CRAC Class C (2025) — Questions 921–930

U1167. When a sinusoidal AC voltage is applied across a coil, the current through it is a sinusoidal AC current. The phase of this current:

在线圈两端施加正弦交流电压，则流过其中的电流是正弦交流电流。该电流的相位：

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In an inductor (coil) the current lags the voltage by 90° (电流滞后电压90°), because the induced EMF opposes changes in current. A is correct; the capacitor is the opposite.

U1168. If a sinusoidal AC current flows through a coil, the voltage across it is a sinusoidal AC voltage. The phase of this voltage:

若正弦交流电流流过线圈，则其两端的电压是正弦交流电压。该电压的相位：

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Inductor restated from the current's viewpoint: since current lags voltage by 90°, the voltage leads the current by 90°. A is correct and consistent with U1167.

U1169. An R–C series circuit is suddenly connected to a DC voltage U; the voltage across capacitor

将电阻R和电容C串联后突然接到直流电压U上，电容C两端的电压会：

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On suddenly connecting an RC series to DC, the capacitor voltage starts at 0 (it cannot change instantly) and charges toward U along an exponential curve Vc = U(1−e^(−t/τ)). A is correct; discharging (B) is the disconnect case.

U1170. Given a resistor R and capacitor C, the product of the resistance and capacitance is called the time constant τ. When R and

给定电阻R和电容C，则其阻值和电容量的乘积称为时间常数 τ。将R和C串联后突然接到直流电压U上，电容C两端的电压经过τ时间后大约为U的：

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After one time constant τ = RC, the capacitor reaches 1 − e^(−1) ≈ 0.632 = 63% of the final voltage U. A is correct; 37% is the remaining gap (the value for the decaying quantity at τ).

U1171. When an R–C series circuit is suddenly connected to a DC voltage U, the voltage across resistor R will:

将电阻R和电容C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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At the instant of connection the capacitor is uncharged (acts like a short), so the full U appears across R; then as the capacitor charges, VR = U·e^(−t/τ) decays exponentially to 0. A is correct.

U1172. Given a resistor R and capacitor C, the product RC 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和电容C，则其阻值和电容量的乘积称为时间常数 τ。将R和C串联后突然接到直流电压U上，电阻R两端的电压经过τ时间后大约为U的：

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After one τ the resistor voltage has decayed to e^(−1) ≈ 0.368 ≈ 37% of its initial U. A is correct — this is the complement of the capacitor's 63% at the same instant.

U1173. When an R–C series circuit is suddenly connected to DC voltage U, the current through resistor R will:

将电阻R和电容C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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Current equals resistor voltage divided by R, so it jumps to U/R at t=0 (capacitor short) and then decays as I = (U/R)·e^(−t/τ) to 0 as the capacitor charges. A is correct. (The branch current through R and C is the same series current.)

U1174. When an R–C series circuit is suddenly connected to DC voltage U, the current through capacitor

将电阻R和电容C串联后突然接到直流电压U上，流过电容C的电流会：

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In a series circuit the capacitor current equals the resistor current; it jumps to U/R at t=0 and decays to 0 as the capacitor finishes charging (I = C·dVc/dt → 0 when Vc stops changing). A is correct.

U1175. An R–C parallel circuit is connected to a DC supply of voltage U. When the supply is suddenly disconnected, the voltage across 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，电容C两端的电压会：

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With the supply removed, the charged capacitor discharges through the parallel resistor: Vc = U·e^(−t/τ), decaying exponentially from U to 0. A is correct; the capacitor voltage cannot jump (it is stored energy).

U1176. An R–C parallel circuit is connected to a DC supply of voltage U. When the supply is suddenly disconnected, the voltage across 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，电阻R两端的电压会：

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In parallel, the resistor and capacitor share the same voltage, so on disconnect the resistor voltage also decays exponentially from U to 0 along with the capacitor discharge. A is correct.