Chapter 3: Current Electricity Assertion-Reason Questions | physics Class 12 CBSE

Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 1

Logic Check

Assertion (A): The drift velocity of electrons in a metallic conductor is of the order of millimetres per second, yet large currents can flow in conductors. Reason (R): The current depends on the product of electron number density, charge, drift velocity, and cross-sectional area (I = nevdA), and the electron number density in metals is extremely large (~10²⁸–10²⁹ m⁻³).

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 2

Logic Check

Assertion (A): When a circuit is closed, current is established throughout the circuit almost instantly. Reason (R): Electrons from the negative terminal travel to the positive terminal at nearly the speed of light.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 3

Logic Check

Assertion (A): Ohm's law is a fundamental law of nature applicable to all materials. Reason (R): Materials like GaAs and diodes do not obey Ohm's law, showing it is an empirical rule valid only for certain materials under certain conditions.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 4

Logic Check

Assertion (A): The resistivity of a metallic conductor increases with temperature. Reason (R): As temperature increases, the relaxation time τ of electrons decreases due to more frequent collisions with lattice ions, and since ρ = m/(ne²τ), resistivity increases.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 5

Logic Check

Assertion (A): The resistivity of a semiconductor decreases with increasing temperature. Reason (R): In a semiconductor, increasing temperature significantly increases the free electron density n, which more than compensates for the decrease in relaxation time τ, resulting in lower resistivity.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 6

Logic Check

Assertion (A): EMF of a cell is a force that drives the current through the circuit. Reason (R): EMF is actually a potential difference (measured in volts), representing work done per unit charge by the cell; the name 'electromotive force' is historical and misleading.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 7

Logic Check

Assertion (A): The terminal voltage of a cell is always less than its EMF during current discharge. Reason (R): When current I flows, a voltage drop Ir occurs across the internal resistance r of the cell, making terminal voltage V = ε – Ir < ε.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 8

Logic Check

Assertion (A): Power is transmitted at very high voltages (e.g., 200 kV) over long distances. Reason (R): The power wasted in transmission cables is Pc = P²Rc/V², which is inversely proportional to V²; higher transmission voltage drastically reduces cable losses.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 9

Logic Check

Assertion (A): In a balanced Wheatstone bridge, no current flows through the galvanometer, and the result does not depend on the galvanometer's calibration. Reason (R): At balance, the galvanometer only needs to detect zero current (null condition), making the resistance determination independent of galvanometer sensitivity or scale.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 10

Logic Check

Assertion (A): The paths of electrons between successive collisions are straight lines in the absence of an electric field. Reason (R): With no electric field, no net force acts on electrons between collisions, so they travel in straight lines at constant velocity (Newton's first law).

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 11

Logic Check

Assertion (A): In the presence of an electric field, the paths of electrons between successive collisions are curved. Reason (R): The electric field exerts a constant force (–eE) on each electron, causing constant acceleration and hence a curved (parabolic) trajectory between collisions.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 12

Logic Check

Assertion (A): The drift velocity of conduction electrons in a metal is independent of time in steady state, even though the electric field is continuously accelerating the electrons. Reason (R): The average drift velocity is constant because the acceleration gained between collisions is exactly offset, on average, by the complete randomisation of velocity at each collision, yielding vd = –eEτ/m.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 13

Logic Check

Assertion (A): Resistance of a conductor is directly proportional to its length and inversely proportional to its cross-sectional area. Reason (R): R = ρl/A, where doubling length doubles resistance (two slabs in series carry same current at double voltage) and halving area doubles resistance (half-slab carries half current at same voltage).

