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Identification of diode, LED, resistor and capacitor: NEET MCQs with answers

50 questions on this topic of Experimental Skills (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. A student is given an unknown two-terminal electronic component. Physical examination reveals a cylindrical body with a distinct silver-colored metallic band near one of its leads. Which component is this most likely to be, and what does the band designate?

  1. A carbon resistor; the band designates 10% tolerance
  2. An electrolytic capacitor; the band designates the positive lead
  3. A semiconductor diode; the band designates the cathode terminal
  4. An LED; the band designates the anode terminal
Show answer and explanation

Answer: (C) A semiconductor diode; the band designates the cathode terminal

Standard silicon rectifier diodes (such as the 1N4007 series) are encapsulated in cylindrical black plastic packages with a silver or white ring marked near one end. This ring identifies the cathode (n-type terminal) of the diode.

medium

2. A student connects an unidentified component in series with a 12 V battery, a 1 kΩ resistor, and a DC microammeter. The meter registers 0 μA. When the battery terminals are reversed, the current remains strictly 0 μA. However, when the DC battery is replaced by a 12 V, 1 kHz AC source, an AC current of approximately 7.5 mA flows. The component is:

  1. A reverse-biased p-n junction diode
  2. An intact capacitor of approximate value 0.1 μF
  3. An open-circuited resistor of value 1 kΩ
  4. A standard carbon resistor of value 1.6 kΩ
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Answer: (B) An intact capacitor of approximate value 0.1 μF

A capacitor acts as an open circuit to DC (zero steady-state current in both polarities) because its capacitive reactance Xc = 1/(2πfC) is infinite for f = 0. Under AC (f = 1 kHz), it offers finite reactance Xc, allowing an alternating current to flow through the circuit.

hard

3. During a laboratory inspection, a student has to match four components with their distinct electrical properties: (P) Carbon resistor (Q) Electrolytic capacitor (R) Silicon diode (S) Light emitting diode Properties: (1) Emits photons when forward-biased beyond ~2 V (2) Blocks steady DC current after a transient charging time (3) Exhibits identical V-I characteristic for positive and negative voltages (4) Conducts heavily only when anode is positive by ~0.7 V without emitting light The correct match is:

  1. P-2, Q-3, R-1, S-4
  2. P-3, Q-2, R-4, S-1
  3. P-3, Q-4, R-2, S-1
  4. P-4, Q-2, R-3, S-1
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Answer: (B) P-3, Q-2, R-4, S-1

A resistor is a linear, bilateral device with symmetric V-I response (P-3). An electrolytic capacitor blocks DC after charging (Q-2). A silicon diode conducts around 0.7 V without emitting visible light (R-4). An LED emits light when forward-biased above its bandgap voltage (~2 V) (S-1).

medium

4. During a laboratory test, a multimeter set to the highest resistance range (20 MΩ) is connected across a completely discharged capacitor. If the multimeter's internal source charges the capacitor through an internal resistance of 1.0 MΩ, and the capacitance is 4.7 μF, what is the approximate time constant of the charging deflection?

  1. 4.7 s
  2. 0.21 s
  3. 20 s
  4. 94 s
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Answer: (A) 4.7 s

The time constant τ of the transient deflection seen during resistance measurement is given by τ = R_int × C. Substituting R_int = 1.0 MΩ = 1.0 × 10⁶ Ω and C = 4.7 μF = 4.7 × 10⁻⁶ F gives τ = (1.0 × 10⁶) × (4.7 × 10⁻⁶) = 4.7 s.

easy

5. A student tests an unmarked two-terminal component using an ohmmeter. Swapping the probe polarities gives 4.7 kΩ in one orientation and 4.7 kΩ in the reversed orientation. This behavior demonstrates that the component is:

  1. a p-n junction diode
  2. a light emitting diode
  3. an ordinary resistor
  4. a leaky capacitor
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Answer: (C) an ordinary resistor

A resistor is a linear, bilateral passive component that offers equal resistance irrespective of the polarity of the applied test voltage. In contrast, diodes and LEDs are unilateral and display starkly different forward and reverse resistances.

hard

6. In a practical physics examination, a candidate is given a circuit containing an unknown component Z in series with an ammeter and a variable DC source. When the voltage is raised from 0 V to 0.4 V, the current remains strictly zero. At 0.7 V, the current rises steeply to 15 mA. When the polarity of Z is reversed, the current remains zero even up to 10 V. The component Z and its state are:

