MPSC 2026 Prelims Paper-I · Question 52 of 100

Correct — option (1), "electric current". The stem is the textbook definition of electric current, almost word for word. Current is the rate of flow of electric charge across a surface: if a net charge q crosses a given cross-section in time t, the average current is I = q/t, and the instantaneous current is the derivative I = dq/dt. The word 'net' in the stem is doing real work — in an electrolyte or a plasma, positive and negative carriers move in opposite directions, and the current is the algebraic sum of their contributions, not the count of charges in motion. The SI unit of current is the ampere, and one ampere is one coulomb of charge crossing the surface per second. The ampere is one of the seven SI base units; charge is the derived quantity, so the coulomb is defined as the ampere-second rather than the other way round. Since the 2019 revision of the SI, the ampere is fixed by assigning an exact numerical value to the elementary charge, e = 1.602176634 × 10⁻¹⁹ coulomb. A point worth carrying: current is a scalar even though we speak of its direction. The test of a vector is whether quantities add by the parallelogram law, and currents meeting at a junction do not — they add algebraically, which is exactly what Kirchhoff's junction rule states. The vector quantity in this family is current density, J, defined as current per unit cross-sectional area and directed along the flow. Now look at what makes this question quicker than it appears. Options (2) 'potential difference' and (3) 'voltage difference' are the same physical quantity under two names — voltage is potential difference, and any physics text will use the words interchangeably. Exactly one option can be credited on this paper. If two options are synonyms, then either both would have to be right or neither is, and since both cannot be, neither can. That eliminates half the paper's choices on a purely logical ground, before any physics is applied. Scanning an option set for duplicates costs a few seconds and occasionally hands you a question you did not know.

• (2)potential difference — Potential difference is not a rate of flow of charge; it is the work done per unit charge in carrying a charge between two points, measured in volts, where one volt is one joule per coulomb. It is the cause rather than the flow — the electrical 'pressure' that drives charge through a conductor, related to the current by Ohm's law, V = IR, in the ohmic case. Confusing the two is the most basic error in current electricity, and it is why the examiner has put the quantity here.
• (3)voltage difference — Voltage is simply another word for electric potential, so 'voltage difference' names the same quantity as option (2), potential difference. Two options that say the same thing cannot be distinguished by any answer key, and since exactly one option is credited, neither of them can be the credited one. Beyond that logical point, the physics is the same as for option (2): a potential difference is energy per unit charge in joules per coulomb, not charge per unit time in coulombs per second.
• (4)charge motion — 'Charge motion' is a description, not a defined physical quantity, and it has no unit, no symbol and no defining equation. Charge in motion is what constitutes a current, but the quantity the stem defines is a rate — how much net charge crosses the surface per unit time — and a rate needs a number attached to it. This is the plausible-English-phrase distractor that appears whenever the three real quantities on offer are not enough to fill four slots.

Electric current is the rate at which net charge crosses a surface: I = dq/dt, measured in amperes, where one ampere is one coulomb per second. In a metallic conductor the carriers are free electrons, which in the absence of a field move randomly at high speed with no net transport; applying a potential difference superimposes on that random motion a slow systematic drift, and it is the drift that constitutes the current. The relation is I = nAve, where n is the number of free electrons per unit volume, A the cross-sectional area, v the drift velocity and e the electronic charge. Drift velocities are strikingly small — of the order of a fraction of a millimetre per second in ordinary household wiring — which raises the classic puzzle of why a light comes on the instant the switch is thrown. The answer is that the electric field is established through the circuit at close to the speed of light, so electrons everywhere in the wire begin drifting almost simultaneously; nobody waits for an electron to travel from the switch to the bulb. Conventional current is defined as the direction in which positive charge would flow, which in a metal is opposite to the actual motion of the electrons — a historical convention fixed before the electron was discovered and retained ever since.

