CRAC Class B (2025) — Questions 921–930
U0996. A method that can be used to roughly judge whether a dry- cell battery has failed is:
要大致判断一节干电池是否已经失效，可用的方法是：
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A simple go/no-go test is to measure the no-load terminal voltage; a healthy cell reads near its nominal voltage, while a failed one reads much lower (A). B's needle method is unreliable, C risks a large short-circuit current through the meter, and D is nonsense (polarity cannot "disappear").
U0997. Using a multimeter's resistance range to measure a quarter- wavelength grounded antenna with an impedance of 50 ohms gives a reading of 0 ohms.
用万用表的电阻档测量一副阻抗为50欧姆的四分之一波长接地 天线，读数为0欧。可能的情况是：
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A quarter-wave grounded antenna behaves as a DC short only if there is a DC path to ground — here, plausibly via a loading/inductor coil that is DC-continuous. So 0 Ω suggests such a DC path may exist (A). It does not prove the antenna is damaged (B) or mismatched (C/D); those need RF measurements.
U0998. Using a multimeter's resistance range to measure a quarter- wavelength grounded antenna with an impedance of 50 ohms gives a reading of infinity.
用万用表的电阻档测量一副阻抗为50欧姆的四分之一波长接地 天线，读数为无穷大。可能的情况是：
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An infinite reading means no DC path to ground exists (no inductor shunt), so A is the correct inference. It does not prove the antenna is open-damaged (B) or mismatched (C/D); those require RF testing.
U0999. Using a multimeter's resistance range to measure the center- feed-point impedance of a home-built quarter-wavelength- wire dipole antenna.
用万用表的电阻档测量一副用四分之一波长的导线自制的偶极 天线的中心馈电点阻抗。可能的情况是：
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A center-fed dipole is balanced: its two halves are separated by the feed-point insulator, so there is no DC path between them and the DC resistance reads infinity (A). The 50 Ω (B) is the RF feed-point impedance at resonance, not a DC reading; C/D are irrelevant to a DC ohmmeter.
U1000. Using a multimeter's resistance range to measure the resistance between the center conductor and the shield of an ideal 50-ohm coaxial cable of arbitrary length with an open- circuited (terminated open) end.
用万用表的电阻档测量一条终端开路的任意长度理想50欧同轴 电缆的中心导体和屏蔽层之间的电阻。可能的情况是：
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Open-circuited coax has its center conductor and shield disconnected at both ends, so the DC resistance is infinite (A). The 50 Ω is the RF characteristic impedance, not a DC measurement, and for an ideal open line it does not depend on length (so C/D are wrong).
U1001. Using a multimeter's resistance range to measure the resistance between the center conductor and the shield of an ideal 50-ohm coaxial cable of arbitrary length with a short- circuited end.
用万用表的电阻档测量一条终端短路的任意长度理想50欧同轴 电缆的中心导体和屏蔽层之间的电阻。可能的情况是：
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A shorted end ties the center conductor to the shield, so the DC resistance is 0 Ω (A). The 50 Ω characteristic impedance is an RF property; an ideal short reads 0 Ω regardless of length, so C/D are wrong.
U1002. Which soldering material is relatively suitable for amateur radio construction and repair?
哪一种焊接材料比较适合业余无线电制作和维修？
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Rosin-core solder is standard for electronics: the rosin flux cleans the joint without being corrosive. Acid-core solder (D) is for plumbing and attacks electronics; silver/copper brazing rods (B/C) are for metal joining, not PCB soldering. So A is correct. 欧姆定律与电阻 Ohm's Law and Resistance
U1003. Connect a load of resistance R to a source of voltage U. Regarding the current I in the load and the power P consumed by the load, which of the following descriptions is correct? ('x＾m' means 'x to the power m')
将一个电阻为R的负载接到电压为U的电源上。关于负载中的电 流I及负载所消耗的功率Ｐ，以下描述正确的是：（“x＾m” 表示“x的m次方”）
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By Ohm's law I = U/R, and power P = U·I = U²/R. Option A gives both correctly. B wrongly uses P = U/R, and C/D swap the resistance relationship.
U1004. A current I flows through a load of resistance R. Regarding the voltage U across the load and the power P consumed by the load, which of the following is correct? ('x＾m' means 'x to the power m')
一个电阻为R的负载中流过的电流为I。关于负载两端的电压U 及负载所消耗的功率Ｐ，以下描述正确的是：（“x＾m”表示 “x的m次方”）
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U = I·R (Ohm's law) and P = U·I = I²·R. Option A is correct; B misstates power, and C/D invert the relations.
U1005. A resistive load has voltage U across it and current I flowing through it. Regarding the load's resistance R and consumed power P, which is correct? ('x＾m' means 'x to the power m')
一个电阻负载两端电压为U，流过的电流为I。关于该负载的电 阻R和所消耗的功率P，以下描述正确的是：（“x＾m”表示 “x的m次方”）
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R = U/I and P = U·I are the basic definitions, so A is correct. Options B/C/D wrongly equate R with U·I.