Q&A: practice questions with answers

Concept: Intel 8051 (MCS-51 family, 1980) is an 8-bit microcontroller: its ALU, accumulator and data bus are 8 bits wide.

1. Features: 4 KB on-chip ROM, 128 bytes RAM, 32 I/O lines (4 ports), two 16-bit timers, one serial port.
2. Only the program counter and DPTR are 16-bit (for addressing 64 KB memory); the data width remains 8-bit.

Yaad rakhein: 8051 → 8-bit; 8085 → 8-bit microprocessor; 8086 → 16-bit.

Answer: (D) 8-bit microcontroller

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

1. The thermometer always reads exactly $2^{\circ }\text{C}$ high – readings are consistent, so it is precise.
2. Every reading is $2^{\circ }\text{C}$ away from the true value – a systematic error, so it is not accurate.

Yaad rakhein: 'Consistently wrong' = precise but not accurate; random scatter = not precise.

Answer: (A) precise but not accurate

Concept: A conductor of length $l$ moving with speed $v$ perpendicular to a field $B$ develops emf $\epsilon =Blv$.

1. Induced emf $\propto v$ – faster motion cuts more flux per second (Faraday's law $\epsilon =-\frac{d\varphi }{dt}$).
2. Resistance decides the induced current ($I=\epsilon /R$), not the emf; area and temperature do not appear in $Blv$.

Yaad rakhein: $\epsilon =Blv$ – B, length, velocity.

Answer: (C) the speed of the conductor's motion through the magnetic field

Concept: With resistance constant, $P=I^{2}R$.

1. New current $=2I$ → $P^{\prime }=(2I{)}^{2}R=4I^{2}R=4P$.
2. Power becomes four times (quadruples), not double, because current is squared.

Trick: Double $I$ → $4\times$ power; double $V$ (at fixed $R$) → also $4\times$ ($P=V^2/R$).

Answer: (B) Power will quadruple.

Concept: In parallel, all resistors are connected between the same two points, so the voltage across each is the same; the current divides.

1. Same voltage → option D is true; 'same current' (C) is the property of series connection.
2. $\frac{1}{R_{eq}}=\frac{1}{R_1}+\frac{1}{R_2}+\dots$ → $R_{eq}$ is smaller than the smallest resistance, so A and B are wrong (B is the series formula).

Yaad rakhein: Series → same current, R adds; Parallel → same voltage, R decreases.

Answer: (D) They have the same voltage.

Concept: For quantities added together, the absolute limiting errors add: $\delta R=\delta R_1+\delta R_2+\delta R_3$.

1. $R=30+60+50=140\Omega$.
2. Absolute errors: $5\%$ of 30 $=1.5$, of 60 $=3$, of 50 $=2.5$; total $=7\Omega$.
3. Result $=140\pm 7\Omega$ (i.e. still $\pm 5\%$).

Trick: When every part has the same percentage error, the sum has the same percentage error: $5\%$ of 140 $=7$.

Answer: (A) $140\pm 7\Omega$

Concept: Energy $E=VIt$ with $t$ in seconds.

1. $t=5\text{ min}=300\text{ s}$.
2. $E=12\times 2\times 300=7200\text{ J}$.

Trick: Power $=VI=24\text{ W}$; $24\times 300=7200$. Forgetting to convert minutes to seconds gives 120 J – not an option, so convert.

Answer: (B) $7200\text{ J}$

Concept: Deflection sensitivity $s$ of a CRT is the spot deflection per volt of deflecting voltage (mm/V); the deflection factor is its reciprocal.

1. $G=\frac{1}{s}$, expressed in V/mm – the voltage needed for 1 mm deflection.
2. Typical values: $s\approx 0.1$–$1$ mm/V, so $G\approx 1$–$10$ V/mm.

Yaad rakhein: Sensitivity high ⇔ factor low; $G=1/s$.

Answer: (B) $G=\frac{1}{s}$

Concept: Eddy currents are circulating currents induced in a bulk conductor when the magnetic flux through it changes (Faraday's law).

1. A changing (time-varying) field or relative motion between conductor and field → changing flux → eddy currents.
2. A uniform/constant field with no motion produces no change of flux, hence no induced current; an electrostatic field does not induce currents in this way.

Yaad rakhein: No change in flux → no induction. Laminated cores are used to reduce eddy-current loss.

Answer: (A) a changing magnetic field

Concept: Astronomical distances are expressed in light-years: the distance light travels in one year, $\approx 9.46\times {10}^{15}\text{ m}$.

1. 1 astronomical unit (AU) = mean Earth–Sun distance $\approx 1.5\times {10}^{11}\text{ m}$ – used within the solar system, far too small for stars and galaxies.
2. Metre and kilometre are everyday units; the nearest star is about 4.2 light-years ($\approx 4\times {10}^{13}\text{ km}$) away, so the light-year is the practical unit.

Yaad rakhein: AU → solar system; light-year / parsec → stars and galaxies ($1\text{ pc}\approx 3.26$ ly).

Answer: (D) light-year

Concept: The standard Intel 8051 has 128 bytes of internal data RAM (addresses 00H–7FH).

1. This RAM holds 4 register banks (32 bytes), a bit-addressable area (16 bytes) and general-purpose scratch pad (80 bytes).
2. The 8052 variant has 256 bytes; 4 KB is the on-chip program ROM, not RAM.