  1. A silicon diode in forward bias in the first arrangement
  2. A carbon resistor of 46.7 Ω
  3. A germanium diode in reverse bias in the first arrangement
  4. An LED emitting infrared radiation at 0.3 V
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Answer: (A) A silicon diode in forward bias in the first arrangement

Silicon p-n junction diodes possess a knee (cut-in) voltage of approximately 0.7 V. Below 0.5 V, the forward current is practically negligible, but around 0.7 V, the current increases rapidly. In reverse bias, the diode blocks conduction (reverse saturation current is on the order of nanoamperes, registering zero on a standard lab ammeter).

easy

7. Which of the following physical appearances correctly helps in distinguishing an electrolytic capacitor from a standard carbon resistor without using any electrical meter?

  1. The capacitor has a cylindrical metal can with polarity markings and leads of unequal length.
  2. The capacitor has distinct colored bands encircling a small cylindrical ceramic rod.
  3. The capacitor always has three terminals, whereas a resistor always has two terminals.
  4. The capacitor has a transparent glass encapsulation showing a visible p-n junction boundary.
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Answer: (A) The capacitor has a cylindrical metal can with polarity markings and leads of unequal length.

Electrolytic capacitors are typically enclosed in cylindrical aluminum cans, display a printed stripe indicating the negative terminal, and have leads of unequal length to identify polarity. In contrast, carbon composition resistors are small cylinders marked with 4 or 5 colored bands and have non-polarized leads of equal length.

medium

8. A technician connects a digital multimeter in the diode test mode across a component. In forward connection, the multimeter screen reads 1.95 V, while in reversed connection, it reads 'OL' (overload). Which component is under test?

  1. Silicon rectifier diode
  2. Red light emitting diode
  3. Germanium signal diode
  4. Carbon resistor of value 1.95 kΩ
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Answer: (B) Red light emitting diode

Digital multimeters in diode test mode display the forward threshold voltage drop. A standard silicon diode shows around 0.6 V to 0.7 V, and a germanium diode shows around 0.2 V to 0.3 V. Light emitting diodes (LEDs) possess wider band gaps and display forward voltage drops between 1.8 V and 3.0 V (red LED is around 1.8 V to 2.0 V).

medium

9. Consider the following statements regarding the physical identification of electronic components: Statement I: The cathode terminal of an LED always has a longer external metallic lead than the anode. Statement II: The cathode side of a semiconductor junction diode is typically identified by a painted silver or white ring near one end of its cylindrical package. Choose the correct option:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (C) Statement I is incorrect but Statement II is correct

Statement I is incorrect because the longer lead of an LED corresponds to the anode (positive terminal), whereas the shorter lead (and flat notch on the rim) corresponds to the cathode. Statement II is correct because standard p-n junction diodes have a printed band/ring designating the n-region (cathode) side.

easy

10. A student checks an unknown two-terminal component using an ohmmeter. In one orientation of the test probes, the meter shows very high resistance (open circuit). Upon reversing the probes, the meter shows a low resistance of about 25 Ω, but no visible light is emitted from the component. The component is most likely:

  1. a ceramic disc capacitor
  2. a light emitting diode (LED)
  3. a carbon composition resistor
  4. a p-n junction rectifier diode
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Answer: (D) a p-n junction rectifier diode

A standard p-n junction diode conducts current only in the forward-biased direction, showing a low resistance, and blocks current in reverse bias, showing very high resistance. Unlike an LED, an ordinary silicon or germanium rectifier diode does not emit visible light when forward biased.

medium

11. When checked with a multimeter, a component shows low resistance in one direction and very high resistance in the other. It is a:

  1. Capacitor
  2. Wire
  3. Diode
  4. Resistor
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Answer: (C) Diode

A diode conducts in forward bias (low resistance) and blocks in reverse bias (high resistance), so its resistance depends on the direction of the test current.

NCERT Class 11

easy

12. A student checks a component visually. It has a tiny transparent bulbous encapsulation with two unequal length leads, and looking inside, one lead connects to a noticeably larger anvil-shaped electrode while the other connects to a smaller post. The component is:

  1. a simple junction diode
  2. a ceramic disc capacitor
  3. a carbon composition resistor
  4. a light emitting diode (LED)
Show answer and explanation

Answer: (D) a light emitting diode (LED)

An LED has a transparent or translucent epoxy lens. Internally, the cathode terminal connects to the larger anvil-shaped lead frame and has a shorter lead externally, while the anode is connected to the smaller post.

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