Indian Constitution — Fundamentals & Governance

Why This Chapter Matters

This chapter is where Basic Science stops being pure theory and starts looking exactly like the trade syllabus for Technician (Electrical), Technician (Mechanical & Electronics), and much of the applied portion tested for ALP as well. RRB papers routinely draw 4 to 6 questions from resistors, capacitors, transformers, diodes, transistors, and measuring instruments, and unlike some physics topics, this material is almost entirely factual recall with a handful of straightforward numericals, meaning it is some of the highest return-on-effort content in your entire prep.

The single biggest mistake aspirants make is treating "electrical" and "electronic" components as one blurred category, unable to say cleanly why a resistor is passive but a transistor is active, or why a capacitor stores charge while an inductor stores it differently. This chapter draws those lines clearly. We move from basic components, through electrical safety, into the beginnings of semiconductor electronics, and finish with the measuring instruments you will actually hold and read in a workshop. Read every section once for understanding, then drill the tables until the names and units come without hesitation.

1. Electrical Components — The Building Blocks

Resistor

A resistor is a passive component that opposes the flow of electric current, deliberately introduced into a circuit to limit current or divide voltage. Resistors are measured in ohms (Ω), and a huge range of everyday resistors are colour-coded with bands that tell you their resistance value and tolerance without needing a multimeter, using the standard resistor colour code (black, brown, red, orange, yellow, green, blue, violet, grey, white for digits 0 through 9).

Resistors are broadly of two types: fixed resistors, with one unchanging resistance value, and variable resistors (also called rheostats or potentiometers), whose resistance can be adjusted, commonly used as volume controls, dimmer switches, and speed regulators.

Analogy: a resistor is like a narrow section deliberately built into an otherwise wide water pipe. It does not stop the flow, but it restricts it, controlling how much water gets through in a given time, exactly the way a resistor controls how much current flows for a given voltage.

Capacitor

A capacitor is a passive component that stores electrical energy in the form of an electric field between two conductive plates separated by an insulating material called a dielectric. Its capacity to store charge is called capacitance, measured in farads (F), though because a farad is an enormous unit, real capacitors are usually rated in microfarads (µF), nanofarads (nF), or picofarads (pF).

C = Q / V

where Q is the charge stored and V is the voltage across the capacitor. A capacitor charges when connected to a voltage source, storing energy, and discharges when that stored energy is released back into a circuit.

Analogy: a capacitor behaves like a small water tank connected to a pipe. It fills up (charges) when water flows in, and it can release that stored water in a quick burst (discharges) when needed, unlike a resistor, which never stores anything, only restricts the ongoing flow.

Capacitors are used for smoothing out voltage fluctuations in power supplies, storing energy for camera flashes, starting single-phase induction motors, and improving power factor in industrial electrical systems, a use directly relevant to railway substations and traction equipment where power factor correction reduces wasted reactive power.

Exam trap: do not confuse capacitance's unit, the farad, with resistance's unit, the ohm, or with charge's unit, the coulomb. All three are separate quantities that appear together in the same capacitor formula, and RRB has tested "unit of capacitance" as a standalone one-liner repeatedly.

Inductor

An inductor is a passive component, usually a coil of wire, that stores energy in a magnetic field when current flows through it, and it opposes any sudden change in current, a property called inductance, measured in henries (H), named after Joseph Henry. Where a capacitor resists changes in voltage, an inductor resists changes in current. This complementary pairing appears often in comparison-based questions.

Transformer

A transformer is a static electrical device that transfers electrical energy from one circuit to another through electromagnetic induction, changing the voltage level in the process, without changing the frequency. It consists of two coils, the primary winding (connected to the input supply) and the secondary winding (connected to the output load), both wound around a common laminated iron core, with no direct electrical (conductive) connection between them.

The relationship between the windings is given by the transformer equation:

V1 / V2 = N1 / N2

where V1 and V2 are the primary and secondary voltages, and N1 and N2 are the number of turns in the primary and secondary windings respectively.

A step-up transformer has more turns on the secondary than the primary (N2 > N1), increasing voltage from primary to secondary. A step-down transformer has fewer turns on the secondary than the primary (N2 < N1), decreasing voltage.

Worked example: A transformer has 200 turns on the primary and 1000 turns on the secondary. If the primary voltage is 230 V, find the secondary voltage.

V2 = V1 × (N2/N1) = 230 × (1000/200) = 230 × 5 = 1150 V

This is a step-up transformer, since the secondary voltage is higher.

