Current Electricity — Class 12 MCQs with Answers

Current Electricity — important questions & MCQs with answers (Class 12 Physics)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   The SI unit of resistivity (ρ) is:

   AohmBohm/metreCohm-metreDsiemens
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   Answer: ohm-metre

   Explanation: Resistivity is defined by R = ρL/A, so ρ = RA/L carries units Ω·m² ÷ m = Ω·m. A common slip is picking plain ohm, confusing resistance (one wire's property) with resistivity, a material constant.

2. Q2 of 6Easy

   Kirchhoff's current law (KCL) is based on conservation of:

   AEnergyBChargeCMassDMomentum
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   Answer: Charge

   Explanation: Kirchhoff's current law says ΣI_in = ΣI_out at any junction — charge can't pile up or vanish there, so it's a statement of charge conservation, not energy or momentum (that's KVL's job).

3. Q3 of 6Medium

   A cell of emf ε and internal resistance r drives current I through external resistance R. The terminal voltage is:

   AV = εBV = ε + IrCV = IR + IrDV = ε − Ir
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   Answer: V = ε − Ir

   Explanation: As current flows out of a cell, some EMF is used up driving charge through its own internal resistance, so terminal voltage V = ε − Ir is always less than ε on discharge — V = ε + Ir only applies while charging.

4. Q4 of 6Medium

   Assertion (A): Free electrons in a metal move with a steady average drift velocity, even though the applied electric field exerts a constant force on each of them. Reason (R): An electron gains velocity from the field only until its next collision with an ion, which wipes out this gain, so on average the gain builds up only for the relaxation time τ.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true and R is the correct explanation of A

   Explanation: A is true: the drift velocity vd = −eEτ/m does not change with time. R is true and explains it: an electron speeds up only until its next collision, so averaging over all electrons, whose last collisions were on average a time τ ago, gives a constant drift velocity of size eEτ/m.

5. Q5 of 6Hard

   In the Wheatstone bridge shown, the bridge is balanced (no current through G) when:

   

   AP + Q = R + SBP × Q = R × SCP − R = Q − SDP/Q = R/S
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   Answer: P/Q = R/S

   Explanation: Balance means zero galvanometer current, which by Kirchhoff's laws forces the ratio condition P/Q = R/S — not a sum or product relation, which is why those distractors look tempting but aren't dimensionally the balance rule.

6. Q6 of 6Hard

   Assertion (A): Two cells of emf 2.0 V and 4.0 V, each of internal resistance 2.0 Ω, are joined in parallel with like terminals together. The combination is equivalent to a single cell of emf 3.0 V and internal resistance 1.0 Ω. Reason (R): For cells in parallel, 1/r_eq = 1/r₁ + 1/r₂ and ε_eq/r_eq = ε₁/r₁ + ε₂/r₂.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true and R is the correct explanation of A

   Explanation: R is true; these are the rules for cells in parallel. Using them, 1/r_eq = 1/2.0 + 1/2.0 gives r_eq = 1.0 Ω, and ε_eq = 1.0 × (2.0/2.0 + 4.0/2.0) = 3.0 V. So A is true and follows directly from R.

Current Electricity notes: key concepts (Class 12 Physics)

Charge in motion through conductors: current and drift velocity, Ohm's law with resistance and resistivity and their temperature dependence, series and parallel networks, EMF and internal resistance, Kirchhoff's two rules, the Wheatstone bridge with the metre bridge and potentiometer, and electrical power and Joule heating.

Electric current

The rate of flow of charge through a cross-section. Conventional current is taken along the flow of positive charge, opposite to the actual electron drift.

Drift velocity

The slow net velocity electrons acquire in a field, of order 10⁻⁴ m s⁻¹ — far slower than the near-light-speed at which the electric signal itself travels.

Current and drift velocity

Related by I = neAv_d, which is what links the macroscopic current to the microscopic motion of the charge carriers.

Mobility

The drift velocity acquired per unit electric field. It is higher for electrons than for holes, which is why n-type devices tend to be faster.

Ohm's law

Current is proportional to potential difference at constant temperature. It is an empirical rule, not a universal law — diodes and semiconductors disobey it.

Resistance and resistivity

Resistance depends on the sample's dimensions, while resistivity is the material property independent of shape and size.

Temperature dependence

Resistivity of a metal RISES with temperature as lattice vibrations scatter electrons; for a semiconductor it FALLS, since more carriers are freed.

Series and parallel circuits

In a series circuit the resistances add and the same current flows through each; in a parallel circuit the reciprocals add and each resistor has the same voltage, the total always being less than the smallest resistor.

EMF and terminal voltage

EMF is the full work per unit charge done by the cell; terminal voltage is what is left after the drop across the internal resistance, so it is lower while current flows.

Internal resistance

The resistance of the cell's own electrolyte. An ideal cell would have zero, which is why the terminal voltage of a fresh cell barely dips under load.

Cells in series and parallel

Series connection adds EMFs and internal resistances, giving higher voltage; parallel connection keeps the EMF and lowers internal resistance, giving higher current.

Kirchhoff's junction rule

The total current entering a junction equals the total leaving it — a direct statement of conservation of charge.

Kirchhoff's loop rule

The algebraic sum of potential changes around any closed loop is zero — a direct statement of conservation of energy.

Wheatstone bridge

Four resistors in a loop with a galvanometer across the middle. At balance no current flows through the galvanometer and the ratios of the arms are equal.

Metre bridge and potentiometer

Practical forms of the balance principle. A potentiometer measures EMF without drawing current, so it beats a voltmeter, which always loads the cell.

Power and Joule heating

Electrical power is VI, and in a resistor it appears as heat at the rate I²R — the basis of every heater, fuse and filament lamp.