Physics
36 Solved Past Papers
2012 – 2024 Archives
Electronics Past Papers
Solved past paper MCQs for Electronics from official UHS, NUMS, SZABMU, DUHS, and KMU examinations. Includes verified distractor autopsies and step-by-step cognitive explanations.
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[PMC Sample 3] What is the typical range of the forbidden energy band gap in electrical conductors?
A
100 - 1000 eV
B
0.5 - 10 eV
C
10000 - 10000000 eV
D
0.01 - 0.1 eV
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
Band theory of solids divides materials into conductors, semiconductors, and insulators based on the gap between the valence and conduction bands.
Solution:
In excellent conductors (metals), the valence and conduction bands overlap, meaning the band gap is effectively zero. In certain weak conductors or semi-metals, a very narrow energy gap exists, which typically ranges from 0.01 to 0.1 eV.
Why other options are incorrect:
0.5 to 10 eV describes the band gap of typical semiconductors and insulators.
Other extremely high values are physically non-existent for solid-state molecular structures.
[PMC Practice 2] If a half-wave rectifier is connected to a 50 Hz AC mains, the number of DC pulses generated per second in the output voltage is:
A
25
B
50
C
100
D
75
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
The ripple frequency of a half-wave rectifier is exactly equal to the frequency of the input AC supply.
Solution:
A half-wave rectifier permits only one polarity of current (one peak) to pass through per input cycle, while blocking the other. Since the input frequency is \( 50\text{ Hz} \) (50 full cycles per second), there will be exactly 50 pulses of DC output per second.
Why other options are incorrect:
100 is the output pulse frequency of a full-wave rectifier.
25 and 75 do not correspond to the physics of half-wave rectification.
[PMC Practice 5] A transformer is commonly used prior to the rectification stage in a DC power supply to:
A
Step up the voltage
B
Step down the voltage
C
Equalize the voltage
D
None of these
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
Standard household mains voltage is typically very high (e.g., 220V AC), while most digital and solid-state electronic devices operate on low DC voltages (e.g., 5V to 12V).
Solution:
The primary function of the transformer in a consumer electronic power supply is to step down the high AC mains voltage to a safe, low AC level before passing it through the rectifying diodes and filters.
Why other options are incorrect:
Step up is incorrect because electronic devices do not require hundreds or thousands of volts to operate.
[PMC Practice 23] A basic half-wave rectifier passes only the:
A
Upper half-cycle
B
Lower half-cycle
C
Both half-cycles
D
None of these
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
A half-wave rectifier operates by selectively allowing only half of the input AC waveform to pass to the load.
Solution:
Under standard conventional setups, the diode is arranged to be forward-biased during the positive phase. It therefore permits only the positive (upper) half-cycle of the wave to pass while completely cutting off the negative (lower) half-cycle.
Why other options are incorrect:
Passing both half-cycles describes a full-wave rectifier.
The conversion of A.C into D.C is called rectification and circuit is called rectifier. Which component of electronics acts as a rectifier?
A
Diode
B
Transistor
C
Transformer
D
Inductor
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
Rectification relies on a component that acts as a one-way electrical valve.
Solution:
The core function of a P-N junction diode is to allow current to pass heavily in the forward direction and block it almost entirely in the reverse direction.
This exact characteristic is what "rectifies" an alternating current into a direct current.
Why other options are incorrect:
Transformer: Changes the voltage amplitude but maintains the AC waveform.
Transistor: Used as a switch or amplifier.
Inductor: Opposes sudden changes in current, used mostly in filtering.
Three diodes connected in bridge type arrangements
C
Four diodes connected in bridge type arrangements
D
Two diodes connected in bridge type arrangements
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
A full-wave bridge rectifier is a highly efficient topology that rectifies the full AC wave without needing a specialized center-tapped transformer.
Solution:
The "bridge" configuration explicitly consists of a diamond-like arrangement of diodes.
To ensure that both the positive and negative halves of the AC cycle are directed identically through the load, precisely four diodes are required.
During any half cycle, two diodes conduct while the other two block.
Why other options are incorrect:
You cannot build a functional bridge topology with 1, 2, or 3 diodes; they would result in either half-wave rectification, no rectification, or a dead short.
In electronics, the behavior of a component is best understood by observing how the current flowing through it changes as the voltage across it changes.
Solution:
The characteristic curve of a diode (often called the V-I curve) is a fundamental graphical representation of its operation.
The x-axis conventionally represents the applied Potential Difference (Voltage).
The y-axis represents the resulting Current.
Therefore, it is a plot between Voltage and current.
Why other options are incorrect:
Voltage/Current and time: This would describe an oscilloscope waveform, not a component's internal characteristic curve.
