Which one of the following statements regarding Ohm's law is not correct?
- (a)Ohm's law is an assumption that current through a conductor is always directly proportional to the potential difference applied to it.
- (b)A conducting device obeys Ohm's law when the resistance of a device is independent of magnitude and polarity of applied potential difference.
- (c)A conducting material obeys Ohm's law when the resistance of material is independent of the magnitude and direction of applied electric field.
- (d)All homogeneous materials obey Ohm's law irrespective of whether the field is within range or strong.
Correct — D. Ohm's law is not a universal law of nature: many materials (semiconductors, diodes, electrolytes, gases) are non-ohmic, and even ohmic conductors depart from linearity when the applied field is very strong or the temperature changes. So the claim that all homogeneous materials obey Ohm's law regardless of whether the field is weak or strong is false — that is the statement asked for. Options (a), (b) and (c) correctly state Ohm's law and the condition (constant resistance) under which a device obeys it.
- (a)Ohm's law is an assumption that current through a conductor is always directly proportional to the potential difference applied to it. — This correctly frames Ohm's law as the empirical proportionality of current to applied potential difference — a true statement, so it is not the one asked for.
- (b)A conducting device obeys Ohm's law when the resistance of a device is independent of magnitude and polarity of applied potential difference. — This gives the right condition for a device to be ohmic — its resistance stays constant regardless of the size or polarity of the applied voltage; a true statement.
- (c)A conducting material obeys Ohm's law when the resistance of material is independent of the magnitude and direction of applied electric field. — This is the same condition expressed in field terms — resistance independent of the magnitude and direction of the applied field; true, so not the answer.
Ohm's law (V = IR) is an empirical relation that holds only for ohmic conductors and only over a limited range. A material is ohmic just when its resistance stays constant no matter the magnitude or direction of the applied voltage or field; it is not obeyed by all materials nor at every field strength.
The false statement is the one that overreaches — claiming every homogeneous material obeys the law even in strong fields. Genuine physics restricts the law to constant-resistance conductors within a working range; diodes, transistors and gases break it outright.
- Ohm's law: V = IR, giving a straight-line current–voltage graph through the origin for an ohmic conductor.
- A conductor is ohmic only if its resistance is constant with voltage magnitude, polarity and direction.
- Non-ohmic examples include diodes, transistors, filament lamps, electrolytes and gases.
- Strong fields or temperature changes make even ohmic conductors deviate from the law.
Ohm's law holds only where resistance is constant — not for all materials or all field strengths.
- Treating Ohm's law as a universal law rather than an empirical rule with a limited range.
- Forgetting that resistance itself changes with temperature and strong fields.
Asked as 'which statement about Ohm's law is not correct' — the false option is usually the one that claims universal validity.
Two wires have their lengths, diameters and resistivities, all in the ratio of 1 : 2. If the resistance of the thinner wire is 10 ohms, the resistance of the thicker wire is
- (a) 10 ohms
- (b) 5 ohms
- (c) 20 ohms
- (d) 40 ohms
Answer(a) 10 ohms
Same concept — resistance and the relation R = ρL/A that underlies Ohm's law. UPSC tests how resistance depends on length, area and resistivity; this NDA item tests when the V = IR relation itself applies.
If three resistors of 1 Ohm each connect in parallel to each other the resultant resistance is
- (a) 1 Ohm
- (b) 1⁄3 Ohm
- (c) 3 Ohm
- (d) 9 Ohm
Answer(b) 1⁄3 Ohm
Same area — resistance in a circuit governed by Ohm's law. That NDA item tests combining resistors in parallel; this one tests the validity and conditions of Ohm's law itself.
- practice — not a real PYQ
Which one of the following is a non-ohmic device?
- (a)A metal wire at constant temperature
- (b)A semiconductor diode
- (c)A carbon resistor
- (d)A nichrome coil at fixed temperature
Answer(b) A semiconductor diode — its current–voltage graph is non-linear, so it does not obey Ohm's law.
- practice — not a real PYQ
For an ohmic conductor, the graph of current (I) against potential difference (V) is
- (a)a curve bending towards the V-axis
- (b)a straight line through the origin
- (c)a horizontal line
- (d)a parabola
Answer(b) a straight line through the origin — constant resistance means I ∝ V.