UKPSC Physics: Electricity, Magnetism & Electronics

1. High-Yield Micro-Notes

• Conductors, Resistance & Heating Elements: Electrical resistance follows the relation R = ρ·L / A, depending directly on conductor length (L), inversely on cross-sectional area (A), and intrinsically on material resistivity (ρ) and temperature. Silver (Ag) possesses the highest electrical conductivity of all metals (≈ 6.30 × 10⁷ S/m). Traditional incandescent bulb filaments use Tungsten (W) due to its exceptionally high melting point (≈ 3422 °C) and high resistivity. Pencil “lead” contains no elemental lead—it is a non-metallic conductor composed of graphite mixed with clay.
• Electromagnetism & Coil Calculations: The magnetic field at the center of a flat, tightly wound circular coil with N turns carrying current I is given by B = (μ₀·N·I) / (2·r). Substituting μ₀ = 4π × 10⁻⁷ T·m/A, N = 100 turns, I = 1 A, and r = 0.1 m yields B = 6.28 × 10⁻⁴ Tesla. A choke coil in fluorescent lighting provides a high inductive voltage surge momentarily to strike the gas discharge, then limits steady-state operating current without wasting energy.
• Semiconductor Devices & Photovoltaics: A solar cell operates on the photovoltaic effect and is fundamentally a specialized semiconductor p-n junction diode that converts radiant photon energy into DC electric current. Silicon (Si) is the standard semiconductor material for solar cells and Integrated Circuit (IC) microchips. A standard semiconductor diode conducts current primarily in one direction, serving as a rectifier to convert Alternating Current (AC) into Direct Current (DC).
• Charge Dynamics & Electrical Protection: One Coulomb of electric charge equals 6.242 × 10¹⁸ electrons (Q = n·e). Transferring 10⁹ electrons per second requires approximately 200 years (≈ 198 years) to accumulate 1 Coulomb on a receiving body. Lightning conductors (pointed copper rods invented by Benjamin Franklin) protect tall buildings by conducting accumulated atmospheric charge harmlessly into the deep earth. Electrostatic Precipitators (ESP) remove airborne fine particulate matter, specifically PM 2.5 and PM 10, from industrial flue gas emissions using high-voltage corona discharge.

2. Important Electronic Devices

3. Master Questions

Q1. A tightly wound circular coil of radius 10 cm comprises 100 turns and carries a steady current of 1 A. What is the magnitude of the magnetic field induced at the center of the coil? (Take μ₀ = 4π × 10⁻⁷ T·m/A)

[UKPSC Pre 2024] [Statement-Based]

(a) 6.28 × 10⁻⁴ T

(b) 6.28 × 10⁻³ T

(c) 6.68 × 10⁻⁴ T

(d) 6.82 × 10⁻² T

Q2. In solid-state electronics, a photovoltaic solar cell unit is structurally and functionally classified as a:

[UKPSC AE 2013] [Direct Fact]

(a) Semiconductor Triode

(b) Semiconductor Diode

(c) Junction between two metallic conductors

(d) Solid-state Thermistor

Q3. In a traditional fluorescent tube-light circuit, the primary electrical function of the ‘choke’ is to:

[UKPSC Revenue Inspector 2023] [Direct Fact]

(a) Continuously decrease operating circuit current

(b) Continuously step up operating current

(c) Decrease circuit voltage momentarily during startup

(d) Induce a high voltage surge momentarily to strike the gas discharge

Q4. Which tetravalent semiconductor element is primarily used in manufacturing photovoltaic solar cells and microchip Integrated Circuits (ICs)?

[UKPSC Pre 2006-07 | UKPSC Pre 2012] [Direct Fact]

(a) Silicon

(b) Beryllium

(c) Cerium

(d) Vanadium

Q5. A material that contains a very high concentration of mobile conduction electrons and offers exceptionally low ohmic resistance to electric current is termed a:

[UKPSC Pre 2021] [Direct Fact]

(a) Dielectric Insulator

(b) Ideal Inductor

(c) Electrical Conductor

(d) Intrinsic Semiconductor

Q6. Consider the following statements:

Assertion (A): Lightning conductors installed on top of buildings safeguard structures from catastrophic lightning damage.

Reason (R): They establish a low-resistance path that discharges atmospheric electrostatic charge safely into the earth.