Reverse voltage forward voltage: Plotting a voltage against a voltage is nonsensical for describing component behavior.
Most electronic devices operate on low-voltage Direct Current (DC), but power grids supply high-voltage Alternating Current (AC).
Solution:
Mains AC is typically \( 220\text{V} \) or \( 110\text{V} \). Electronic circuits (like phones and laptops) require much lower voltages (e.g., \( 5\text{V} \), \( 12\text{V} \)).
Before rectification occurs, the high mains voltage must be lowered to a safe, usable level to prevent destroying the diodes.
A Step down transformer is utilized at the very beginning of the power supply to achieve this voltage reduction.
Why other options are incorrect:
Step up: Would dangerously increase the voltage, destroying the circuit.
Low input voltage / Low input resistance: Do not describe the fundamental function of the transformer in this context.
The circuit required for change of AC voltage to DC voltage is called:
A
Rectifier
B
Amplifier
C
Detector
D
Emitter
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
Power supply design heavily relies on specific circuits to convert alternating currents to direct currents.
Solution:
The dedicated electronic circuit constructed (usually with diodes) to convert bidirectional AC into unidirectional DC is known universally as a Rectifier.
Why other options are incorrect:
Amplifier: Boosts the power of an input signal.
Detector: Extracts information from a modulated radio wave.
Emitter: A physical terminal on a bipolar junction transistor, not a circuit type.
For the positive half cycle i.e., \( 0 - T/2 \) the diode \( D \):
A
Shows maximum resistance
B
Behaves as open
C
Is reverse biased
D
Is forward biased
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
In a standard half-wave rectifier diagram, a single diode is connected in series with the AC source and the load.
Solution:
The total time period of the AC wave is \( T \). The first half cycle exists from \( 0 \) to \( T/2 \) and is typically the positive half.
During the positive half cycle, the P-region of the diode connects to the positive potential and the N-region to the negative potential.
This correctly aligns with the condition for the diode to conduct, meaning it is forward biased.
Why other options are incorrect:
Reverse biased, maximum resistance, and behaving as an open circuit all perfectly describe the diode's state during the negative half cycle (\( T/2 \) to \( T \)), not the positive half.
A single diode blocks one polarity of current and permits the other.
Solution:
Because a single diode can only conduct during one half of an AC cycle (either positive or negative depending on orientation), it effectively chops off half the wave.
Therefore, a single semiconductor diode acts purely as a half wave rectifier.
A full wave rectifier strictly requires a minimum of two diodes (with a center tap) or four diodes (bridge).
Why other options are incorrect:
Full wave rectifier: Requires multiple diodes.
Amplifier: Requires a transistor (active device with gain).
Transmitter: Requires complex oscillator and antenna circuits.
There are two primary ways to achieve full-wave rectification: Center-tapped (\( 2 \) diodes) and Bridge (\( 4 \) diodes). Without specifying "center-tapped", the industry standard topology is the bridge rectifier.
Solution:
A Full Wave Bridge Rectifier does not require an expensive center-tapped transformer.
It utilizes an arrangement of exactly \( 4 \) diodes configured in a bridge loop.
During the positive half cycle, two diodes conduct. During the negative half cycle, the other two conduct.
for this specific NUMS question, \( 4 \) is the expected answer.
Why other options are incorrect:
\( 1 \): Makes a half-wave rectifier.
\( 2 \): Valid only if a "center-tapped transformer" is explicitly mentioned.
\( 3 \): Not a functional rectifier configuration.
When the temperature of semiconductor suddenly drops to zero kelvin, then a semiconductor acts as:
A
Super conductor
B
Semi-conductor
C
Conductor
D
Insulator
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
The electrical conductivity of intrinsic semiconductors is highly temperature-dependent. Thermal energy is required to break covalent bonds and excite electrons from the valence band to the conduction band.
Solution:
At Absolute Zero (\( 0\text{ K} \)), there is zero thermal energy available in the lattice.
Consequently, no covalent bonds are broken, and absolutely no electrons can cross the bandgap into the conduction band.
With zero free electrons and zero holes, the material has infinite resistivity. Therefore, it acts perfectly as an insulator.
Why other options are incorrect:
Conductor / Super conductor: Conductivity drops to zero at \( 0\text{ K} \) for semiconductors, unlike metals which become better conductors or superconductors at ultra-low temperatures.
Semi-conductor: At \( 0\text{ K} \), it loses its defining semi-conductive property and behaves purely as an insulator.
A process in which only one half of alternating current is converted into direct current, such process is called:
A
Half wave rectification
B
Magnification
C
Amplification
D
Full wave rectification
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
Rectification is the process of converting bidirectional alternating current (AC) into unidirectional direct current (DC).