Railway relevance: the 25 kV AC carried on overhead traction equipment cannot be used directly by a locomotive's low-voltage control circuits or its traction motors in that raw form. Onboard locomotive transformers step this high voltage down to levels usable by the traction system, exactly the reverse of how a step-up transformer at a generating station first raises voltage for efficient long-distance transmission before it ever reaches the railway network.

Exam trap: a transformer changes voltage but never changes frequency; if the primary is 50 Hz AC, the secondary remains 50 Hz AC. Also remember a transformer works purely on electromagnetic induction and therefore functions only on AC, never on steady DC, since DC produces no changing flux.

Diode (Preview)

A diode is a semiconductor device allowing current to flow in essentially one direction only. It bridges this chapter's component list into the deeper semiconductor discussion later, so hold this definition in mind; we return to diodes with full detail in Section 4.

2. Circuit Symbols and Basic Wiring Concepts

Every technician must recognise the standard symbols used in circuit diagrams. While full symbol memorisation is more a workshop-practical skill than an exam-heavy topic, a handful appear regularly in RRB's diagram-based questions.

• A cell is drawn as one long and one short parallel line, long line positive, short line negative.
• A battery (multiple cells) is drawn as a repeated pattern of long-short lines.
• A resistor is drawn as a rectangle or a zig-zag line.
• A switch is drawn as a break in the line with a small circle at each end and a diagonal line representing the open contact.
• A fuse is typically drawn as a rectangle or oval placed within the line, sometimes labelled "F".
• An ammeter is drawn as a circle with the letter "A" inside; a voltmeter as a circle with "V" inside.

Rule to remember: an ammeter is always connected in series in a circuit, because it must carry the same current that flows through the component it is measuring. A voltmeter is always connected in parallel across a component, because it must experience the same voltage as that component without diverting the main current path. We expand on this rule fully in Section 5, but fix it in memory now, since it recurs across the whole electrical portion of your syllabus.

3. Electrical Safety

Electrical safety is tested both as a factual topic and as a genuinely important workplace skill for anyone who will work around live railway electrical systems, so treat this section as more than exam filler.

Common Electrical Hazards

• Electric shock: current passing through the human body, dangerous even at surprisingly low levels; currents as low as about 10 to 20 milliamperes through the chest can cause muscles to lock, preventing a person from releasing a live conductor, and currents in the range of roughly 100 milliamperes through the heart can be fatal by disrupting normal heart rhythm.
• Short circuit: an unintended low-resistance path between live and neutral (or live and earth), causing a sudden surge of current, generating heat, and often triggering fires if not interrupted quickly by a fuse or circuit breaker.
• Overloading: drawing more current than a circuit or wire is rated for, causing overheating, insulation damage, and fire risk.
• Earth leakage: current escaping to earth through a fault, often through a person's body if they touch a faulty appliance casing.

Protective Measures

• Fuses and circuit breakers, discussed in Chapter 7, break the circuit automatically when current exceeds a safe limit.
• Earthing (grounding) provides fault current a safe, low-resistance path to the ground, away from a person's body.
• Insulation, materials like rubber, PVC, and certain plastics, prevents current from reaching unintended surfaces, including the outer casing of wires and tools.
• Residual Current Device (RCD), also called an Earth Leakage Circuit Breaker (ELCB), detects a mismatch between current flowing out through the live wire and current returning through the neutral wire; any such mismatch signals leakage current, likely through a person's body, and the RCD trips the circuit within milliseconds, well before a dangerous shock can develop fully.
• Personal Protective Equipment (PPE): insulated gloves, rubber-soled boots, and insulated tools used by electricians and technicians working near live equipment.

Exam trap: never touch a person receiving an electric shock directly with bare hands, since the current can pass through you as well. The standard safe first step is to cut off the power supply immediately, or use a non-conductive object like a dry wooden stick or dry cloth to separate the person from the source, before attempting any physical contact.

Railway-Specific Safety Note

Overhead traction equipment carries 25,000 volts, a lethal voltage at any contact distance, not merely on direct touch. High voltage can arc across an air gap of a metre or more depending on conditions, which is exactly why railway safety protocol enforces strict minimum clearance distances near OHE wires for any staff working on rooftops, signal gantries, or near overhead structures, and why de-energising and earthing the OHE section is mandatory procedure before any maintenance work is carried out on or near it.

