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. 83Electrical Resistance
8 MCQs1 revision card9-step worked example
Source: NCERT Current ElectricityOfficial key: NTA-verifiedLast updated: 21 Sep 2026
Electrical Resistance, explained for NEET
The common confusion: writing V = IR and calling it "Ohm's law." NCERT Class 12 Physics Part I, Chapter 3, Points to Ponder (page 105) is explicit: V = IR on its own is not a statement of Ohm's law. It defines resistance, R = V/I, for any device. Ohm's law is the further claim that the I–V relation is linear — R stays the same whatever V is applied.
Definition. Ohm's law is written V = R I (NCERT Class 12 Physics Part I, Chapter 3, equation 3.3, page 83). The constant R is the resistance of the conductor. From R = V/I, the SI unit is volt per ampere, called the ohm (Ω): 1 Ω = 1 V/A. The law is used for an ohmic conductor at constant temperature.
Where it fails. NCERT Chapter 3, section 3.6 (page 89) lists three kinds of deviation:
- V stops being proportional to I.
- The relation depends on the sign of V — reversing V changes the current's magnitude, as in a diode.
- The relation is not unique — more than one value of V for the same current I (NCERT's example is GaAs).
For such devices you can still compute V/I at one operating point, but that ratio is not a fixed resistance.
Bridge to NEET. The basic pattern is "given two of V, I, R, find the third." Two slips cost marks here:
- Inverting the ratio: writing I = R/V instead of I = V/R. Check the units: V/A gives Ω, but A/V does not.
- Mixing up graph slopes: on a V (y-axis) vs I (x-axis) graph, the slope is R. On an I (y-axis) vs V (x-axis) graph, the slope is 1/R. So the steeper line on an I–V graph has the smaller resistance.
Watch out: for an ohmic resistor at constant temperature, doubling V doubles I. R does not change. If a question keeps temperature fixed and changes V, find R first from the given pair, then reuse it.
Can you answer these Electrical Resistance 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
The SI unit of electrical resistance, the ohm (Ω), is equivalent to:
Show answer and why every option is right or wrong
Answer: C. R = V/I, so 1 Ω = 1 V/A (NCERT Class 12 Physics Part I, Chapter 3, page 83).
Why A is wrong: A is wrong because it multiplies V by I instead of dividing. R = V/I, so the unit is a ratio, not a product.
Why B is wrong: B is wrong because it inverts the ratio (I/V). That is the unit of 1/R, not of resistance.
Why D is wrong: D is wrong because joule per second is the watt, a unit of power. It has nothing to do with the ratio V/I that defines resistance.
MCQ 2Easy RecallPractice
According to NCERT, which statement about the relation V = IR is correct?
Show answer and why every option is right or wrong
Answer: A. NCERT Class 12 Physics Part I, Chapter 3, Points to Ponder (page 105): V = IR defines resistance for all devices, while Ohm's law asserts that I is proportional to V (linear I–V relation).
Why B is wrong: B is wrong because it treats the defining relation V = IR as Ohm's law. Devices such as diodes do not have a fixed R, so they do not obey Ohm's law, even though V/I can still be computed.
Why C is wrong: C is wrong because V = IR is the definition of resistance for any device. It is Ohm's law (linearity) that holds only for certain conductors.
Why D is wrong: D is wrong because Ohm's law says R is constant, so V is proportional to I. A resistance that grows with current would describe a non-linear device.
MCQ 3Easy RecallPractice
Which of the following is listed by NCERT as a deviation from Ohm's law?
Show answer and why every option is right or wrong
Answer: C. NCERT Class 12 Physics Part I, Chapter 3, section 3.6 (page 89) lists a relation that depends on the sign of V (a diode) as one of the ways Ohm's law fails.
Why A is wrong: A is wrong because I proportional to V at constant temperature is exactly what Ohm's law predicts. It is ohmic behaviour, not a deviation.
Why B is wrong: B is wrong because a V/I ratio that does not depend on polarity is ohmic behaviour. The deviation is when reversing V changes the relation.
Why D is wrong: D is wrong because a straight line through the origin is the signature of an ohmic conductor, not a failure of Ohm's law.
MCQ 4Direct ApplicationPractice
A potential difference of 12 V is applied across an ohmic resistor of 4.0 Ω. The current through it is:
Show answer and why every option is right or wrong
Answer: B. I = V/R = 12 V / 4.0 Ω = 3.0 A (Ohm's law, NCERT Class 12 Physics Part I, Chapter 3, page 83).
Why A is wrong: A is wrong because 48 A comes from multiplying V × R (12 × 4.0). Current is V divided by R.
Why C is wrong: C is wrong because 0.33 A comes from the inverted ratio R/V (4.0/12). The units of R/V are not amperes.
Why D is wrong: D is wrong because 8.0 A comes from subtracting R from V (12 − 4.0). Volts and ohms cannot be subtracted.
