CRAC Class A (2025) — Questions 591–600

电流 (current) is charge flow per unit time — here described as electrons passing per unit time (A). B is not a standard quantity; C lacks the "per unit time" rate; D is drift speed, not current.

电阻 (resistance) is a circuit's opposition to current flow between two points (A). B and C describe energy (that is power dissipation, not resistance itself); D describes a time, which resistance is not.

功率 (power) is work (energy) per unit time — here, the work done by current per unit time (A). B and C are energy (not rate); D is not a quantity of power.

Ohm's law: I = U/R, so current is proportional to voltage and inversely proportional to resistance (A); equivalently U = I·R, so voltage is proportional to both current and resistance (B). C wrongly makes current proportional to R; D wrongly makes voltage inversely proportional to R.

Every real current encounters resistance (including RF, audio, and DC), so there is no kind of current free of resistive opposition (A). B/C/D all do experience resistance.

阻抗 (impedance) is the general AC opposition a circuit presents between two points — combining resistance and reactance (A). B describes a capacitor-like behavior, C an inductor-like behavior, D a filter behavior; none is the general definition.

电动势 (EMF) is measured in volts (伏/特) (A) — it is a voltage- like quantity. Ampere is current, watt is power, ohm is resistance.

电压 (voltage) is measured in volts (伏/特) (A).

电流 (current) is measured in amperes (安/培) (A).

电阻 (resistance) is measured in ohms (欧/姆) (A).