Vi Characteristics

8 MCQs9-step worked example
Source: NCERT Current ElectricityOfficial key: NTA-verifiedLast updated: 7 Oct 2026

Try this first

Which device does NCERT give as an example of a V–I relation that depends on the sign of the applied voltage?
  1. A.A good conductor
  2. B.A diode
  3. C.GaAs
  4. D.A conductor obeying the linear Ohm's law
Tap to see the answer

Answer: B. B is correct: in a diode, reversing the direction of V at the same magnitude does not produce a current of the same magnitude in the opposite direction (NCERT Class 12 Physics, Chapter 3, page 89).

A is wrong: A is wrong because a good conductor is NCERT's example of type (a), where V stops being proportional to I; it is not the sign-dependent case.

C is wrong: C is wrong because GaAs is the example of a non-unique relation, with more than one V for the same I.

D is wrong: D is wrong because a conductor obeying the linear Ohm's law gives the same magnitude of current for either sign of V.

All practice questions for this lesson →

Vi Characteristics, explained for NEET

The question first: a device is tested at 1.0 V, 2.0 V and 3.0 V and draws 0.10 A, 0.30 A and 0.60 A. Is it an ohmic conductor? V ÷ I gives 10 Ω, 6.7 Ω and 5.0 Ω. The ratio is not constant, so the answer is no, and that one check is the whole topic.

The concept. Ohm's law holds over a large class of materials, but NCERT lists devices where the proportionality of V and I does not hold. The deviations are of three types (NCERT Class 12 Physics, Chapter 3, page 89):

  • (a) V ceases to be proportional to I. The V–I curve of a good conductor bends away from the straight line of the linear Ohm's law (Fig. 3.5).
  • (b) The relation between V and I depends on the sign of V. Reversing V with its magnitude fixed does not give the same current in the opposite direction. A diode does this (Fig. 3.6).
  • (c) The relation between V and I is not unique: there is more than one value of V for the same current I. GaAs behaves this way (Fig. 3.7).

NCERT adds that such materials and devices are widely used in electronic circuits (page 89).

Bridge to NEET. An ohmic conductor has a straight V–I line through the origin, so V ÷ I is the same at every point. For a non-ohmic device V ÷ I changes from point to point. Read the table of readings and test the ratio.

Watch-out: type (c) is the one that gets inverted. GaAs gives more than one voltage for the same current, not more than one current for the same voltage. No past-paper item in the corpus is tagged to this subtopic, so this lesson rests on the NCERT page and the NTA syllabus line "V-I characteristics of Ohmic and non-ohmic conductors".

How do you solve a Vi Characteristics question? A worked example

  1. 1

    Given

    Two devices P and Q are tested at three voltages. P: (1.2 V, 0.30 A), (2.4 V, 0.60 A), (3.6 V, 0.90 A). Q: (1.2 V, 0.20 A), (2.4 V, 0.50 A), (3.6 V, 1.00 A).

  2. 2

    Required

    Decide which device is ohmic, give its resistance, and name the kind of deviation shown by the other.

  3. 3

    Concept

    An ohmic conductor has the same V ÷ I at every reading. Where V ÷ I changes, V has ceased to be proportional to I, the first type of deviation in NCERT Class 12 Physics, Chapter 3, page 89. No sign of V changes in these tables, so only that type can be tested here.

  4. 4

    Formula

    R = V ÷ I at each reading (a prerequisite relation; its formula entry belongs to the Ohm's law topic and is not cited as a source here).

  5. 5

    Substitution

    P: 1.2 ÷ 0.30, 2.4 ÷ 0.60, 3.6 ÷ 0.90.
    Q: 1.2 ÷ 0.20, 2.4 ÷ 0.50, 3.6 ÷ 1.00.

  6. 6

    Calculation

    P: 4.0 Ω, 4.0 Ω, 4.0 Ω. Q: 6.0 Ω, 4.8 Ω, 3.6 Ω.
    All numbers here are measured readings; there are no exact constants, so no significant-figure adjustment is needed.

  7. 7

    Final answer

    P is ohmic with R = 4.0 Ω. Q is non-ohmic: V ÷ I falls from 6.0 Ω to 3.6 Ω as the current rises, so V is not proportional to I.

  8. 8

    Common trap

    Seeing that the current rises with voltage in Q and calling it ohmic. Every conductor passes more current at a higher voltage; ohmic means the ratio stays fixed. A second slip is averaging the three ratios of Q to get one "resistance"; a non-ohmic device has no single resistance.

  9. 9

    Similar NEET-style question

    A device carries 0.20 A at 2.0 V and 0.60 A at 4.0 V. Is it ohmic? What is R(4.0 V) ÷ R(2.0 V)? (Answer: R at 2.0 V = 2.0 ÷ 0.20 = 10 Ω; R at 4.0 V = 4.0 ÷ 0.60 = 6.67 Ω ≈ 6.7 Ω. The ratio is not constant, so it is non-ohmic. R(4.0 V) ÷ R(2.0 V) = 6.67 ÷ 10 = 0.67.)