MCQ 5Direct ApplicationPractice
A current of 0.50 A flows through a resistor when a potential difference of 6.0 V is applied across it. Its resistance is:
Show answer and why every option is right or wrong
Answer: A. R = V/I = 6.0 V / 0.50 A = 12 Ω (NCERT Class 12 Physics Part I, Chapter 3, page 83).
Why B is wrong: B is wrong because 0.083 Ω comes from the inverted ratio I/V (0.50/6.0), which gives 1/R, not R.
Why C is wrong: C is wrong because 3.0 Ω comes from multiplying V × I (6.0 × 0.50). Resistance is V divided by I.
Why D is wrong: D is wrong because 6.5 Ω comes from adding V and I (6.0 + 0.50). Quantities with different units cannot be added.
MCQ 6Direct ApplicationPractice
A current of 2.5 A flows through an ohmic resistor of 8.0 Ω. The potential difference across the resistor is:
Show answer and why every option is right or wrong
Answer: D. V = IR = 2.5 A × 8.0 Ω = 2.0 × 10¹ V (NCERT Class 12 Physics Part I, Chapter 3, page 83).
Why A is wrong: A is wrong because 3.2 V comes from dividing R by I (8.0/2.5). The voltage is the product I × R.
Why B is wrong: B is wrong because 0.31 V comes from dividing I by R (2.5/8.0). The voltage is the product I × R.
Why C is wrong: C is wrong because 10.5 V comes from adding I and R (2.5 + 8.0). Amperes and ohms cannot be added.
MCQ 7CalculationPractice
An ohmic resistor carries a current of 1.5 A when 6.0 V is applied across it. At the same temperature, what current flows when the applied voltage is raised to 8.0 V?
Show answer and why every option is right or wrong
Answer: B. Step 1: R = V/I = 6.0 V / 1.5 A = 4.0 Ω. Step 2: at constant temperature R stays the same, so I = 8.0 V / 4.0 Ω = 2.0 A (NCERT Class 12 Physics Part I, Chapter 3, page 83).
Why A is wrong: A is wrong because it assumes the current stays fixed when the voltage changes. For an ohmic resistor, R is constant, so I rises in proportion to V.
Why C is wrong: C is wrong because 32 A comes from multiplying the new voltage by the resistance (8.0 × 4.0) instead of dividing.
Why D is wrong: D is wrong because 0.50 A comes from the inverted ratio R/V (4.0/8.0), which does not give a current.
MCQ 8Concept TrapPractice
Two ohmic conductors P and Q are kept at the same constant temperature. Their graphs of potential difference V (y-axis) against current I (x-axis) are straight lines through the origin, and line P is steeper than line Q. Which statement is correct?
Show answer and why every option is right or wrong
Answer: D. On a V (y-axis) vs I (x-axis) graph, slope = V/I = R. The steeper line P therefore has the larger resistance (Ohm's law, NCERT Class 12 Physics Part I, Chapter 3, page 83).
Why A is wrong: A is wrong because it reads the graph as if it were I against V, where slope = 1/R. With V on the y-axis, slope = R, so steeper means larger resistance.
Why B is wrong: B is wrong because the relative slopes are enough. Slope = R, so comparing steepness directly compares resistance.
Why C is wrong: C is wrong because passing through the origin only shows each conductor is ohmic. The value of R is the slope, and the slopes differ.
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Electrical Resistance: quick recall before you leave
How do you solve a Electrical Resistance question? A worked example
- 1
Given
A potential difference V = 12 V is applied across an ohmic resistor at constant temperature. The current measured is I = 0.40 A.
- 2
Required
The resistance R of the resistor.
- 3
Concept
For an ohmic conductor, V is proportional to I, and the constant of proportionality is the resistance (Ohm's law, NCERT Class 12 Physics Part I, Chapter 3, page 83).
- 4
Formula
V = IR, rearranged to R = V/I
- 5
Substitution
R = 12 V / 0.40 A
- 6
Calculation
R = 12 / 0.40 = 30, so R = 3.0 × 10¹ Ω (written in scientific notation so the trailing zero is not ambiguous). Unit check: V/A = Ω. Both given values have 2 significant figures, so the answer carries 2 significant figures. No exact constants are used in this calculation, so nothing extra limits the significant-figure count.
- 7
Final answer
R = 3.0 × 10¹ Ω
- 8
Common trap
Inverting the ratio: I/V = 0.40/12 ≈ 0.033 is 1/R (unit A/V), not R. A second slip is multiplying V × I = 4.8, which has units of V·A, not ohms. Check the unit before choosing: V/A = Ω.
- 9
Similar NEET-style question
"A current of 0.25 A flows through an ohmic resistor of 8.0 Ω. Find the potential difference across it."
Strategy: V = IR = 0.25 A × 8.0 Ω = 2.0 V (unit check: A × Ω = A × V/A = V).
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What to remember before solving Electrical Resistance questions
Which Electrical Resistance 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.
Electrical Resistance questions from past NEET papers
No question in our NEET 2020–2025 set targets this topic directly.
How does NEET ask about Electrical Resistance?
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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