Law
Ohm's law
Current through a conductor is directly proportional to potential difference: V = I R, where R is resistance. Holds for ohmic conductors at constant temperature.
-- NCERT Class 12 Physics, Ch. 3, p. 83Ohm's Law
8 MCQs1 revision card9-step worked example
Source: NCERT Current ElectricityOfficial key: NTA-verifiedLast updated: 21 Sep 2026
Ohm's Law, explained for NEET
The trap: writing V = IR and calling it "Ohm's law." NCERT Class 12 Physics Part I, Chapter 3 (Points to Ponder, page 105) is explicit: the relation V = IR is not a statement of Ohm's law. It defines resistance, and it can be written for any device. Ohm's law is the stronger claim that the current–voltage relation is linear — R stays the same whatever V you apply.
The law. For a conductor obeying Ohm's law, the potential difference V across it is proportional to the current I through it:
V = R I (NCERT Chapter 3, Eq. 3.3, page 83)
R is the resistance; its SI unit is the ohm, 1 Ω = 1 V/A. The law applies to an ohmic conductor held at constant temperature.
How to tell ohmic from non-ohmic. Compute V/I at several points. If the ratio is the same everywhere (the I–V graph is a straight line through the origin), the device obeys Ohm's law. If the ratio changes, V = IR still gives a resistance at that point, but the device does not obey Ohm's law.
Limitations (NCERT Section 3.6, page 89). Ohm's law is not a fundamental law of nature. It fails when:
- V stops being proportional to I;
- the V–I relation depends on the sign of V (as in a diode);
- the V–I relation is not unique — more than one value of V for the same current (NCERT's example is GaAs).
Bridge to NEET. The basic pattern gives two of V, I, R and asks for the third. The mistakes that produce distractors are arithmetic, not physics: dividing where you should multiply (I/R or R/I instead of IR), or inverting V/I. A related slip is assuming the current stays fixed when the voltage across a fixed ohmic resistor is changed.
Watch out: on an I–V graph (I on the vertical axis) the slope is 1/R, not R. A steeper line means a smaller resistance.
Can you answer these Ohm's Law 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
According to NCERT's Points to Ponder for Current Electricity, which statement correctly separates the relation V = IR from Ohm's law?
Show answer and why every option is right or wrong
Answer: D. NCERT Class 12 Physics Part I, Chapter 3, Points to Ponder (page 105): V = IR is a definition of resistance valid for all devices; Ohm's law is the claim that the I–V relation is linear, i.e. R does not depend on V.
Why A is wrong: A is wrong because it is exactly the trap this lesson names: V/I can be computed for any device, including non-ohmic ones, so being able to write V = IR does not mean Ohm's law is obeyed.
Why B is wrong: B is wrong because Ohm's law is a linear relation, V proportional to I, not to I squared.
Why C is wrong: C is wrong because Ohm's law says R is constant for an ohmic conductor at constant temperature; it does not make R grow with current.
MCQ 2Easy RecallPractice
Which of the following is one of the limitations of Ohm's law listed in NCERT Section 3.6?
Show answer and why every option is right or wrong
Answer: A. NCERT Class 12 Physics Part I, Chapter 3, Section 3.6 (page 89) lists sign-dependence of the V–I relation (diode) as a departure from Ohm's law, along with non-linearity and non-unique V–I relations (GaAs).
Why B is wrong: B is wrong because an ohmic conductor at constant temperature has the same V/I at every voltage; that constancy is what Ohm's law asserts, so this describes the law, not a limitation.
Why C is wrong: C is wrong because NCERT states Ohm's law is not a fundamental law of nature; diodes and GaAs are given as materials or devices that do not obey it.
Why D is wrong: D is wrong because NCERT's Points to Ponder say V = IR defines resistance for all devices, ohmic or not.
MCQ 3Easy RecallPYQ Pattern
From V = IR, the SI unit of resistance (the ohm) is equivalent to:
Show answer and why every option is right or wrong
Answer: A. Rearranging V = IR gives R = V/I, so 1 Ω = 1 V/A (NCERT Class 12 Physics Part I, Chapter 3, page 83).
Why B is wrong: B is wrong because it inverts the ratio (I/V instead of V/I); A/V is the unit of 1/R, not of R.
Why C is wrong: C is wrong because it multiplies volt by ampere instead of dividing; this is the multiply-instead-of-divide slip in rearranging V = IR.
Why D is wrong: D is wrong because R = V/I has current to the first power; squaring the ampere has no basis in V = IR.
MCQ 4Direct ApplicationPYQ Pattern
An ohmic resistor of 12 Ω carries a steady current of 0.50 A. What is the potential difference across it?
Show answer and why every option is right or wrong
Answer: B. V = IR = 0.50 A × 12 Ω = 6.0 V (Ohm's law, NCERT Class 12 Physics Part I, Chapter 3, Eq. 3.3, page 83).
Why A is wrong: A is wrong because 24 V comes from R/I = 12/0.50, dividing resistance by current instead of multiplying.
Why C is wrong: C is wrong because 0.042 V comes from I/R = 0.50/12, dividing current by resistance instead of multiplying.
Why D is wrong: D is wrong because 12.5 comes from adding R and I, which have different units and cannot be added.
MCQ 5Direct ApplicationPYQ Pattern
A steady current of 1.5 A flows through an ohmic conductor when a potential difference of 9.0 V is applied across it. What is its resistance?
Show answer and why every option is right or wrong
Answer: B. R = V/I = 9.0 V / 1.5 A = 6.0 Ω (NCERT Class 12 Physics Part I, Chapter 3, Eq. 3.3, page 83).
Why A is wrong: A is wrong because 13.5 comes from V × I = 9.0 × 1.5; multiplying V and I gives a product in watts, not a resistance.
