Use a multimeter set to diode/continuity mode. Resistor: same resistance both directions. Capacitor: short circuit when uncharged in DC, then OPEN. Diode: low-resistance forward, high-resistance reverse. LED: same as diode but glows when forward biased above the threshold.
-- NCERT Physics Lab Manual Class 12, Part 11, p. 150Experiment 18 Component Identification
8 MCQs9-step worked example
Source: NCERT Experimental SkillsOfficial key: NTA-verifiedLast updated: 24 Sep 2026
Experiment 18 Component Identification, explained for NEET
The trap in this experiment is treating "conducts" and "does not conduct" as a two-way sort. A mixed tray gives you four behaviours, not two, and two of them look alike for the first second.
Put the multimeter on a resistance range, touch the two leads of the unknown component, then reverse the leads. Four outcomes, each decisive:
- Resistor — the same finite reading both ways. It has no polarity, so reversing changes nothing.
- Junction diode — a low reading one way, an overload (effectively infinite) the other way. Opaque body, a band marking the cathode end.
- LED — the same one-way conduction, but the forward reading corresponds to a larger voltage drop, and on a diode-test range the junction may glow faintly. The give-away before any test is the clear or tinted plastic dome and the unequal lead lengths, the longer lead being the anode.
- Capacitor — a low reading that climbs steadily to infinity as the capacitor charges through the meter, in both directions. A capacitor left charged then reads infinity until it is discharged.
The confusion to name out loud: a capacitor caught in its first instant looks like a forward-biased diode, and a charged capacitor looks like a reverse-biased diode. Watching the reading move is what separates them; a single glance at the display does not. This is why the NCERT Physics Laboratory Manual has you reverse the leads and watch, rather than record one number (NCERT Physics Lab Manual Class 12, Part 11, page 150).
In NEET this arrives as a short table of readings with a "which component is which" stem, occasionally with a tolerance check attached. Light arithmetic, heavy on reading the evidence correctly.
Watch-out: discharge a capacitor before testing it, and never conclude from one lead orientation.
Can you answer these Experiment 18 Component Identification MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
Which component in a mixed collection gives the same finite resistance reading whichever way round its two leads are connected to the multimeter?
Show answer and why every option is right or wrong
Answer: C. C is correct: a resistor is a non-polar component, so reversing the leads leaves the reading unchanged at its rated value (NCERT Physics Lab Manual Class 12, Part 11, page 150).
Why A is wrong: A is wrong: a junction diode conducts in one direction only, so the two readings differ sharply.
Why B is wrong: B is wrong: an LED is also a junction device and conducts one way only.
Why D is wrong: D is wrong: a capacitor's reading is not steady at all, since it climbs towards infinity as the capacitor charges.
MCQ 2Easy RecallPractice
On the resistance range, a component reads low at the instant of contact and then climbs steadily towards infinity, and it does the same when the leads are reversed. The component is:
Show answer and why every option is right or wrong
Answer: A. A is correct: the meter's internal cell charges the capacitor, so the charging current falls with time and the indicated resistance rises to infinity in either lead orientation (NCERT Physics Lab Manual Class 12, Part 11, page 150).
Why B is wrong: B is wrong: a resistor holds a steady finite reading and does not drift.
Why C is wrong: C is wrong: a forward-biased diode settles at a low reading and stays there, rather than climbing to infinity.
Why D is wrong: D is wrong: a reverse-biased diode reads very high from the first instant, with no low starting value.
MCQ 3Easy RecallPractice
Before any meter is used, an LED can be told apart from an ordinary junction diode in a mixed tray by:
Show answer and why every option is right or wrong
Answer: D. D is correct: the light must escape, so an LED is packaged in a clear or tinted dome, and the longer lead marks the anode (NCERT Physics Lab Manual Class 12, Part 11, page 150).
Why A is wrong: A is wrong: the opaque body with a cathode band describes the ordinary junction diode, not the LED.
Why B is wrong: B is wrong: equal leads in a metal can describe neither device, and the LED's leads are deliberately unequal.
Why C is wrong: C is wrong: both are two-terminal devices, so a third terminal would indicate something else entirely.
MCQ 4Direct ApplicationPractice
A student touches the meter leads to an unmarked component and reads 4.68 × 10² ohm. On reversing the leads the reading is 4.68 × 10² ohm again, and it is steady. The component is:
Show answer and why every option is right or wrong
Answer: B. B is correct: equal and steady readings in both orientations is the resistor signature, since the component has no preferred direction (NCERT Physics Lab Manual Class 12, Part 11, page 150).
Why A is wrong: A is wrong: a diode would show an overload in one of the two orientations.
Why C is wrong: C is wrong: an LED, being a junction device, would also conduct in one direction only.
Why D is wrong: D is wrong: a capacitor's reading would drift upward rather than sit steady at one value.
MCQ 5Direct ApplicationPractice
An unmarked component has an opaque black body with a band at one end. It gives a low, steady reading with the leads one way and an overload indication with the leads reversed, and it emits no light in either case. The component is:
Show answer and why every option is right or wrong
Answer: D. D is correct: steady one-way conduction, with an opaque banded body and no emission, identifies the ordinary junction diode, the band marking its cathode (NCERT Physics Lab Manual Class 12, Part 11, page 150).