Yaad rakhein: 8051 → 128 B RAM, 4 KB ROM; 8052 → 256 B RAM, 8 KB ROM.

Answer: (B) 128-byte

Concept: From rest, $s=\frac{1}{2}a{t}^2$, so distance $\propto t^2$.

1. Distance in first $t$: $s_1=\frac{1}{2}a{t}^2$.
2. Distance in total time $3t$: $s=\frac{1}{2}a(3t{)}^2=\frac{9}{2}a{t}^2$.
3. Distance in the next $2t$: $s_2=\frac{9}{2}a{t}^2-\frac{1}{2}a{t}^2=4a{t}^2$.
4. $s_1:s_2=\frac{1}{2}:4=1:8$.

Trick: Distances in intervals $t,2t$ from rest are $1^2:(3^2-1^2)=1:8$.

Answer: (A) $1:8$

Concept: Principle of homogeneity: every term added, subtracted or equated in a physical equation must have the same dimensions.

1. Dimensional consistency means the equation obeys this principle – option B.
2. It is a necessary but not sufficient test: a dimensionally consistent equation may still be wrong (numerical constants, $\sin \theta$ etc. are dimensionless), so A and C overstate it.

Yaad rakhein: Consistent ≠ correct; inconsistent = definitely wrong.

Answer: (B) the equation respects the principles of homogeneity of dimensions

Concept: Resistivity $\rho$ is the reciprocal of electrical conductivity: $\rho =\frac{1}{\sigma }$. It is a material property set by how freely charge carriers move.

1. High conductivity (copper, silver) → low resistivity; insulators have very low conductivity → high resistivity.
2. Temperature coefficient only tells how $\rho$ changes with temperature; density and melting point do not fix $\rho$ (silver is denser than copper yet a better conductor).

Yaad rakhein: $\rho =1/\sigma$; unit $\Omega \text{m}$ and $\text{S/m}$.

Answer: (B) Electrical conductivity

Concept: When force changes with position, work is the integral (area under the F–x graph): $W=\int Fdx$.

1. $W=Fd$ and $W=Fd\cos \theta$ apply only to a constant force.
2. $W=\frac{1}{2}m{v}^2$ is kinetic energy (work-energy theorem result), not the definition of work by a variable force.

Yaad rakhein: Variable → integrate; constant → multiply.

Answer: (C) $W=\int Fdx$

Concept: LED brightness depends on the forward current through it; a series resistor sets $I=\frac{V_s-V_D}{R}$.

1. When the source voltage $V_s$ is much greater than the diode drop $V_D$ ($\approx 2$ V), $I\approx \frac{V_s}{R}$ and small changes in $V_D$ hardly alter the current, so brightness stays nearly constant.
2. Temperature affects LED output only slightly; it is not the controlling variable.

Yaad rakhein: LED = current-driven device; $V_s\gg V_D$ → constant current.

Answer: (C) current; greater

Concept: A load cell or proving ring is an elastic member; the applied force produces a strain, and bonded strain gauges convert that strain (change in resistance) into an electrical signal, thereby measuring the force.

1. Sequence: force → strain in the elastic element → strain gauge → force reading, i.e. 'force by the strain'.
2. LVDT measures displacement, piezoelectric transducers suit dynamic forces, capacitive transducers measure displacement/pressure – not the standard load-cell element.

Yaad rakhein: Load cell = strain gauges in a Wheatstone bridge.

Answer: (B) Strain gauges; force by the strain

Concept: The Periodic Labour Force Survey (PLFS) of the NSO reports the unemployment rate (usual status) separately for rural and urban areas.

1. PLFS 2018–19: overall unemployment rate $5.8\%$; rural $5.0\%$; urban $7.7\%$.
2. Urban unemployment is always reported higher than rural because of fewer self-employment/agricultural fall-back options.
3. $5.2\%$ and $6.8\%$ are close to other years' figures (PLFS 2017–18 urban was $7.8\%$, 2019–20 urban $6.9\%$), not 2018–19.

Trick: PLFS 2018–19 → "5.8 overall, 5.0 rural, 7.7 urban".

Answer: (A) 7.7

Concept: Article 43A (Part IV, Directive Principles) says the State shall take steps to secure the participation of workers in the management of undertakings and industries.

1. It was inserted by the 42nd Constitutional Amendment Act, 1976.
2. Article 42 – just and humane conditions of work and maternity relief; Article 44 – Uniform Civil Code; Article 45 – early childhood care and education.
3. Articles 45B, 44C and 42A do not exist.

Yaad rakhein: 43 = living wage, 43A = workers in management, 43B = co-operative societies.

Answer: (D) Article 43A

Concept: After the Revolt of 1857, the British Parliament passed the Government of India Act, 1858, ending the rule of the East India Company and transferring power directly to the British Crown.

1. The post of Secretary of State for India was created and the Governor-General became the Viceroy (Lord Canning first).
2. Pitt's India Act 1784 only set up a Board of Control over the Company; the Charter Act of 1853 was the last charter renewal; "Regulating Act of 1813" is a mix-up (Regulating Act is 1773, Charter Act is 1813).

Yaad rakhein: 1857 revolt → 1858 Act → Company out, Crown in.

Answer: (A) Government of India Act of 1858