    ---

Can you answer these Vi Characteristics 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 device does NCERT give as an example of a V–I relation that depends on the sign of the applied voltage?

Show answer and why every option is right or wrong

Answer: B. B is correct: in a diode, reversing the direction of V at the same magnitude does not produce a current of the same magnitude in the opposite direction (NCERT Class 12 Physics, Chapter 3, page 89).

Why A is wrong: A is wrong because a good conductor is NCERT's example of type (a), where V stops being proportional to I; it is not the sign-dependent case.

Why C is wrong: C is wrong because GaAs is the example of a non-unique relation, with more than one V for the same I.

Why D is wrong: D is wrong because a conductor obeying the linear Ohm's law gives the same magnitude of current for either sign of V.

Report an error · Every fix is public: corrections log

MCQ 2Easy RecallPractice

NCERT names one material whose V–I relation is not unique, with more than one value of V for the same current I. Which is it?

Show answer and why every option is right or wrong

Answer: A. A is correct: NCERT names GaAs as the material exhibiting a relation between V and I that is not unique (NCERT Class 12 Physics, Chapter 3, page 89).

Why B is wrong: B is wrong because the diode is NCERT's example of a relation that depends on the sign of V, not of a non-unique relation.

Why C is wrong: C is wrong because the good conductor illustrates V ceasing to be proportional to I, while still giving one V for each I.

Why D is wrong: D is wrong because a linear resistor gives exactly one V for every I.

Report an error · Every fix is public: corrections log

MCQ 3Easy RecallPractice

A relation between V and I that is "not unique" (type c) means which of the following?

Show answer and why every option is right or wrong

Answer: C. C is correct: NCERT describes the non-unique relation as having more than one value of V for the same current I (NCERT Class 12 Physics, Chapter 3, page 89).

Why A is wrong: A is wrong because it swaps the roles: it describes several currents for one voltage, which is the inverse of the statement in NCERT.

Why B is wrong: B is wrong because a current that reverses with V is normal behaviour, and the sign-dependent type (b) is about the magnitudes being unequal.

Why D is wrong: D is wrong because proportionality is Ohm's law itself, the case with no deviation.

Report an error · Every fix is public: corrections log

MCQ 4Direct ApplicationPractice

A device gives I = 0.25 A at V = 1.0 V, I = 0.50 A at V = 2.0 V and I = 0.75 A at V = 3.0 V. Which conclusion is correct?

Show answer and why every option is right or wrong

Answer: B. B is correct: V ÷ I = 1.0 ÷ 0.25 = 2.0 ÷ 0.50 = 3.0 ÷ 0.75 = 4.0 Ω at every reading, so V is proportional to I and the device obeys Ohm's law (NCERT Class 12 Physics, Chapter 3, page 89, describes the deviations that are absent here).

Why A is wrong: A is wrong because the current is supposed to change with voltage in any conductor; what matters is whether V ÷ I stays constant, and it does.

Why C is wrong: C is wrong because 0.25 is the current at 1.0 V, i.e. I ÷ V; the resistance is the inverse, V ÷ I = 4.0 Ω.

Why D is wrong: D is wrong because V ÷ I being non-zero says nothing about linearity; the test is whether the ratio is the same at every reading.

Report an error · Every fix is public: corrections log

MCQ 5Direct ApplicationPractice

Readings of a device are (1.0 V, 0.10 A), (2.0 V, 0.30 A) and (3.0 V, 0.60 A). Which conclusion is correct?

Show answer and why every option is right or wrong

Answer: C. C is correct: V ÷ I = 10 Ω, 6.7 Ω, 5.0 Ω, which is not constant, so V is not proportional to I. This is type (a) in NCERT Class 12 Physics, Chapter 3, page 89.

Why A is wrong: A is wrong because 10 Ω is only the first ratio; the other two readings give different values.

Why B is wrong: B is wrong because an average does not rescue the law; an ohmic device has the same ratio at every reading, not a mean of unequal ones.

Why D is wrong: D is wrong because every reading has a positive V, so nothing about the sign of V can be concluded from this table.

Report an error · Every fix is public: corrections log

MCQ 6Direct ApplicationPractice

A diode carries 12 mA at V = +0.70 V but only a few microamperes at V = −0.70 V. Which type of deviation from Ohm's law is this?

Show answer and why every option is right or wrong

Answer: D. D is correct: reversing V at the same magnitude does not give the same magnitude of current, which is NCERT's type (b), the case it attributes to a diode (NCERT Class 12 Physics, Chapter 3, page 89).

Why A is wrong: A is wrong because the currents at +0.70 V and −0.70 V differ by orders of magnitude, so V is not proportional to I.