Solution:
If a circuit utilizes both the positive and negative halves of the AC cycle, it is Full Wave Rectification.
If a circuit uses a single diode to block one half of the AC cycle (letting only the positive or negative half pass), it is termed Half wave rectification.
Why other options are incorrect:
Full wave rectification: Converts both halves.
Amplification & Magnification: Refers to increasing signal strength/size, completely unrelated to AC/DC conversion.
Efficiency of full wave rectifier circuit is almost _ than half wave rectifier circuit:
A
Four times
B
Double
C
Sixteen times
D
Same as
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
Rectifier efficiency (\( \eta \)) represents the ratio of DC output power to the applied AC input power.
Solution:
A Half-Wave Rectifier wastes an entire half-cycle of the input wave. Its maximum theoretical efficiency is roughly \( 40.6\% \).
A Full-Wave Rectifier utilizes both the positive and negative half-cycles of the input wave. Its maximum theoretical efficiency is roughly \( 81.2\% \).
Comparing the two: \( 81.2\% \) is exactly double of \( 40.6\% \).
Why other options are incorrect:
Four times / Sixteen times: These drastically overestimate the efficiency ratio.
Same as: Incorrect, because utilizing the blocked half-cycle inherently increases power efficiency.
The defining function of any rectifier is to transform bidirectional current into current that flows in only one direction.
Solution:
A half-wave rectifier uses a diode to block the negative half of the AC cycle.
The resulting output consists of pulses of positive current separated by gaps of zero current.
Even though the magnitude of the current fluctuates (pulsates), it never drops below zero to flow backward. Thus, it is purely a unidirectional current.
Why other options are incorrect:
AC current: AC flows in both directions; the rectifier has eliminated this.
Straight line parallel to vertical axis: This describes an infinite slope, which represents zero physical reality in this context (an ideal steady DC would be parallel to the horizontal axis).
Zero always: Incorrect, as it conducts during the positive half-cycle.
Negligible in comparison to the resistance of half wave rectifier
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
The total internal forward resistance of a rectifier circuit depends on the number of diodes in the conduction path during any given half-cycle.
Solution:
In a Half-Wave Rectifier, current flows through only \( 1 \) diode during the active half-cycle. Therefore, the forward resistance is \( R_d \).
In a Full-Wave Bridge Rectifier (the most common un-tapped configuration), current must pass through \( 2 \) diodes in series during every half-cycle. Therefore, the forward resistance is \( 2 R_d \).
Since \( 2 R_d > R_d \), the internal resistance of the FWR circuit is more than the resistance of a half wave rectifier.
Why other options are incorrect:
A single diode will physically offer less resistance than two diodes in series, making "less than" or "equal to" incorrect when comparing bridge to half-wave architectures.
The potential difference applied across a diode to reduce depletion layer is:
A
Junction potential
B
Reverse biasing
C
Forward biasing
D
Biasing voltage
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
The depletion layer in a P-N junction acts as a barrier to charge carriers. Applying external voltage (biasing) manipulates the width of this layer.
Solution:
When the positive terminal of a battery is connected to the P-type material and the negative terminal to the N-type material, holes and electrons are pushed toward the junction.
This external potential opposes the built-in junction potential.
As a result, the width of the depletion layer decreases, allowing current to flow. This specific configuration is known as Forward biasing.
Why other options are incorrect:
Reverse biasing: Increases the width of the depletion layer.
Junction potential: The internal built-in barrier itself, not an externally applied voltage.
Biasing voltage: Too generic; could refer to either forward or reverse bias.
The conversion of alternating current into direct current is called:
A
Amplification
B
Rectification
C
Magnification
D
Resolution
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
Standard terminology defines the various manipulations of electrical signals.
Solution:
The deliberate process of blocking or inverting the reverse-flowing portion of an alternating current (AC) so that it flows exclusively in one direction (DC) is termed Rectification.
Why other options are incorrect:
Amplification: Increasing the magnitude of a signal.
Magnification / Resolution: Terms primarily used in optics and imaging, completely unrelated to AC/DC conversion.
The ripple factor (\( \gamma \)) measures the smoothness of the DC output. A lower ripple factor means a smoother DC output with fewer AC fluctuations.
Solution:
For a Half-Wave Rectifier (HWR), the standard ripple factor is approximately \( 1.21 \) (or \( 121\% \)), meaning the AC ripple is actually larger than the DC component.
For a Full-Wave Rectifier (FWR), the standard ripple factor is approximately \( 0.48 \) (or \( 48\% \)).
Since \( 0.48 < 1.21 \), the Full wave rectifier has significantly less ripple factor.
Why other options are incorrect:
Half-wave rectifiers have huge gaps between pulses, leading to massive ripples.
