RRB Technician Grade I (Signal) 2024, 2025 PYQs

Concept: Transmission-line conductors need low resistivity plus low cost, light weight and good strength.

1. Silver has the lowest resistivity but is too costly; gold also costly.
2. Copper ($\rho \approx 1.7\times {10}^{-8}\Omega \text{m}$) and aluminium ($\approx 2.8\times {10}^{-8}\Omega \text{m}$) are cheap and conductive — aluminium (ACSR) for overhead lines, copper for cables.
3. Tungsten, iron, lead, tin have much higher resistivity.

Yaad rakhein: Resistivity order: Ag $<$ Cu $<$ Au $<$ Al — practical choice: Cu and Al.

Answer: (D) Copper and aluminium

Concept: Inside a long current-carrying solenoid the magnetic field is uniform.

1. Uniform field ⇒ field lines are parallel straight lines, equally spaced, along the axis.
2. Outside the solenoid the field is weak and the lines spread out like a bar magnet's.
3. Circular lines are for a straight wire, not inside a solenoid.

Yaad rakhein: Solenoid inside = uniform = parallel straight lines ($B={\mu }_{0}nI$).

Answer: (C) They are in the form of parallel straight lines

Concept: $F=BIL\sin \theta$, where $\theta$ is the angle between the current and the magnetic field.

1. $\sin \theta$ is maximum ($=1$) at $\theta ={90}^{\circ }$ → $F_{max}=BIL$.
2. Parallel or anti-parallel ($0^{\circ }$ or ${180}^{\circ }$): $\sin \theta =0$, no force.
3. The conductor need not move — the force is on the current, not on motion.

Yaad rakhein: Perpendicular → maximum force (Fleming's left-hand rule).

Answer: (C) When the direction of the current is at right angles to the magnetic field

Concept: If $Q\propto m^n$, then $\frac{\Delta Q}{Q}=n\frac{\Delta m}{m}$.

1. $\frac{\Delta m}{m}=\frac{0.0001}{0.005}=0.02=2\%$.
2. For $Q\propto m^2$: $\frac{\Delta Q}{Q}=2\times 2\%=4\%$.

Trick: Power $n$ multiplies the percentage error by $n$.

Answer: (D) 4%

Concept: The depletion layer width depends on the net barrier potential across the junction.

1. Reverse bias adds to the built-in potential → more ions uncovered → depletion layer widens.
2. Forward bias opposes the barrier → depletion layer narrows and current flows.
3. The width changes in both cases, so options saying "remains constant" are wrong.

Yaad rakhein: Reverse = wider, Forward = narrower.

Answer: (A) Depletion layer gets wider when it is reverse biased.

Concept: Work done by the net force equals the change in kinetic energy: $W=\Delta KE$.

1. Constant force $F$ over distance $d$: $W=F\times d$.
2. Car starts from rest, so $K{E}_{initial}=0$ → $KE=F\times d$.
3. Check: $v^2=2ad=\frac{2Fd}{m}$, so $\frac{1}{2}m{v}^2=Fd$.

Yaad rakhein: From rest, KE gained = work done = $Fd$ (mass cancels out).

Answer: (B) $F\times d$

Concept: In an intrinsic (pure) semiconductor the number of electrons in the conduction band equals the number of holes in the valence band.

1. Equal electron and hole concentrations place the Fermi level at the centre of the forbidden gap.
2. n-type doping shifts it up towards the conduction band; p-type shifts it down towards the valence band.

Yaad rakhein: Intrinsic → middle; n-type → near CB; p-type → near VB.

Answer: (D) lies mid-way between the conduction band and valance band

Concept: PCB = Printed Circuit Board — an insulating board (usually glass-epoxy FR-4) with copper tracks etched on it to connect components.

1. Components are soldered onto the board; the copper "printed" tracks replace loose wiring.
2. Other expansions (Processor Control Board etc.) are not standard electronics terms.

Yaad rakhein: The green board inside every gadget = Printed Circuit Board.

Answer: (B) Printed Circuit Board

Concept: Random errors vary unpredictably between readings; systematic errors shift every reading the same way.

1. Random errors are reduced by repeating observations, averaging and controlling conditions (temperature, vibration).
2. Calibration corrects a systematic (instrumental) error — it does not reduce the scatter of random errors.

Yaad rakhein: Random → average it out; Systematic → calibrate it out.

Answer: (A) Calibrating the instrument

Concept: Accuracy = closeness to the true value; precision = closeness of repeated readings to each other.

1. Hitting the same point repeatedly = high precision (readings agree).
2. That point being far from the centre = low accuracy (systematic offset).
3. Option B is high precision and high accuracy; option C is low in both.

Yaad rakhein: Tight group off-centre = precise but not accurate.

Answer: (D) Hitting the same point far from the centre repeatedly

Concept: Motional EMF $\epsilon =Blv\sin \theta$.