Why B is wrong: B is wrong because each current here belongs to one voltage; non-uniqueness needs two voltages for one current.

Why C is wrong: C is wrong because this device also treats +V and −V differently, which is the sign dependence and not simply a loss of proportionality.

Report an error · Every fix is public: corrections log

MCQ 7CalculationPractice

An ohmic resistor carries 0.50 A at 2.0 V. A non-ohmic device also carries 0.50 A at 2.0 V but only 0.80 A at 5.0 V. At 5.0 V, what is the ratio of the current in the ohmic resistor to the current in the non-ohmic device?

Show answer and why every option is right or wrong

Answer: A. A is correct: the ohmic resistor has R = 2.0 ÷ 0.50 = 4.0 Ω, so at 5.0 V its current is 5.0 ÷ 4.0 = 1.25 A. The ratio is 1.25 ÷ 0.80 = 1.56, which rounds to 1.6. The ohmic device keeps a constant V ÷ I, the non-ohmic one does not (NCERT Class 12 Physics, Chapter 3, page 89).

Why B is wrong: B is wrong because the two devices agree only at 2.0 V; at 5.0 V the ohmic resistor carries 1.25 A while the other carries 0.80 A.

Why C is wrong: C is wrong because it is the inverse ratio, 0.80 ÷ 1.25, taking the non-ohmic current over the ohmic current.

Why D is wrong: D is wrong because it is the voltage ratio 5.0 ÷ 2.0, which would be the current ratio only if both devices were ohmic.

Report an error · Every fix is public: corrections log

MCQ 8CalculationPractice

A device shows V = 2.0 V at I = 0.40 A and V = 6.0 V at I = 0.60 A. What are the values of V ÷ I at the two readings, and what follows?

Show answer and why every option is right or wrong

Answer: D. D is correct: 2.0 ÷ 0.40 = 5.0 Ω and 6.0 ÷ 0.60 = 10 Ω. The ratio is not constant, so V is not proportional to I (NCERT Class 12 Physics, Chapter 3, page 89).

Why A is wrong: A is wrong because V = IR can always be used to define the resistance at one reading; Ohm's law needs the same R at every reading.

Why B is wrong: B is wrong because 6.0 ÷ 0.60 is 10 Ω, not 15 Ω; the 15 comes from dividing 6.0 by 0.40, mixing readings.

Why C is wrong: C is wrong because 0.20 and 0.10 are I ÷ V (0.40 ÷ 2.0 and 0.60 ÷ 6.0), the inverse of the resistance.

Report an error · Every fix is public: corrections log

Free NEET study resources

Get a structured 30-day Mechanics plan and a complete formula booklet — delivered to your inbox instantly.

What to remember before solving Vi Characteristics questions

3 NCERT lines
Key Fact

Limitations of Ohm’s law: V not proportional to I, depends on sign of V, or not unique (GaAs)

Although Ohm’s law has been found valid over a large class of materials, there do exist materials and devices used in electric circuits where the proportionality of V and I does not hold. The deviations broadly are one or more of the following types: (a) V ceases to be proportional to I (Fig. 3.5). (b) The relation between V and I depends on the sign of V. In other words, if I is the current for a certain V, then reversing the direction of V keeping its magnitude fixed, does not produce a current of the same magnitude as I in the opposite direction (Fig. 3.6). This happens, for example, in a diode which we will study in Chapter 14. (c) The relation between V and I is not unique, i.e., there is more than one value of V for the same current I (Fig. 3.7). A material exhibiting such behaviour is GaAs.

-- NCERT Class 12 Physics, Ch. 3, p. 89
Key Fact

Figure captions: good conductor, diode and GaAs V-I curves

FIGURE 3.5 The dashed line represents the linear Ohm’s law. The solid line is the voltage V versus current I for a good conductor. FIGURE 3.6 Characteristic curve of a diode. Note the different scales for negative and positive values of the voltage and current. FIGURE 3.7 Variation of current versus voltage for GaAs.

-- NCERT Class 12 Physics, Ch. 3, p. 89

Vi Characteristics: NEET previous year questions (PYQs) with answers

20 questions in Current Electricity

No question in our NEET 2020–2025 set targets this topic directly.

All 20 past-paper questions from Current Electricity →

More in Current Electricity: 3 exam traps and mistakes · 7 formulas · 6 question patterns from its other lessons.

Sources

NCERT refs: Class 12 Physics Chapter 3, p.89

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.

Report an error · Every fix is public: corrections log

Every past-paper question from this chapter:

Practice the whole chapter →
Temperature Dependence ResistanceWheatstone Bridge

Prefer to watch this?

  • 8 real NEET Current Electricity questions (2022–2025) on animated circuits: watch the current flow (15 min)

    Watch on YouTube
Practice this topic8 questions