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 14

Logic Check

Assertion (A): Nichrome, Manganin, and Constantan are used to make standard wire-wound resistors. Reason (R): These alloys have a very weak (near-zero) temperature dependence of resistivity, so their resistance values remain stable over a wide range of temperatures.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 15

Logic Check

Assertion (A): Kirchhoff's junction rule states that the sum of currents entering a junction equals the sum leaving it. Reason (R): This rule is based on the conservation of energy around a closed loop.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 16

Logic Check

Assertion (A): Kirchhoff's loop rule states that the algebraic sum of potential changes around any closed loop is zero. Reason (R): Electric potential is a single-valued function of position; returning to the starting point along any closed path results in zero net change in potential, reflecting conservation of energy.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 17

Logic Check

Assertion (A): Bending or reorienting a wire does not change the validity of Kirchhoff's junction rule. Reason (R): Kirchhoff's junction rule is based on conservation of charge, which is a topological property of the circuit — it depends on how elements are connected, not on the physical shape of the wires.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 18

Logic Check

Assertion (A): Electric current is a scalar quantity. Reason (R): Currents obey the law of vector addition at a junction.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 19

Logic Check

Assertion (A): The equivalent internal resistance of cells in parallel is less than the internal resistance of any individual cell. Reason (R): For parallel combination, 1/req = 1/r₁ + 1/r₂ + ..., so req is less than the smallest individual internal resistance, allowing higher current without large internal voltage drop.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 20

Logic Check

Assertion (A): A diode does not obey Ohm's law. Reason (R): In a diode, the resistance R = V/I remains constant and independent of the applied voltage V.

Reasoning Options

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Chapter sections

Chapter 3: Current Electricity | Assertion-Reason

Assertion (A): When a circuit is closed, current is established throughout the circuit almost instantly. Reason (R): Electrons from the negative terminal travel to the positive terminal at nearly the speed of light.

Assertion (A): Ohm's law is a fundamental law of nature applicable to all materials. Reason (R): Materials like GaAs and diodes do not obey Ohm's law, showing it is an empirical rule valid only for certain materials under certain conditions.

Assertion (A): The resistivity of a metallic conductor increases with temperature. Reason (R): As temperature increases, the relaxation time τ of electrons decreases due to more frequent collisions with lattice ions, and since ρ = m/(ne²τ), resistivity increases.

Assertion (A): The resistivity of a semiconductor decreases with increasing temperature. Reason (R): In a semiconductor, increasing temperature significantly increases the free electron density n, which more than compensates for the decrease in relaxation time τ, resulting in lower resistivity.

Assertion (A): EMF of a cell is a force that drives the current through the circuit. Reason (R): EMF is actually a potential difference (measured in volts), representing work done per unit charge by the cell; the name 'electromotive force' is historical and misleading.

Assertion (A): The terminal voltage of a cell is always less than its EMF during current discharge. Reason (R): When current I flows, a voltage drop Ir occurs across the internal resistance r of the cell, making terminal voltage V = ε – Ir < ε.

Assertion (A): Power is transmitted at very high voltages (e.g., 200 kV) over long distances. Reason (R): The power wasted in transmission cables is Pc = P²Rc/V², which is inversely proportional to V²; higher transmission voltage drastically reduces cable losses.

Assertion (A): The paths of electrons between successive collisions are straight lines in the absence of an electric field. Reason (R): With no electric field, no net force acts on electrons between collisions, so they travel in straight lines at constant velocity (Newton's first law).

Assertion (A): In the presence of an electric field, the paths of electrons between successive collisions are curved. Reason (R): The electric field exerts a constant force (–eE) on each electron, causing constant acceleration and hence a curved (parabolic) trajectory between collisions.

Assertion (A): Nichrome, Manganin, and Constantan are used to make standard wire-wound resistors. Reason (R): These alloys have a very weak (near-zero) temperature dependence of resistivity, so their resistance values remain stable over a wide range of temperatures.

Assertion (A): Kirchhoff's junction rule states that the sum of currents entering a junction equals the sum leaving it. Reason (R): This rule is based on the conservation of energy around a closed loop.

Assertion (A): Electric current is a scalar quantity. Reason (R): Currents obey the law of vector addition at a junction.

Assertion (A): A diode does not obey Ohm's law. Reason (R): In a diode, the resistance R = V/I remains constant and independent of the applied voltage V.

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