1. It depends on field $B$, length $l$, speed $v$ and the angle $\theta$ between $v$ and $B$.
2. Resistance of the conductor affects only the induced current ($I=\epsilon /R$), not the EMF itself.

Yaad rakhein: EMF is "cause", current is "effect" — $R$ changes the effect only.

Answer: (C) The resistance of the conductor

Concept: Lenz's law: the induced current opposes the change that produces it (energy conservation).

1. As the ring enters/moves through the field, flux through it changes and eddy/induced current flows.
2. The induced current experiences a magnetic force opposing the motion — a retarding force upward.
3. So the ring falls slower than free fall; it is not pushed up or made faster.

Yaad rakhein: Induction always "brakes" the motion (magnetic braking).

Answer: (B) It slows down as it falls.

Concept: A thermistor is a temperature-sensitive resistor; it may be NTC (resistance falls with temperature) or PTC (resistance rises).

1. Whether the coefficient is positive or negative is decided by the construction material (metal oxides etc.) and the temperature region.
2. The temperature change can come from self-heating by the current, or from the surroundings — which may heat or cool it.

Yaad rakhein: NTC for temperature sensing; PTC for self-resetting fuses/over-current protection.

Answer: (B) positive or negative temperature coefficients; heating or cooling

Concept: The barrier (knee) potential of a p-n junction depends on the semiconductor material.

1. Germanium: about $0.3\text{V}$ at ${25}^{\circ }\text{C}$; silicon: about $0.7\text{V}$.
2. Barrier potential decreases by roughly $2\text{mV}$ per ${}^{\circ }\text{C}$ rise in temperature.

Yaad rakhein: Ge = 0.3 V, Si = 0.7 V.

Answer: (A) 0.3 V

Concept: The MCS-51 (8051) family includes 8051, 8031 (ROM-less), 8751 (EPROM) and their CMOS versions 80C51BH, 80C31BH, 87C51.

1. 80C51BH, 80C31BH and 87C51 are standard 8051-series members.
2. 8044AH is Intel's RUPI-44 serial communication controller (8051 core with an SDLC/HDLC interface) — a separate product line, not listed as an 8051-series microcontroller.

Yaad rakhein: 8051 family numbers contain "51" or "31"; 8044 is the odd one.

Answer: (B) 8044AH

Concept: Lenz's law gives the direction of induced EMF/current: it opposes the change in magnetic flux that causes it.

1. This is a consequence of conservation of energy — the induced current must do work against the cause.
2. The direction depends on whether flux is increasing or decreasing, not on the amount of flux (option D wrong).

Yaad rakhein: The minus sign in $\epsilon =-\frac{d\varphi }{dt}$ is Lenz's law.

Answer: (A) The induced current always opposes the change in magnetic flux that produced it.

Concept: The forbidden energy gap $E_g$ between the valence band and the conduction band decides whether a material conducts.

1. Conductors: valence and conduction bands overlap – no forbidden gap, electrons flow freely.
2. Semiconductors (Si, Ge): narrow gap of about $1\text{eV}$, so a little energy (heat/light) lifts electrons into the conduction band.
3. Insulators: wide gap ($>5\text{eV}$), so electrons cannot cross at ordinary temperatures.

Yaad rakhein: Gap order – conductor $0$ < semiconductor $\approx 1\text{eV}$ < insulator $>5\text{eV}$.

Answer: (B) Conductors have no forbidden gap, semiconductors a narrow gap and insulators a wide gap.

Concept: A thermocouple works on the Seebeck effect – an emf is produced when its two junctions are at different temperatures.

1. Over its working range the emf is approximately proportional to the temperature difference: $E\approx \alpha (T_1-T_2)$, where $\alpha$ is the Seebeck coefficient.
2. Exactly, $E=a\theta +b{\theta }^2$ (slightly parabolic), but the second term is small, so the relation is treated as linear for measurement.

Yaad rakhein: Thermocouple → Seebeck effect → emf ∝ temperature difference (linear).

Answer: (D) linear

Concept: For a long solenoid the field inside is $B={\mu }_{0}nI$ ($n$ = turns per unit length) – it does not depend on the position inside.

1. The fields of the closely wound turns add up to give parallel, equally spaced field lines inside – a uniform field along the axis.
2. At the ends the field drops to about half ($\frac{1}{2}{\mu }_{0}nI$), and outside it is nearly zero – so options (A), (C), (D) are wrong.

Yaad rakhein: Long solenoid ≈ bar magnet with a uniform field inside, almost zero outside.

Answer: (B) uniform

Concept: Faraday's law: induced emf $e=-\frac{d\varphi }{dt}$ – it depends on the rate of change of flux, not on the flux itself.

1. Flux increases linearly: $\varphi =kt$.
2. $e=-\frac{d\varphi }{dt}=-k$ = a constant value.
3. It would be zero only if the flux did not change at all.

Trick: Flux ∝ $t$ ⇒ slope constant ⇒ emf constant.

Answer: (A) constant