4. Semiconductor Basics

What Makes a Semiconductor Different

Materials are broadly classed by how well they conduct electricity. Conductors (like copper and aluminium) allow current to flow easily, having many free electrons. Insulators (like rubber and glass) resist current flow almost completely, having very few free electrons. Semiconductors sit between these two extremes; at low temperature they behave almost like insulators, but their conductivity increases significantly as temperature rises or when small, controlled amounts of impurity atoms are deliberately added, a process called doping.

The two most common semiconductor elements are silicon (Si) and germanium (Ge), both having four electrons in their outermost shell, which is exactly what allows the doping process to work as it does.

Analogy: think of a pure semiconductor crystal as a nearly empty train platform at a quiet hour; almost nobody is around to carry a current of foot traffic. Doping is like a sudden targeted announcement that draws a controlled number of passengers (extra charge carriers) onto that platform, dramatically increasing the crowd's movement (conductivity) in a specific, engineered way.

Doping: N-type and P-type

Doping introduces a small, controlled quantity of impurity atoms into a pure semiconductor crystal to modify its conductivity.

N-type semiconductor: formed by doping with a pentavalent impurity (five outer electrons, such as phosphorus or arsenic). The extra fifth electron from each impurity atom becomes a free charge carrier, giving the material an excess of negative charge carriers (electrons). "N" stands for negative, referring to the majority charge carrier type.

P-type semiconductor: formed by doping with a trivalent impurity (three outer electrons, such as boron or gallium). Each impurity atom creates a "hole", a missing electron, which behaves as a positive charge carrier as neighbouring electrons shift to fill it, passing the hole along like a vacancy moving down a queue. "P" stands for positive, referring to the majority charge carrier type.

Memory hook: "N for Negative electrons, extra from a 5-electron dopant; P for Positive holes, missing from a 3-electron dopant." The "5 for N, 3 for P" pairing is the exact fact RRB likes to test through a mixed-up option list.

The P-N Junction and the Diode

When a P-type and an N-type semiconductor are joined together, the boundary between them is called a P-N junction, and this junction is the basic structure of a diode.

A diode allows current to flow easily in one direction (called forward bias, where the positive terminal of the source connects to the P-side and negative to the N-side) and blocks current almost completely in the opposite direction (called reverse bias). This one-way behaviour makes diodes essential for rectification, converting AC to DC, since AC current reverses direction and a diode only lets the "correct-direction" half of each cycle through.

• A half-wave rectifier uses a single diode, passing only one half of each AC cycle, producing a pulsing DC output with gaps.
• A full-wave rectifier uses multiple diodes (commonly four, in a bridge rectifier arrangement) to use both halves of the AC cycle, producing a smoother, more continuous pulsing DC output.

A special type of diode, the Light Emitting Diode (LED), emits light when forward biased, and has become the dominant lighting technology in modern railway coaches, signal indicators, and station lighting because of its low power consumption and long operating life compared to older incandescent and fluorescent options.

Another special type, the Zener diode, is specifically designed to operate safely in reverse bias beyond a certain voltage (the Zener breakdown voltage), and is used deliberately for voltage regulation, holding a steady output voltage even as input voltage fluctuates.

Exam trap: an ordinary diode is destroyed if pushed into reverse breakdown, but a Zener diode is specifically designed to withstand and exploit reverse breakdown safely; this is the opposite of how a general-purpose diode is meant to be used, and questions distinguishing "ordinary diode" from "Zener diode" behaviour in reverse bias appear regularly.

The Transistor

A transistor is a semiconductor device with three layers and three terminals, used mainly for amplifying electrical signals or for switching current on and off. Invented in 1947 at Bell Labs by John Bardeen, Walter Brattain, and William Shockley, an achievement that earned the trio the Nobel Prize in Physics in 1956, the transistor is widely regarded as one of the most important inventions of the twentieth century, since it eventually replaced bulky, fragile vacuum tubes and made all of modern compact electronics, from radios to computers to control circuits in modern locomotives, possible.

A Bipolar Junction Transistor (BJT), the most commonly taught type at this level, comes in two configurations: NPN and PNP, referring to the sequence of doped layers. Its three terminals are the Emitter (E), Base (B), and Collector (C). A small current at the base terminal controls a much larger current flowing between the emitter and collector, which is exactly the mechanism behind signal amplification: a weak input signal at the base produces a much stronger, shaped output current through the emitter-collector path.