Choose the correct answer:

[UKPSC Mains 2002-03] [Assertion-Reason]

(a) Both (A) and (R) are true and (R) is the correct explanation of (A)

(b) Both (A) and (R) are true but (R) is not the correct explanation of (A)

(c) (A) is true but (R) is false

(d) (A) is false but (R) is true

Q7. The full name of the energy-efficient lighting unit abbreviated as ‘C.F.L.’ is:

[UKPSC Mains 2010-11] [Direct Fact]

(a) Complete Fluorescent Lamp

(b) Clean Fluorescent Luminaire

(c) Compact Fluorescent Lamp

(d) Care Free Lamp

Q8. The writing core or ‘lead’ in modern pencils is composed of:

[UKPSC UDA/LDA Pre 2003] [Direct Fact]

(a) Elemental Lead (Pb)

(b) Antimony

(c) Graphite mixed with clay

(d) Carbon black

Q9. In an electric circuit, an electrochemical battery functions primarily to:

[UKPSC RO/ARO Pre 2022] [Direct Fact]

(a) Directly measure current amperage

(b) Maintain a potential difference to drive current through the circuit

(c) Restrict total circuit current like a fuse

(d) Block current flow completely like a dielectric

Q10. A semiconductor p-n junction diode is commonly used in electronic circuits to:

[UKPCS Lower Pre 2021] [Direct Fact]

(a) Amplify weak alternating signals

(b) Convert Alternating Current (AC) into Direct Current (DC)

(c) Generate continuous radio frequencies

(d) Produce penetrating gamma rays

Q11. The electrical resistance (R) of a uniform metallic conductor depends on:

[UKPSC FRO/ACF Pre 2021] [Direct Fact]

(a) Its linear length (L)

(b) Its cross-sectional surface area (A)

(c) The nature and resistivity of its material (ρ)

(d) All of the above factors

Q12. The glowing filament inside a traditional incandescent electric bulb is manufactured from:

[UKPSC Pre 2012] [Direct Fact]

(a) Tungsten

(b) Nichrome

(c) Lead

(d) Aluminum

Q13. Which of the following solid elements exhibits the highest electrical conductivity at room temperature (20 °C)?

[UKPSC RO/ARO Mains 2006] [Direct Fact]

(a) Diamond

(b) Silver

(c) Graphite

(d) Copper

Q14. In consumer goods classifications upheld by judicial precedent, household automatic voltage stabilizers are categorized as:

[UKPSC APO 2009] [Direct Fact]

(a) Electrical goods

(b) Electronic goods

(c) Electrochemical goods

(d) Pure electromagnetic goods

Q15. In industrial air pollution abatement systems, an Electrostatic Precipitator (ESP) is most widely deployed to capture:

[UKPSC Revenue Inspector 2023] [Direct Fact]

(a) Coarse dust only (PM 50)

(b) Fine particulate matter (PM 2.5)

(c) Gaseous sulfur dioxide (SO₂)

(d) Oxides of nitrogen (NOx)

Q16. If a steady stream of 10⁹ electrons transfers from one body to another every second, approximately how long will it take for the recipient body to accumulate a net charge of 1 Coulomb? (Take 1 year ≈ 3 × 10⁷ seconds)

[UKPSC AE 2013] [Direct Fact]

(a) About 2000 years

(b) About 200 years

(c) About 20 years

(d) About 2 years

4. Master Explanations & Answer Key

1. Ans. (a)

• Direct Answer: The magnetic field at the center of a circular loop is B = (μ₀·N·I) / (2·r) = (4π × 10⁻⁷ × 100 × 1) / (2 × 0.1) = 2π × 10⁻⁴ T ≈ 6.28 × 10⁻⁴ Tesla.
• Exam Trap / Distractor Alert: Ensure the radius is converted from centimeters to meters (10 cm = 0.1 m); omitting this conversion leads to an error factor of 100, matching distractor (d).
• High-Yield Extra Facts: The SI unit of magnetic flux density is the Tesla (1 T = 1 Weber/m² = 10⁴ Gauss); the field along the central axis of a long solenoid is given by B = μ₀·n·I (where n is turns per unit length).

2. Ans. (b)

• Direct Answer: A solar cell is fundamentally a large-area semiconductor p-n junction diode optimized to absorb solar photons and separate photo-generated electron-hole pairs across its depletion zone.
• Exam Trap / Distractor Alert: Do not confuse a solar diode with a triode (a three-terminal amplifying vacuum tube or transistor); solar cells have no internal power gain and do not use triode architecture.
• High-Yield Extra Facts: The open-circuit voltage (Voc) of a single conventional silicon solar cell is typically 0.5 to 0.6 Volts; connecting cells in series inside a photovoltaic module increases output voltage.

