Electricity — Set 6
Physics · विद्युत · Questions 51–60 of 60
In alternating current (AC), the direction of current?
Correct Answer: B. Changes periodically with time
• **Changes periodically with time** = AC current reverses direction periodically, completing a full cycle at the supply frequency. • **India supply: 50 Hz → reverses 100 times per second** — This periodic reversal distinguishes AC from DC. • 💡 Wrong-option analysis: Is always zero: zero current means no supply; Remains constant: constant direction describes DC; Flows only in one direction: a resistor carries current in whatever direction the voltage drives it.
Power factor in an AC circuit is equal to?
Correct Answer: C. cosφ
• **cosφ** = Power factor equals the cosine of the phase angle φ between voltage and current. • **P = Vrms Irms cosφ; cosφ = 1 for pure resistive load** — Low power factor means more reactive current for the same real power. • 💡 Wrong-option analysis: 1/cosφ: inverse is secant, not power factor; tanφ: ratio of reactive to real power; sinφ: relates to reactive power Q = VIsinφ.
An AC circuit has Vrms = 100 V, Irms = 2 A, and power factor 0.8. What is the average power?
Correct Answer: A. 160 W
• **160 W** = Using P = Vrms × Irms × cosφ = 100 × 2 × 0.8 = 160 W. • **P = 100 × 2 × 0.8 = 160 W** — Apparent power = 200 VA; real power = 160 W due to 0.8 power factor. • 💡 Wrong-option analysis: 200 W: apparent power (Vrms×Irms) without power factor; 250 W: no formula gives this; 80 W: forgets Irms = 2 multiplier (computes 100×0.8 only).
The unit commonly used in household electricity bills for energy is?
Correct Answer: D. Kilowatt-hour
• **Kilowatt-hour** = Domestic electricity bills measure energy in kilowatt-hours; 1 kWh = 3.6×10⁶ J. • **1 kWh = 1 billing unit = 3.6 MJ** — A 1000 W device running for 1 hour uses exactly 1 unit. • 💡 Wrong-option analysis: Ampere: unit of current, not energy; Volt: unit of potential, not energy; Watt: unit of power (rate of energy), not energy itself.
Fleming's left-hand rule is used to find the direction of?
Correct Answer: C. Force on a current-carrying conductor in a magnetic field
• **Force on a current-carrying conductor in a magnetic field** = Fleming's left-hand rule (FBI): thumb=Force, index=B field, middle=current I. • **Left hand → motors (force); right hand → generators (induced current)** — Used in electric motor design. • 💡 Wrong-option analysis: Electric field due to charge: found via Coulomb's law; Induced current in a generator: that is Fleming's right-hand rule; Resistance: a scalar, not determined by any hand rule.
Two capacitors 3 μF and 6 μF are connected in series. What is the equivalent capacitance?
Correct Answer: A. 2 μF
• **2 μF** = Using 1/Ceq = 1/3 + 1/6 = 2/6 + 1/6 = 3/6 = 1/2, so Ceq = 2 μF. • **1/Ceq = 1/3 + 1/6 = 1/2 → Ceq = 2 μF** — Series capacitance (2 μF) is less than the smaller capacitor (3 μF). • 💡 Wrong-option analysis: 18 μF: product 3×6, no valid formula; 9 μF: sum 3+6 which is actually the parallel rule; 4.5 μF: arithmetic mean (3+6)/2, no circuit basis.
A superconductor below its critical temperature has?
Correct Answer: C. Zero electrical resistivity
• **Zero electrical resistivity** = Below the critical temperature, a superconductor exhibits ρ = 0, allowing current to flow without any energy loss. • **ρ = 0 → R = 0 → no Joule heating** — MRI superconducting magnets maintain huge currents indefinitely using this property. • 💡 Wrong-option analysis: Infinite capacitance: capacitance is unrelated to superconductivity; Very high resistance: exactly the opposite; Zero charge: charge is not zero, it flows without loss.
Two capacitors 2 μF and 5 μF are connected in parallel. What is the equivalent capacitance?
Correct Answer: C. 7 μF
• **7 μF** = In parallel, capacitances add: Ceq = 2 + 5 = 7 μF. • **Ceq = C1 + C2 = 2 + 5 = 7 μF** — Parallel combination always gives larger capacitance than either capacitor alone. • 💡 Wrong-option analysis: 10 μF: product 2×5, not the parallel formula; 3 μF: difference 5-2, irrelevant; 1.4 μF: result of series formula (10/7≈1.43).
The permittivity of free space is denoted by ɛ0 and is approximately?
Correct Answer: B. 8.85×10^-12 C^2/(N m^2)
• **8.85×10^-12 C^2/(N m^2)** = ε₀ is the permittivity of free space; Coulomb's constant k = 1/(4πε₀). • **ε₀ ≈ 8.85×10⁻¹² F/m** — A larger ε₀ would mean weaker electrostatic forces for the same charges. • 💡 Wrong-option analysis: 1.6×10⁻¹⁹ C: elementary charge of one electron; 9×10⁹ N m²/C²: that is Coulomb's constant k; 3×10⁸ m/s: speed of light (though c = 1/√(ε₀μ₀)).
An appliance uses 600 W when connected to 200 V. What current does it draw?
Correct Answer: A. 3 A
• **3 A** = Using I = P/V = 600/200 = 3 A. • **I = P/V = 600/200 = 3 A** — Higher wattage at the same voltage means proportionally higher current draw. • 💡 Wrong-option analysis: 1.5 A: divides 300 by 200 (half the power); 6 A: multiplies instead of divides; 0.33 A: computes V/P instead of P/V.