The net electric charge that passes through the surface per unit time is called ________.

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (b) 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.  (c) 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.  (d) '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.

Concept: 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.

State PCS 2026 Q52 — Science & Technology PYQ with Answer

State PCS 2026 Q52 — Science & Technology PYQ with Answer