Electricity — Set 4
Physics · विद्युत · Questions 31–40 of 60
Which material is typically a good insulator?
Correct Answer: A. Rubber
• **Rubber** = Rubber has very high resistivity (≈10¹³ Ω m) compared to metals, making it an excellent insulator. • **Rubber ρ ≈ 10¹³ Ω m vs copper ≈ 1.7×10⁻⁸ Ω m** — This 21-order-of-magnitude difference is why rubber insulates copper wires. • 💡 Wrong-option analysis: Iron: metal with free electrons, low resistivity conductor; Copper: one of the best conductors; Aluminium: metal conductor used in overhead lines.
Electric current in a metal is mainly due to the motion of?
Correct Answer: A. Electrons
• **Electrons** = In metallic conductors, free outer-shell electrons drift under an electric field, constituting the current. • **Free electron drift speed ≈ 10⁻⁴ m/s** — Even though electrons drift slowly, the electric signal propagates near the speed of light. • 💡 Wrong-option analysis: Neutrons: electrically neutral, cannot carry charge; Protons in nucleus: bound tightly, cannot move through the lattice; Positive ions: fixed in the crystal lattice, do not drift.
The SI unit of electric power is?
Correct Answer: B. Watt
• **Watt** = Electric power is the rate of energy transfer; 1 W = 1 J/s. • **1 W = 1 J/s = 1 V·A** — A 100 W bulb converts 100 J of electrical energy per second into light and heat. • 💡 Wrong-option analysis: Volt: unit of potential difference; Joule: unit of energy, not the rate; Coulomb: unit of electric charge.
A transformer works on the principle of?
Correct Answer: D. Mutual induction
• **Mutual induction** = A transformer transfers energy between two coils through mutual electromagnetic induction caused by changing magnetic flux. • **Vs/Vp = Ns/Np (turns ratio)** — Works only on AC because DC produces no changing flux. • 💡 Wrong-option analysis: Photoelectric effect: emission of electrons by light, unrelated to transformers; Nuclear fission: splitting of heavy nuclei, used in reactors; Electrolysis: chemical decomposition by current, unrelated to voltage transformation.
A transformer has 200 turns in the primary and 50 turns in the secondary. If primary voltage is 240 V, what is secondary voltage?
Correct Answer: B. 60 V
• **60 V** = Using Vs = Vp × (Ns/Np) = 240 × (50/200) = 240 × 0.25 = 60 V. • **Turns ratio = 50/200 = 1/4; Vs = 240/4 = 60 V** — Fewer secondary turns → step-down transformer. • 💡 Wrong-option analysis: 960 V: applies inverse ratio Np/Ns (step-up); 240 V: assumes 1:1 ratio; 120 V: uses ratio 1:2 instead of 1:4.
The SI unit of electric potential is?
Correct Answer: C. Volt
• **Volt** = Electric potential difference is measured in volt; 1 V = 1 J/C. • **1 V = 1 J/C** — Named after Alessandro Volta who invented the first battery. • 💡 Wrong-option analysis: Farad: unit of capacitance; Ohm: unit of resistance; Ampere: unit of electric current.
In electrostatic equilibrium, the electric field inside a conductor is?
Correct Answer: A. Zero
• **Zero** = In electrostatic equilibrium, free charges redistribute on the surface until the net internal field cancels to zero. • **E_inside = 0** — This is the Faraday cage effect that blocks external electric fields from reaching the interior. • 💡 Wrong-option analysis: Infinite: no physical field is infinite inside a normal conductor at equilibrium; Equal to surface field: the surface has a non-zero field perpendicular to it, but the interior is zero; Maximum: the field is a minimum (zero) inside, maximum at sharp exterior surfaces.
A resistor of 6 Ω is connected across 12 V. What power is dissipated? (Use P = V^2/R)?
Correct Answer: C. 24 W
• **24 W** = Using P = V²/R = 144/6 = 24 W. • **P = V²/R = 144/6 = 24 W** — Same result from P = VI = 12×2 = 24 W where I = 12/6 = 2 A. • 💡 Wrong-option analysis: 2 W: computes V/R not V²/R; 72 W: multiplies V²×R instead of dividing; 6 W: uses R as the answer.
In a series circuit, which quantity remains the same in all components?
Correct Answer: A. Current
• **Current** = In a series path, there is only one loop, so the same current flows through every component. • **I₁ = I₂ = I₃ = ... (series law)** — Voltage divides in proportion to resistance; current is uniform. • 💡 Wrong-option analysis: Voltage: divides across each resistor proportional to its R; Resistance: different components can have different R values; Power: P = I²R varies with R even though I is the same.
A short circuit causes very large current mainly because?
Correct Answer: C. Resistance becomes very low
• **Resistance becomes very low** = In a short circuit the path resistance is nearly zero, so I = V/R becomes extremely large. • **I = V/R → R ≈ 0 means I → very large** — This very large current causes rapid heating and possible fire. • 💡 Wrong-option analysis: Voltage becomes zero everywhere: supply voltage remains; capacitance becomes infinite: capacitance is unrelated; power becomes negative: P = I²R is always positive.