CRAC Class C (2025) — Questions 971–980

Large electrolytics have high ESR/ESL and work well at low frequencies but degrade at RF; small ceramics/foils stay effective at high frequencies. Paralleling them lets the combination bypass/filter across a wide band (A). B is a minor side benefit, C is not the purpose (parallel value isn't more "accurate"), and D is false — parallel capacitors do NOT add voltage rating.

A 1000 µF capacitor across a 13.8 V DC supply is a normal filtering/bypass situation (DC sees it as an open, harmless). Across an HF antenna socket (B) it would short out RF signals; across an 8 Ω speaker (C) it would block audio and could stress the amplifier; across 220 V AC (D) a 1000 µF cap draws huge reactive current and risks explosion. Only A is safe and benign.

Both an 电感器 (inductor) and a 电抗器 (reactor) are fundamentally a coil of wire that stores energy in a magnetic field. A capacitor is made of plates/dielectric, and a resistor is made of resistive material — neither is a coil, so C and D are excluded.

Intentionally used RF inductors are 空芯电感 (air-core) and 磁芯 电感 (magnetic/ferrite-core). 分布电感 (distributed inductance) and 寄生电感 (parasitic inductance) are unintended, unwanted effects of layout/wiring, not components you choose to use, so C and D are excluded.

Q = ωL / R_loss (quality factor, 品质因数). A low Q means higher loss resistance, so power is wasted as heat and the inductor degrades the selectivity/bandwidth and gain of the surrounding circuit (A). B is wrong (it affects more than just power); C is wrong (the LC tank Q is strongly influenced by component Q); D is wrong (Q depends on frequency, core, wire, etc., not just L).

A magnetic core raises permeability, so far fewer turns (and thus less copper wire) are needed for the same inductance; less wire means lower DC/AC resistance and lower I²R loss (A). The core does not "reflect energy" (C) nor primarily fix skin effect (D); B's heat-sinking claim is not the real reason.

At HF, a 100 µH inductor presents a high reactance (X_L = 2πfL, e.g. ≈3.8 kΩ at 6 MHz) so across an antenna socket it behaves nearly as an open and does little harm (A). Across 13.8 V DC (C) the coil is a near-short, causing a large DC current and possibly burning out; across 8 Ω speaker (D) it shorts the audio; across 220 V AC (B) it draws large current and risks fire. Only A is benign.

A transformer winding can step voltage up via its turns ratio and, like any coil, produce an inductive kick; a relay coil (衔铁线 圈) generates a high back-EMF when current is interrupted — both can deliver a shock well above 4.2 V. A capacitor only stores the 4.2 V it was charged with (C), and a diode does not generate voltage (D). So A and B are the culprits.

A 按钮开关 (pushbutton) is a mechanical switch; a 继电器 (relay) is an electrically controlled switch; a 晶体管 (transistor) acts as a solid-state switch. A 电位器 (potentiometer) is a variable resistor used for setting levels, not an on/off switch, so D is excluded.

SPDT (单刀双掷) has one movable contact (the pole) that can connect to either of two fixed contacts, i.e., it selects one of two circuits for a single line (A). B describes an SPST (on/off), C is vague, and D describes a DPDT, so they are wrong.