3. Ans. (d)

• Direct Answer: The choke (inductor) creates a sudden, high inductive voltage spike (self-induced EMF, e = -L·di/dt) when the starter opens, ionizing the argon-mercury gas to strike an arc, and then limits the operating current through inductive reactance.
• Exam Trap / Distractor Alert: Many assume a choke only limits current; its initial purpose is to generate a high inductive voltage pulse, after which it limits steady-state current without the high thermal power loss of a resistor.
• High-Yield Extra Facts: In modern lighting fixtures, wound magnetic chokes are largely replaced by high-frequency electronic ballasts, which eliminate stroboscopic flicker and reduce power loss.

4. Ans. (a)

• Direct Answer: Silicon (atomic number 14, Group 14) is the semiconductor used in over 90% of commercial photovoltaic cells and serves as the primary substrate for monolithic Integrated Circuits (ICs).
• Exam Trap / Distractor Alert: Germanium is also a Group 14 semiconductor, but silicon is preferred in commercial microelectronics because of its wider bandgap (1.1 eV vs. 0.67 eV for Ge), lower leakage current, and stable native oxide layer (SiO₂).
• High-Yield Extra Facts: Silicon is the second most abundant element in Earth’s crust (≈ 28% by mass, after oxygen); the microelectronics region in Bengaluru is known as the “Silicon Valley of India.”

5. Ans. (c)

• Direct Answer: Electrical conductors possess high concentrations of mobile valence electrons (≈ 10²⁸ to 10²⁹ free electrons/m³ in metals) that move under an applied electric field, providing low electrical resistance.
• Exam Trap / Distractor Alert: Insulators have tightly bound valence electrons and an energy bandgap greater than 3 eV; semiconductors have an intermediate bandgap (≈ 1 eV) and conduct noticeably only when thermally excited or doped with impurities.
• High-Yield Extra Facts: In metallic conductors, electrical resistance increases with rising temperature due to increased lattice phonon collisions, which lowers carrier mobility.

6. Ans. (a)

• Direct Answer: Both statements are correct and (R) explains (A): the pointed metal rod channels atmospheric electrostatic charge directly into the ground through a heavy copper conductor, preventing explosive heating of the building.
• Exam Trap / Distractor Alert: Lightning conductors do not simply “repel” clouds; they leak charge via point corona discharge to reduce localized field stress and provide a low-resistance path to ground if a strike occurs.
• High-Yield Extra Facts: The lightning conductor was invented in 1749 by Benjamin Franklin; proper grounding requires burying copper grounding plates in moist, charcoal-and-salt-treated earth.

7. Ans. (c)

• Direct Answer: CFL stands for Compact Fluorescent Lamp, an energy-efficient fluorescent bulb shaped to fit into standard incandescent lamp fixtures.
• Exam Trap / Distractor Alert: Look out for similar-sounding options like “Clean Fluorescent” or “Complete Fluorescent”; the term specifically refers to folding or coiling a fluorescent tube into a compact size.
• High-Yield Extra Facts: CFLs contain a small dose of mercury vapor and argon gas; passing current through this vapor emits ultraviolet radiation, which strikes an internal phosphor coating that fluoresces as visible white light.

8. Ans. (c)

• Direct Answer: Pencil cores contain no metallic lead; they are composed of a baked mixture of powdered graphite (a crystalline allotrope of carbon) and fine clay binder.
• Exam Trap / Distractor Alert: The term “lead pencil” is an ancient misnomer stemming from the 1564 discovery of a massive graphite deposit in Borrowdale, England, which locals mistakenly called “black lead” or plumbago.
• High-Yield Extra Facts: Graphite conducts electricity because each carbon atom forms three planar covalent bonds (sp² hybridization), leaving one delocalized pi electron free to move along its hexagonal sheets.

9. Ans. (b)

• Direct Answer: A chemical battery converts stored chemical energy into electrical potential energy, maintaining a voltage across circuit terminals to drive a continuous electric current.
• Exam Trap / Distractor Alert: A battery does not generate new electrons; it pumps existing conduction electrons through the circuit by establishing an electromotive force (EMF).
• High-Yield Extra Facts: Primary batteries (like alkaline dry cells) are non-rechargeable because their chemical reactions are irreversible; secondary batteries (like lead-acid accumulators or lithium-ion cells) are rechargeable.

10. Ans. (b)

• Direct Answer: A semiconductor diode allows current to pass easily in forward bias while blocking it in reverse bias, making it a rectifier that converts bidirectional AC into unidirectional DC.
• Exam Trap / Distractor Alert: A diode does not amplify electrical signals; amplification requires an active three-terminal semiconductor device such as a Bipolar Junction Transistor (BJT) or Field-Effect Transistor (FET).
• High-Yield Extra Facts: A half-wave rectifier uses a single diode, whereas a full-wave bridge rectifier uses four diodes arranged in a diamond bridge network to convert both halves of the AC cycle.