Analogy: think of a transistor's base current as a light touch on a large water tap's control lever at a railway station's water tank. A very small, gentle push on that lever (the base current) controls a huge gush of water through the main pipe (the emitter-collector current). The small control effort governs a disproportionately larger flow, the essence of amplification.

Exam trap: do not confuse a diode's two terminals (anode and cathode) with a transistor's three terminals (emitter, base, collector). Diode questions and transistor questions are frequently mixed together in the same MCQ set specifically to catch students who blur component types.

Passive versus Active Components

This distinction is genuinely fundamental and worth memorising cleanly, since it organises everything covered in this chapter into two clear buckets.

Passive components cannot amplify a signal or introduce gain; they only store, dissipate, or oppose energy as it passes through. Resistors, capacitors, inductors, and transformers all fall in this category.

Active components can amplify signals, control large currents using small ones, or introduce gain into a circuit, generally requiring an external power source to do so. Diodes and transistors, along with more advanced devices like integrated circuits, fall into this category. A diode is sometimes treated as a borderline case since it does not amplify, but it is grouped with active semiconductor devices because its behaviour depends on the internal P-N junction physics rather than simple energy storage or opposition.

5. Electrical Measuring Instruments

Every measuring instrument in this section shares one property: connect it wrongly in a circuit and you either get a wrong reading or you damage the instrument, sometimes dangerously. Get the connection rule for each instrument locked in before exam day.

Ammeter

An ammeter measures electric current, in amperes, and must always be connected in series with the circuit component whose current you want to measure, so that the same current flowing through the component also flows through the ammeter. An ideal ammeter has extremely low (ideally zero) resistance, so that inserting it into the circuit does not itself reduce the current being measured. Connecting an ammeter in parallel by mistake creates a short-circuit-like low-resistance path, which can draw dangerously high current and damage the instrument.

Voltmeter

A voltmeter measures potential difference (voltage), in volts, and must always be connected in parallel across the component whose voltage you want to measure. An ideal voltmeter has extremely high (ideally infinite) resistance, so that it draws negligible current from the main circuit and does not disturb the very voltage it is trying to measure. Connecting a voltmeter in series by mistake would insert a huge resistance into the circuit's main path, drastically reducing current and giving a meaningless reading.

Memory hook: "Ammeter Aligns in series (current must pass through it); Voltmeter Visits in parallel (it only observes, sitting alongside)." Picture the ammeter standing in the queue with the current, and the voltmeter standing off to the side watching two points at once.

Galvanometer

A galvanometer is a sensitive instrument that detects and measures very small electric currents, often used as the core movement inside both ammeters and voltmeters. By adding a low-resistance shunt in parallel with a galvanometer, it can be converted into an ammeter capable of measuring larger currents. By adding a high-resistance multiplier in series with a galvanometer, it can be converted into a voltmeter capable of measuring larger voltages. This shared origin explains why ammeters and voltmeters, despite opposite connection rules, are often built from the same basic sensing mechanism.

Multimeter

A multimeter is a versatile, combined instrument capable of measuring voltage, current, and resistance (and often additional quantities like continuity, diode condition, and capacitance in advanced models), selectable through a rotary dial or buttons. Multimeters come in two broad types: analog multimeters, which use a needle moving across a printed scale, and digital multimeters (DMM), which display readings numerically on an LCD screen, generally offering greater precision and easier reading, especially valuable in field conditions with poor lighting, a genuinely practical consideration for railway maintenance staff working in yards and under coaches.

Exam trap: a multimeter set to measure resistance must never be connected to a live, powered circuit; resistance measurement works by the multimeter sending its own small test current through the component, and an external voltage source interferes with this measurement and can damage the meter. Always de-energise a circuit before testing resistance or continuity with a multimeter.

Wattmeter

A wattmeter measures electric power directly, in watts, combining the functions of current sensing and voltage sensing internally, since power depends on both quantities (P = VI). It typically has two sets of coils, a current coil connected in series (like an ammeter) and a voltage/pressure coil connected in parallel (like a voltmeter), working together to compute and display power.

Energy Meter

An energy meter, the device fitted at your home or any consumer's electrical connection, measures the total electrical energy consumed over time, typically in kilowatt-hours (kWh), the "unit" your electricity bill charges you for. Modern energy meters are largely digital, though older electromechanical models used a rotating aluminium disc whose spin rate depended on power consumption, driving a mechanical counter dial.