Why A is wrong: A is wrong: a resistor reads the same both ways instead of overloading in one orientation.
Why B is wrong: B is wrong: a capacitor climbs to an overload in both orientations, not in one only.
Why C is wrong: C is wrong: an LED is housed in a clear or tinted dome and glows when its junction is forward biased.
MCQ 6Direct ApplicationPractice
On the diode-test range a component shows about 1.9 V with one lead orientation and an overload indication when reversed, and a faint glow is visible from its body during the conducting test. The component is:
Show answer and why every option is right or wrong
Answer: A. A is correct: visible emission together with a forward drop well above the silicon diode's value identifies the LED (NCERT Physics Lab Manual Class 12, Part 11, page 150).
Why B is wrong: B is wrong: an ordinary silicon diode emits no visible light and shows a distinctly smaller forward drop.
Why C is wrong: C is wrong: a resistor conducts both ways and would never overload in one orientation.
Why D is wrong: D is wrong: a capacitor neither emits light nor holds a fixed forward reading.
MCQ 7Concept TrapPractice
A good capacitor and a component with an internal break both settle at an infinite resistance reading. What separates them during the test?
Show answer and why every option is right or wrong
Answer: B. B is correct: the distinguishing evidence is the transient rather than the final value, because the charging current gives the capacitor a low opening reading that a break can never show (NCERT Physics Lab Manual Class 12, Part 11, page 150).
Why A is wrong: A is wrong: it reverses the two behaviours, since the low opening reading belongs to the capacitor.
Why C is wrong: C is wrong: a capacitor does not hold a zero reading, because the reading rises as charge accumulates.
Why D is wrong: D is wrong: a break does not conduct in either orientation.
MCQ 8CalculationPractice
Four unmarked two-terminal components are tested on the same multimeter. P reads 1.0 × 10³ ohm steadily in both orientations. Q reads low at first and climbs to an overload in both orientations. R has an opaque banded body and shows 0.6 V on the diode-test range one way, an overload the other. S has a clear dome and shows 1.9 V one way, an overload the other. In the order P, Q, R, S the components are:
Show answer and why every option is right or wrong
Answer: C. C is correct: the steady two-way reading makes P a resistor, the climbing two-way reading makes Q a capacitor, the opaque banded one-way device with the smaller forward drop is the junction diode, and the clear-domed one-way device with the larger drop is the LED (NCERT Physics Lab Manual Class 12, Part 11, page 150).
Why A is wrong: A is wrong: it swaps P and Q, but a capacitor cannot hold a steady finite reading and a resistor cannot climb to an overload.
Why B is wrong: B is wrong: it swaps R and S, but the larger forward drop and the clear dome belong to the LED.
Why D is wrong: D is wrong: P conducts equally both ways so it cannot be a diode, and R conducts one way only so it cannot be a resistor.
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How do you solve a Experiment 18 Component Identification question? A worked example
- 1
Given
An unmarked two-terminal component from the mixed tray. The multimeter resistance range resolves 1 ohm. The reading is 4.68 × 10² ohm with the leads one way and 4.68 × 10² ohm with them reversed, steady in both cases. A colour code on the body claims 4.7 × 10² ohm with a tolerance of 5% (exact, as a stated tolerance).
- 2
Required
Identify the component, and decide whether the measured value is consistent with the claimed marking.
- 3
Concept
Identification comes first from the pattern of readings, not from their size: equal, steady, finite readings in both lead orientations mean a non-polar conducting element, that is, a resistor. Only after identification does the numerical check mean anything.
- 4
Relation used
The deviation of the measured value from the marked value, expressed as a percentage of the marked value, compared against the stated tolerance band. No dossier formula is required for this step.
- 5
Substitution
Deviation = 4.70 × 10² ohm − 4.68 × 10² ohm = 2 ohm. As a fraction of the marked value: 2 / (4.70 × 10²).
- 6
Calculation
2 / 470 = 4.3 × 10⁻³, which is 0.43%. The factor 100 used to convert a fraction into a percentage and the 5% tolerance figure are exact values and contribute nothing to the significant-figure count; only the measured 4.68 × 10² ohm and the marked 4.7 × 10² ohm do.
- 7
Final answer
The component is a resistor, measured at 4.68 × 10² ohm, deviating 0.4% from its marked value — comfortably inside the 5% band, so the marking is consistent.
- 8
Common trap
Reading only one lead orientation. A single 4.68 × 10² ohm reading is equally consistent with a forward-biased diode until the leads are reversed and the same value appears again. Identification here rests on the pair of readings, and on whether a reading drifts.
- 9
Similar NEET-style question
A component from the same tray reads 6 ohm at the instant of contact, 90 ohm two seconds later, and an overload after five seconds, with the same behaviour when the leads are reversed. Name the component and state the single observation that rules out a forward-biased diode.
What to remember before solving Experiment 18 Component Identification questions
More in Experimental Skills: 6 exam traps and mistakes · 3 formulas · 1 question pattern from its other lessons.
Experiment 18 Component Identification questions from past NEET papers
No question in our NEET 2020–2025 set targets this topic directly.
Sources
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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