11. Ans. (d)

• Direct Answer: The resistance of an ohmic conductor is given by R = ρ·L / A; it is directly proportional to length (L), inversely proportional to cross-sectional area (A), and dependent on the material’s specific resistivity (ρ) and temperature.
• Exam Trap / Distractor Alert: Resistivity (ρ) is an intensive property that depends only on the material and temperature, whereas Resistance (R) is an extensive property that depends on the physical dimensions of the conductor.
• High-Yield Extra Facts: Doubling the length of a wire doubles its resistance (2R); stretching a wire to double its length decreases its area by half, increasing its total resistance fourfold (4R).

12. Ans. (a)

• Direct Answer: Incandescent bulb filaments are made of drawn tungsten wire because tungsten has the highest melting point of any metal (3422 °C) and maintains high tensile strength at glowing incandescence.
• Exam Trap / Distractor Alert: Nichrome (an alloy of nickel and chromium) is used for domestic heating elements (toasters, geysers, room heaters) because it forms a protective oxide layer, not for incandescent bulb filaments.
• High-Yield Extra Facts: Bulb envelopes are evacuated and backfilled with inert gases like argon and nitrogen to suppress tungsten evaporation and extend filament life.

13. Ans. (b)

• Direct Answer: Silver exhibits the highest electrical conductivity (≈ 6.30 × 10⁷ S/m) and lowest resistivity (1.59 × 10⁻⁸ Ω·m) of all known materials at room temperature.
• Exam Trap / Distractor Alert: While silver is the best conductor, copper is more widely used in commercial power grids and building wiring due to its lower cost and high ductility; diamond is an electrical insulator.
• High-Yield Extra Facts: Copper ranks second in electrical conductivity (≈ 5.96 × 10⁷ S/m), followed closely by gold (≈ 4.1 × 10⁷ S/m) and aluminum (≈ 3.5 × 10⁷ S/m).

14. Ans. (b)

• Direct Answer: In commercial sales tax and industrial tariff disputes, the Supreme Court of India ruled that household automatic voltage stabilizers are classified as “electronic goods” because they use solid-state sensor circuits to control voltage.
• Exam Trap / Distractor Alert: While earlier stabilizers relied entirely on electromagnetic step transformers, modern automatic stabilizers use active semiconductor relays and electronic control loops, placing them under electronic tax classifications.
• High-Yield Extra Facts: Modern voltage stabilizers commonly utilize Zener diodes as constant voltage references along with operational amplifiers to trigger switching relays.

15. Ans. (b)

• Direct Answer: Electrostatic Precipitators (ESPs) are filtration devices used in thermal power plants to remove fine particulate matter, especially PM 2.5 and PM 10, with collection efficiencies exceeding 99%.
• Exam Trap / Distractor Alert: ESPs remove solid or liquid particulate aerosols; they do not remove gaseous air pollutants like sulfur dioxide (SO₂, treated via wet flue-gas desulfurization) or nitrogen oxides (NOx, treated via selective catalytic reduction).
• High-Yield Extra Facts: An ESP operates by ionizing dust particles with high-voltage negative corona discharge wires (-50 kV to -100 kV), after which the charged particles migrate to grounded positive collector plates.

16. Ans. (b)

• Direct Answer: Total electrons in 1 Coulomb = 1 / (1.602 × 10⁻¹⁹) ≈ 6.242 × 10¹⁸ electrons. At 10⁹ electrons per second, time t = (6.242 × 10¹⁸) / 10⁹ = 6.242 × 10⁹ seconds. In years: (6.242 × 10⁹) / (3 × 10⁷) ≈ 208 years, which rounds to approximately 200 years.
• Exam Trap / Distractor Alert: Ensure you calculate the ratio using the full electronic charge constant (1.6 × 10⁻¹⁹ C); speed-reading the exponent can lead to answers like 20 years or 2000 years.
• High-Yield Extra Facts: This arithmetic shows that 1 Coulomb is a very large unit of electrostatic charge; practical electrostatic charges encountered in everyday static friction are usually on the scale of microcoulombs (μC) or nanocoulombs (nC).

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Hemant Bhatt

Chief Mentor at Oracle IAS

Hemant Bhatt is a B. Tech from Pantnagar University and has studied Social Sciences at SciencesPo, Paris. He teaches at Oracle IAS, Dehradun. He has given 5 UPSC mains back to back along with 2 interviews.
He is a bibliophile,accomplished writer and a fitness enthusiast.

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