Quick Revision — One-Line Facts

1. A resistor opposes current flow; measured in ohms (Ω); can be fixed or variable (rheostat).
2. A capacitor stores charge in an electric field between two plates; measured in farads (F); C = Q/V.
3. An inductor stores energy in a magnetic field and opposes changes in current; measured in henries (H).
4. A transformer changes AC voltage level via electromagnetic induction; frequency stays the same on both sides.
5. Transformer equation: V1/V2 = N1/N2, where N is number of turns.
6. A step-up transformer has more secondary turns than primary; a step-down transformer has fewer.
7. Transformers work only on AC, since they need changing magnetic flux from electromagnetic induction.
8. An ammeter is connected in series; ideally has zero resistance.
9. A voltmeter is connected in parallel; ideally has infinite resistance.
10. A galvanometer detects small currents and forms the base of both ammeters and voltmeters.
11. A shunt (low resistance, parallel) converts a galvanometer into an ammeter; a multiplier (high resistance, series) converts it into a voltmeter.
12. A multimeter measures voltage, current, and resistance; never test resistance on a live circuit.
13. A wattmeter measures power directly, combining a series current coil and a parallel voltage coil.
14. An energy meter measures total energy consumed, in kilowatt-hours (kWh).
15. Semiconductors have conductivity between conductors and insulators; silicon and germanium are common examples.
16. Doping adds controlled impurity atoms to a semiconductor to change its conductivity.
17. N-type semiconductor: doped with a pentavalent (5-electron) impurity; majority carriers are electrons.
18. P-type semiconductor: doped with a trivalent (3-electron) impurity; majority carriers are holes.
19. A diode conducts easily in forward bias and blocks current in reverse bias.
20. A diode's two terminals are the anode and cathode.
21. A rectifier circuit uses diodes to convert AC to DC; a bridge rectifier commonly uses four diodes.
22. An LED emits light when forward biased; widely used in modern railway coach and signal lighting.
23. A Zener diode is designed to operate safely in controlled reverse breakdown, used for voltage regulation.
24. A transistor has three terminals: Emitter, Base, Collector, and is used for amplification and switching.
25. The transistor was invented in 1947 at Bell Labs by Bardeen, Brattain, and Shockley.
26. A small base current in a transistor controls a much larger emitter-collector current (amplification).
27. Passive components (resistor, capacitor, inductor, transformer) do not amplify signals.
28. Active components (diode, transistor) can control or amplify current/signal, often needing external power.
29. Earthing and fuses are core protective measures against electrical faults and shocks.
30. A Residual Current Device (RCD) trips a circuit quickly on detecting current leakage, protecting against shock.

Memory Tables

Table 1: Passive Electrical Components

Component	Stores/Opposes	Quantity Measured	SI Unit
Resistor	Opposes current flow	Resistance	Ohm (Ω)
Capacitor	Stores energy in electric field	Capacitance	Farad (F)
Inductor	Stores energy in magnetic field	Inductance	Henry (H)
Transformer	Transfers and transforms AC voltage	Turns ratio / Voltage ratio	Volt (V)

Table 2: Diode Types and Uses

Diode Type	Key Behaviour	Main Use
Ordinary (rectifier) diode	Conducts in forward bias, blocks reverse bias	AC to DC conversion (rectification)
Zener diode	Designed for safe reverse breakdown	Voltage regulation
LED (Light Emitting Diode)	Emits light when forward biased	Lighting, indicators, displays

Table 3: Ammeter vs Voltmeter

Feature	Ammeter	Voltmeter
Measures	Current	Voltage (potential difference)
Connection	Series	Parallel
Ideal internal resistance	Zero (very low)	Infinite (very high)
Wrong connection risk	Damage from excess current if placed in parallel	Meaningless reading if placed in series
Built from	Galvanometer + shunt (low resistance, parallel)	Galvanometer + multiplier (high resistance, series)

Table 4: N-type vs P-type Semiconductors

Feature	N-type	P-type
Dopant valence	Pentavalent (5 outer electrons)	Trivalent (3 outer electrons)
Example dopants	Phosphorus, Arsenic	Boron, Gallium
Majority charge carrier	Electrons (negative)	Holes (positive)
Meaning of letter	N for Negative	P for Positive

Practice MCQs

Q1. Which component stores electrical energy in the form of an electric field between two plates? (a) resistor (b) capacitor (c) inductor (d) transformer

