W** — Power grows with the square of current, so doubling I quadruples power. • 💡 Wrong-option analysis: 40 W: mistakenly computes P = 2IR; 10 W: uses P = IR (missing the square); 5 W: uses P = R/I².

Correct Answer: A. Series with the appliance in the live wire

• **Series with the appliance in the live wire** = A fuse is wired in series in the live wire so it breaks the live supply when current exceeds its rating. • **Live → fuse → appliance → neutral** — Placing it in the live wire ensures complete isolation when the fuse blows. • 💡 Wrong-option analysis: Parallel with appliance: a parallel fuse would short-circuit the appliance; Series in neutral only: appliance still connected to live voltage, dangerous; Across mains directly: would immediately short the supply.

Correct Answer: C. H = I^2Rt

• **H = I²Rt** = Joule's law of heating: heat generated equals I²Rt. • **H = I²Rt (joules) — doubling I quadruples heat** — This is why fuses and heaters use the I² relationship. • 💡 Wrong-option analysis: H = V/Rt: dimensionally incorrect; H = IRt: missing the square on I, gives wrong magnitude; H = VIt: valid using V, but the question asks for the form with I and R.