This definition anchors the whole of current electricity, and MPSC's general science section draws on that chapter repeatedly. Around it sit Ohm's law, V = IR, and the distinction between resistance, which is a property of a particular specimen, and resistivity, which is a property of the material; the heating effect of current, H = I²Rt, which underlies the fuse, the electric iron and the incandescent lamp; and Kirchhoff's two rules, of which the junction rule is a statement of conservation of charge and the loop rule of conservation of energy. Worth keeping distinct in memory are the units and what each measures: ampere for current, volt for potential difference, ohm for resistance, coulomb for charge, watt for power and joule for energy. Examiners construct general science questions in this area very largely by swapping one of those pairings, so a candidate who has the quantity-unit-symbol table secure can answer a large share of them without any calculation at all.

• Electric current is the net charge crossing a surface per unit time, I = dq/dt. Its SI unit is the ampere, equal to one coulomb per second.
• The ampere is one of the seven SI base units; the coulomb is derived from it as the ampere-second. Since the 2019 revision of the SI the ampere is fixed by defining the elementary charge as exactly 1.602176634 × 10⁻¹⁹ coulomb.
• Current is a scalar quantity — currents at a junction add algebraically, not by the parallelogram law, which is the content of Kirchhoff's junction rule. Current density, current per unit cross-sectional area, is the corresponding vector quantity.
• In a metal the current is carried by drifting free electrons, with I = nAve. Drift velocity is very small, of the order of a fraction of a millimetre per second, yet a circuit responds almost instantly because the electric field is established throughout it at nearly the speed of light.
• Conventional current is taken to flow in the direction positive charge would move, which in a metallic conductor is opposite to the actual drift of the electrons — a convention fixed before the electron was known.

• Confusing current with potential difference. Current is charge per unit time in coulombs per second; potential difference is energy per unit charge in joules per coulomb. They are different quantities with different units.
• Treating current as a vector because it has a direction. Currents add algebraically at a junction rather than by the parallelogram law, so current is a scalar; current density is the vector quantity in this family.
• Forgetting that conventional current runs opposite to electron flow in a metal. Nearly every circuit diagram and sign convention is written in terms of conventional current.
• Missing duplicate options. 'Potential difference' and 'voltage difference' name one quantity twice, and since only one option can be credited, neither of them can be — a logical elimination that costs no physics at all.

General science questions on current electricity come in three predictable shapes. The first is the definition-and-unit pairing, as here: a quantity is described in words and the candidate must name it, or a quantity is named and the candidate must give its unit. The second is a relation: which formula links current, drift velocity and cross-section, or what Ohm's law states and where it fails. The third is application: how a fuse works, why household appliances are wired in parallel, what an earthing wire does, or which effect of current a given device exploits. All three sit at higher secondary level and none requires calculation beyond one substitution. The most efficient preparation is a single memorised table of quantity, symbol, defining relation and SI unit for the ten or so quantities of this chapter, because the examiner's principal falsification device is to attach the right definition to the wrong name.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which of the following is one of the seven base units of the International System of Units (SI) ?

  • (a)coulomb
  • (b)ampere
  • (c)volt
  • (d)ohm

  Answer(b) ampere. The seven SI base units are the metre, kilogram, second, ampere, kelvin, mole and candela. Current is the base electrical quantity and charge is derived from it — the coulomb is defined as the ampere-second — while the volt and the ohm are likewise derived units, expressible in terms of the base units through the definitions of potential difference and resistance.

• practice — not a real PYQ

  In a metallic conductor, the direction of conventional current is :

  • (a)the same as the direction of drift of the electrons
  • (b)opposite to the direction of drift of the electrons
  • (c)perpendicular to the direction of drift of the electrons
  • (d)undefined, since electrons move randomly

  Answer(b) opposite to the direction of drift of the electrons. Conventional current is defined as the direction in which positive charge would move, a convention fixed before the electron was discovered and retained for consistency with the whole body of circuit theory. Since the carriers in a metal are negatively charged electrons, their actual drift runs opposite to the conventional current, and every circuit diagram is drawn on the conventional basis.