Q2. The SI unit of capacitance is the: (a) ohm (b) henry (c) farad (d) tesla

Q3. A transformer with 100 turns on the primary and 500 turns on the secondary, connected to a 230 V primary supply, will give a secondary voltage of: (a) 46 V (b) 230 V (c) 1150 V (d) 500 V

Q4. A transformer cannot work on steady DC because: (a) DC voltage is always too low (b) DC produces no changing magnetic flux required for induction (c) transformers physically cannot carry DC current at all (d) DC damages the transformer core instantly

Q5. An ammeter must always be connected in a circuit: (a) in parallel with the component (b) in series with the component (c) directly across the battery only (d) it does not matter as long as polarity is correct

Q6. An ideal voltmeter should have: (a) zero resistance (b) very low resistance (c) infinite (very high) resistance (d) resistance equal to the circuit's total resistance

Q7. A semiconductor doped with a pentavalent impurity such as phosphorus becomes: (a) P-type, with holes as majority carriers (b) N-type, with electrons as majority carriers (c) an insulator with no free carriers (d) a pure conductor identical to copper

Q8. A diode conducts current easily when it is: (a) in reverse bias (b) in forward bias (c) disconnected from the circuit (d) connected only to AC supply

Q9. Which diode is specifically designed to operate safely in reverse breakdown for voltage regulation? (a) LED (b) Zener diode (c) rectifier diode (d) photodiode

Q10. The three terminals of a Bipolar Junction Transistor are: (a) Anode, Cathode, Gate (b) Emitter, Base, Collector (c) Source, Drain, Gate (d) Positive, Negative, Neutral

Q11. The transistor was invented in 1947 at Bell Labs by: (a) Faraday, Ohm, and Ampere (b) Bardeen, Brattain, and Shockley (c) Edison, Tesla, and Westinghouse (d) Volta, Galvani, and Oersted

Q12. Which of the following is classified as a passive electrical component? (a) transistor (b) diode (c) resistor (d) integrated circuit

Q13. A multimeter set to measure resistance should be connected to: (a) a live, powered circuit only (b) a de-energised (unpowered) component (c) the mains supply directly (d) any circuit regardless of power state

Q14. A device that provides fault current a safe, low-resistance path to the ground to protect users from electric shock is called: (a) a rectifier (b) an inductor (c) earthing (grounding) (d) a rheostat

Q15. A galvanometer can be converted into an ammeter capable of measuring larger currents by connecting a: (a) high-resistance multiplier in series (b) low-resistance shunt in parallel (c) capacitor in series (d) transformer in parallel

Answer Key

Q	Answer	Reason
1	(b)	A capacitor stores energy in an electric field between two conductive plates separated by a dielectric; resistors and inductors work differently.
2	(c)	Capacitance is measured in farads; ohm measures resistance, henry measures inductance, tesla measures magnetic flux density.
3	(c)	V2 = V1 × (N2/N1) = 230 × (500/100) = 230 × 5 = 1150 V, since secondary turns exceed primary turns (step-up).
4	(b)	Transformers work by electromagnetic induction, which needs continuously changing flux; steady DC current produces no such change.
5	(b)	An ammeter must be in series so the full circuit current passes through it, giving an accurate current reading.
6	(c)	An ideal voltmeter has very high (ideally infinite) resistance so it draws negligible current and does not disturb the voltage being measured.
7	(b)	A pentavalent (5-electron) dopant like phosphorus contributes extra free electrons, making the material N-type with electrons as majority carriers.
8	(b)	A diode conducts easily only when forward biased, with positive terminal on the P-side and negative on the N-side; it blocks current in reverse bias.
9	(b)	A Zener diode is specifically built to safely operate in reverse breakdown at a defined voltage, making it ideal for voltage regulation.
10	(b)	A BJT's three terminals are Emitter, Base, and Collector; Source, Drain, Gate belong to a different device type (FET), not tested at this level.
11	(b)	John Bardeen, Walter Brattain, and William Shockley invented the transistor at Bell Labs in 1947, later earning the 1956 Nobel Prize in Physics.
12	(c)	A resistor is passive, only opposing current without amplifying signals; transistors, diodes, and integrated circuits are active semiconductor devices.
13	(b)	Resistance measurement uses the multimeter's own internal test current; an externally powered circuit interferes and can damage the meter.
14	(c)	Earthing (grounding) gives fault current a safe path to the ground, preventing it from passing through a person touching a faulty appliance.
15	(b)	A low-resistance shunt connected in parallel with a galvanometer diverts most current around it, extending its range to work as an ammeter.