Why C is wrong: C is wrong because 0.17 comes from I/V = 1.5/9.0, inverting the ratio; that is 1/R, not R.
Why D is wrong: D is wrong because 7.5 comes from V − I = 9.0 − 1.5, subtracting quantities with different units.
MCQ 6Direct ApplicationPYQ Pattern
An ohmic conductor at constant temperature carries 0.60 A when 3.0 V is applied across it. If the applied potential difference is raised to 7.5 V, the current becomes:
Show answer and why every option is right or wrong
Answer: D. R = 3.0/0.60 = 5.0 Ω and stays constant for an ohmic conductor at constant temperature, so I = 7.5 V / 5.0 Ω = 1.5 A (equivalently, I is proportional to V: 0.60 × 7.5/3.0 = 1.5 A).
Why A is wrong: A is wrong because it assumes the current stays fixed when the voltage changes; for a fixed ohmic resistance, I rises in proportion to V.
Why B is wrong: B is wrong because 0.24 A = 0.60 × 3.0/7.5 treats I as inversely proportional to V; Ohm's law makes I directly proportional to V.
Why C is wrong: C is wrong because 37.5 comes from V × R = 7.5 × 5.0, multiplying where V must be divided by R.
MCQ 7CalculationPractice
A device gives these readings: 1.0 V → 0.10 A; 2.0 V → 0.40 A; 3.0 V → 0.90 A. Which conclusion is correct?
Show answer and why every option is right or wrong
Answer: C. V/I = 1.0/0.10 = 10 Ω, 2.0/0.40 = 5.0 Ω, 3.0/0.90 ≈ 3.3 Ω. The ratio changes, so the relation is non-linear and the device is non-ohmic, even though V = IR defines a resistance at each point (NCERT Chapter 3, Points to Ponder, page 105; Section 3.6, page 89).
Why A is wrong: A is wrong because being able to compute V/I at each reading is true for any device; Ohm's law needs that ratio to be the same at every reading, and here it is not.
Why B is wrong: B is wrong because 10 Ω is taken from the first reading only; the second and third readings give 5.0 Ω and about 3.3 Ω.
Why D is wrong: D is wrong because 0.10, 0.20 and 0.30 are I/V values (the inverted ratio), not resistances; the actual V/I values fall rather than rise.
MCQ 8CalculationPractice
The I–V graphs of two ohmic conductors, with current I on the vertical axis and potential difference V on the horizontal axis, are straight lines through the origin. Conductor 1's line makes 60° with the V-axis; conductor 2's line makes 30° with the V-axis. The ratio R₁ : R₂ is:
Show answer and why every option is right or wrong
Answer: C. On an I–V graph, slope = I/V = 1/R, so R = 1/tan θ. R₁ = 1/tan 60° = 1/√3 and R₂ = 1/tan 30° = √3, giving R₁ : R₂ = (1/√3) : √3 = 1 : 3. The steeper line has the smaller resistance.
Why A is wrong: A is wrong because 3 : 1 comes from treating the slope of the I–V graph as R; with I on the vertical axis the slope is 1/R, so the ratio inverts.
Why B is wrong: B is wrong because both conductors being ohmic only means each has a constant resistance; it does not make the two resistances equal.
Why D is wrong: D is wrong because 1 : 2 comes from using the angles themselves (30/60) instead of the tangents of the angles.
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Ohm's Law: quick recall before you leave
How do you solve a Ohm's Law question? A worked example
- 1
Given
A potential difference of 4.5 V across an ohmic resistor at constant temperature drives a steady current of 0.30 A. The potential difference is then raised to 12 V, with temperature unchanged.
- 2
Required
(a) The resistance R. (b) The new current at 12 V.
- 3
Concept
For an ohmic conductor at constant temperature, R does not depend on V. Find R from the first reading, then use the same R at the new voltage.
- 4
Formula
V = I R, so R = V/I and I = V/R.
- 5
Substitution
(a) R = 4.5 V / 0.30 A
(b) I = 12 V / R - 6
Calculation
(a) R = 4.5 / 0.30 = 15 Ω
(b) I = 12 / 15 = 0.80 A
No exact constants (counting numbers or mathematical constants) are used here; all values are measured quantities with two significant figures, so both answers are reported to two significant figures. - 7
Final answer
R = 15 Ω; current at 12 V = 0.80 A
- 8
Common trap
Keeping the current at 0.30 A after the voltage change, or computing I/V = 0.067 and calling it the resistance. For a fixed ohmic resistance, I scales with V: 0.30 × (12/4.5) = 0.80 A, which agrees with the answer above.
- 9
Similar NEET-style question
"A potential difference of 2.4 V across an ohmic conductor drives a current of 0.40 A. Find its resistance."
Strategy: R = V/I = 2.4/0.40 = 6.0 Ω. Do not multiply (0.96) or invert (0.17).
---
What to remember before solving Ohm's Law questions
Which Ohm's Law formulas do you need for NEET?
1 formula — click to collapse
Ohm's law
Linear ohmic conductor: voltage = current x resistance at constant T.
| Symbol | Quantity | SI Unit |
|---|---|---|
| V | voltage | V |
| I | current | A |
| R | resistance | Ω |
Valid when
- Ohmic conductor
- Constant temperature
More in Current Electricity: 3 exam traps and mistakes · 6 formulas · 5 question patterns from its other lessons.
Ohm's Law questions from past NEET papers
No question in our NEET 2020–2025 set targets this topic directly.
How does NEET ask about Ohm's Law?
1 recurring pattern from past papers — click to collapse
Simple Ohm's law application: given two of V, I, R find the third.
Direct ApplicationEasy
Common distractors
misuses power formula
Uses P = VI as if I = P/R
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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