CRAC Class C (2025) — Questions 931–940
U1177. An R–C parallel circuit is connected to a DC supply of voltage U. When the supply is suddenly disconnected, the current through resistor R will:
将电阻R和电容C并联后接在电压为U的直流电源上。突然断开 电源，流过电阻R的电流会：
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By Ohm's law the resistor current equals its voltage over R; at disconnect it is U/R and decays as (U/R)·e^(−t/τ) to 0 with the shared voltage. A is correct.
U1178. An R–C parallel circuit is connected to a DC supply of voltage U. When the supply is suddenly disconnected, the current through capacitor
将电阻R和电容C并联后接到电压为U的直流电源上。突然断开 电源，流过电容C的电流会：
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On disconnect the capacitor begins to discharge through R, so its current jumps from 0 (before disconnect there was no DC capacitor current) to U/R in the discharge direction and then decays exponentially to 0. A is correct — the magnitude follows the same (U/R)·e^(−t/τ) decay as the resistor current.
U1179. An R–L series circuit is suddenly connected to a DC voltage U; the voltage across inductor L will:
将电阻R和电感L串联后突然接到直流电压U上，电感L两端的电 压会：
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At t=0 the inductor current is zero (cannot change instantly), so the full U appears across L; as current builds, VL = U·e^(−t/τ) decays exponentially to 0. A is correct.
U1180. Given a resistor R and inductor L, the product of the resistance and inductance is the time constant τ. When R and L are connected in series and suddenly connected to DC voltage U, after time τ the voltage across inductor L is approximately what fraction of U?
给定电阻R和电感L，则其阻值和电感量的乘积称为时间常数 τ。将R和L串联后突然接到直流电压U上。经过τ时间后，电感L 两端的电压大约为U的：
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After one τ the inductor voltage has decayed to e^(−1) ≈ 37% of its initial U (it started at U and falls toward 0). A is correct — the mirror of the RC capacitor's 63% rise.
U1181. Given a resistor R and inductor L, the product RL is the time constant τ. When connected in series and suddenly connected to DC voltage U, after time τ the voltage across resistor R is approximately what fraction of U?
给定电阻R和电感L，则其阻值和电感量的乘积称为时间常数 τ。将R和L串联后突然接到直流电压U上。经过τ时间后，电阻 R两端的电压大约为U的：
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The resistor voltage rises with the current: VR = U(1−e^(−t/τ)). After one τ it reaches 1−e^(−1) ≈ 63% of U. A is correct (the complement of the inductor's 37% at that instant).
U1182. When an R–L series circuit is suddenly connected to a DC voltage U, the voltage across resistor R will:
将电阻R和电感L串联后突然接到直流电压U上，电阻R两端的电 压会：
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By Ohm's law VR = I·R; the current starts at 0 and rises exponentially toward U/R, so VR rises from 0 toward U exponentially. A is correct; the inductor takes the initial U and gives it up.
U1183. When an R–L series circuit is suddenly connected to a DC voltage U, the current through resistor R will:
将电阻R和电感L串联后突然接到直流电压U上，流过电阻R的电 流会：
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Inductor current cannot jump, so the series current starts at 0 and rises as I = (U/R)(1−e^(−t/τ)) toward U/R. A is correct; the resistor current equals the inductor current.
U1184. When an R–L series circuit is suddenly connected to a DC voltage U, the current through inductor L will:
将电阻R和电感L串联后突然接到直流电压U上，流过电感L的电 流会：
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Being a series circuit, the inductor current equals the resistor current: it starts at 0 and rises exponentially to U/R. A is correct; inductor current is continuous and cannot jump.
U1185. An R–L parallel circuit is placed in a DC circuit of current I. When the circuit is suddenly opened, the voltage across inductor L will:
将电阻R和电感L并联后接在电流为I的直流电路中。突然断开电 路，电感L两端的电压会：
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On opening the source, the inductor maintains its current I, which now flows through the parallel resistor, so initially VL = I·R; this voltage then decays exponentially to 0 as the stored energy dissipates. A is correct.
U1186. A series or parallel circuit of a capacitor and an inductor has a state in which, at a particular frequency, the capacitive reactance and the inductive reactance cancel each other and the circuit's reactance disappears. This state is called:
由电容与电感组成的串联或并联电路存在一种状态，此时电容 的容抗与电感的感抗在特定频率下相互抵消，电路中的电抗消 失。该状态称为：
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谐振 (resonance) is the condition XL = Xc so the net reactance is zero. Matching, negative resistance, and phantom are unrelated concepts. A is correct.