CRAC Class B (2025) — Questions 861–870

U0576. After the RF components in amateur radio equipment accumulate dust or become damp, even without breakdown or leakage, they may unexpectedly produce, due to a change in the insulator's physical properties:

业余无线电设备中的射频部件积灰或受潮后，即使没有击穿或漏电，也可能因绝缘体的物理性质发生改变而意外产生：

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Dust/moisture change an insulator's (dielectric's) properties, causing dielectric loss (介质损耗) — energy dissipated in the dielectric under RF (A). Eddy-current (B) and hysteresis (C) losses belong to magnetic cores; magnetic-reluctance loss (D) is not the effect described.

U0577. A semiconductor is a class of substance or material whose conducting ability lies between that of conductors and insulators. Which of the following descriptions of semiconductors is correct? (Choose all that apply.)

半导体是导电能力介于导体与绝缘体之间的一类物质或材料。对半导体的正确描述是：

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All four are correct: semiconductors' conductivity is controllable (A), they are intrinsic vs impurity (P/N) types (B), the P-N junction has a built-in EMF and rectifying (one-way) behavior (C), and silicon/germanium build transistors and ICs (D).

U0578. Which of the following devices are made of semiconductor material? (Choose all that apply.)

下列哪些器件由半导体材料制成？

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Bipolar (A), gallium-nitride (B), and LDMOS (C) transistors are all solid-state semiconductor devices. A vacuum-tube triode (D) is not semiconductor — it is an electron-tube device.

U0579. Electrostatic discharge (ESD) is a common electromagnetic phenomenon that accompanies daily life at all times. However, strong ESD can damage electronic equipment or even endanger human life and must be prevented. Which of the following are related to static electricity? (Choose all that apply.)

静电放电是一种常见电磁现象，时刻伴随日常生活。但是，较强的静电放电却足以损坏电子设备甚至危及人身，需要预防。以下所述与静电有关的是：

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Static-related phenomena include charge buildup on wind- blown guy wires (A), thundercloud charge (B), and atmospheric/static noise QRN heard on receive (C). RF energy fed to the antenna while transmitting (D) is intentional radio emission, not static electricity.

U0580. If a conductor is placed in an electrostatic field, the conductor will exhibit the following characteristics: (Choose all that apply.)

如果将导体置于静电场中，导体将呈现如下特点：

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Statement A is correct physics: at electrostatic equilibrium a conductor is equipotential, internal field is zero, and excess charge resides on the outer surface. The official answer key marks A and B. Study note: in real physics, surface charge concentrates where curvature is large (sharp points), so option D actually states the correct tip-discharge condition; the source key nonetheless marks B, so memorize the keyed answer.

U0581. Direct current (DC) is defined by the direction of charge motion never changing. Which of the following descriptions of direct current is correct? (Choose all that apply.)

直流电（DC）是以电荷的运动方向始终不变来定义的。关于直流电，以下描述正确的是：

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DC has fixed polarity terminals (A) and a battery supplies DC (B). C is false — 13.8 V can deliver dangerous short-circuit current; D is false — pulsating DC still contains an AC (ripple) component even though its average direction is fixed.

U0582. Alternating current (AC) is defined by the direction of charge motion alternating with time. Which of the following descriptions of alternating current is correct?

交流电（AC）是以电荷的运动方向随时间交替变化来定义的。关于交流电，以下描述正确的是：

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AC polarity alternates, so an AC source has no fixed +/− terminals (A). B is false — live (hot) and neutral must not be swapped; C is false — AC can be non-sinusoidal/multi- frequency; D is false — received RF is an AC signal.

U0583. Which of the following terms can be used to describe the number of times per second that alternating current changes polarity?

以下哪一个术语可以用来描述交流电每秒改变极性的次数？

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The number of polarity reversals per second is the frequency (频率), measured in hertz (A). Rate (B), wavelength (C), and pulse rate (D) describe other quantities.

U0584. What is the relationship between the "wavelength" frequently mentioned by amateur radio operators and the frequency of radio waves?

业余无线电爱好者经常提及的"波长"与无线电波的频率有什么关系？

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Wavelength λ = c / f (speed of light ÷ frequency); higher frequency means shorter wavelength (A). B (useless) is false — amateurs use wavelength to name bands (e.g., "20 m"); C and D invent false fixed ratios.

U0585. The physical quantity "electromotive force (EMF)" describes:

物理量"电动势"描述的是：

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电动势 (electromotive force, EMF) is the ability of a source to convert other energy forms into electrical energy (A). B describes voltage; C describes current; D is not a standard quantity.