They cannot be "Both" or "None" as their mathematical constants for ripple factor distinctly differ.
For every \( 1 \) full cycle of AC input, a half-wave rectifier produces exactly \( 1 \) output pulse.
If the input frequency is \( 50 \text{ Hz} \) (meaning \( 50 \) full cycles per second), the output will correspondingly have \( 50 \) pulses per second.
Therefore, the number of pulses is \( 50 \).
Why other options are incorrect:
\( 100 \): This would be the number of pulses for a Full-Wave Rectifier, where \( f_{output} = 2 \times f_{input} \).
\( 60 \) & \( 150 \): Incorrect values not mathematically linked to a \( 50\text{Hz} \) input.
Full-wave rectification requires processing both halves of the AC input waveform, which necessitates more than one diode.
Solution:
A Half-Wave Rectifier uses exactly \( 1 \) diode.
A Full-Wave Center-Tapped Rectifier uses \( 2 \) diodes.
A Full-Wave Bridge Rectifier uses \( 4 \) diodes.
for this specific exam question, the minimum configuration required for full wave rectification—using a center tap—utilizes \( 2 \) diodes.
Why other options are incorrect:
\( 1 \) diode only allows half-wave rectification.
\( 3 \) diodes is not a standard full-wave configuration.
While \( 4 \) diodes are used in a bridge configuration, \( 2 \) is the correct answer historically mapped for the general "full wave rectification" baseline involving center-tapped transformers.
In case of half rectification, resistance of diode during negative half of A.C is:
A
Very low
B
A few ohms
C
Very high
D
Negligible
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
A diode acts as a one-way valve for electrical current based on its biasing.
Solution:
During the positive half cycle of an AC input, the diode is forward-biased, offering very low resistance (ideal: zero).
During the negative half cycle, the polarity reverses, and the diode becomes reverse-biased.
In a reverse-biased state, the depletion region widens, resulting in a very high resistance (ideal: infinite) which blocks current flow, effectively creating the half-wave rectification.
Why other options are incorrect:
Very low, negligible, or a few ohms describe the forward-biased state, not the negative half (reverse-biased) state.
The direction of current through the load resistance of a full-wave rectification circuit:
A
Inverts for positive cycle
B
Remains constant
C
Changes for every cycle
D
Inverts for negative cycle
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
The primary goal of any rectifier (half-wave or full-wave) is to convert bidirectional AC into unidirectional DC.
Solution:
In a full-wave rectifier, different diode pairs conduct during opposite halves of the AC cycle.
However, the physical topology of the circuit routes the current from both halves so that it always enters the load resistance from the exact same terminal.
Because the current always flows through the load in the same direction, the direction remains constant (unidirectional).
Why other options are incorrect:
If the current inverted or changed direction for any cycle, the output would still be alternating current (AC), rendering the rectifier useless.
In the following figure what happens for the positive half cycle of the input?
Full Wave Bridge Rectifier Conductive Path during Positive Half-Cycle
A
\( D_1 \) and \( D_3 \) conducts
B
\( D_1 \) and \( D_2 \) conducts
C
\( D_3 \) and \( D_4 \) conducts
D
\( D_2 \) and \( D_4 \) conducts
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
In a standard full-wave bridge rectifier, diodes conduct in diagonally opposite pairs depending on the instantaneous polarity of the alternating input voltage.
Solution:
During the positive half-cycle of the AC input voltage, the terminal polarity forward-biases the first pair of diagonally opposite diodes, labeled \( D_1 \) and \( D_3 \) in the diagram.
Simultaneously, the opposite pair (\( D_2 \) and \( D_4 \)) is reverse-biased and blocks current.
Therefore, \( D_1 \) and \( D_3 \) conduct during the positive half-cycle.
Why other options are incorrect:
Option D: \( D_2 \) and \( D_4 \) conduct during the alternating negative half-cycle.
Options B and C: Adjacent pairs (such as \( D_1 \) & \( D_2 \), or \( D_3 \) & \( D_4 \)) cannot conduct simultaneously in a bridge rectifier as they would cause an input short circuit.
Conversion of alternating current into direct current is called:
A
Amplification
B
Oscillation
C
Rectification
D
Regeneration
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
The fundamental process of translating an alternating waveform (which changes direction periodically) into a unidirectional waveform is a core electronics principle.
Solution:
An alternating current (AC) flows in both directions, whereas a direct current (DC) flows only in one direction.
The specific process of converting AC to DC using electronic components (like diodes) is formally defined as rectification.
Why other options are incorrect:
Amplification: Increasing the amplitude of a signal.
Oscillation: Converting DC into AC.
Regeneration: Feeding a portion of the output back into the input to boost signal strength.
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