CRAC Class C (2025) — Questions 851–860

1,805,000 Hz = 1.805 × 10^6 Hz = 1.805 MHz. Option A is correct. B (kHz) would be 1,000 times too small, C (mHz = 10^-3 Hz) is absurd, and D (GHz) is 1,000 times too large.

2430 MHz = 2.430 × 10^9 Hz = 2.43 GHz. Option A is correct. B (243 GHz) is 100× too large, C (nHz) is vanishingly small, and D (kHz) is 10^6× too small.

With the resistor directly across the 13.8 V source, P = V²/R = 13.8² / 1 ≈ 190.4 W, about 190 W (A). Options B and C give voltage and current (not power), and D (VA) is a volt-ampere figure inappropriate for a pure resistor.

5 W = 5000 mW. In dBm: 10·log10(5000) ≈ 36.99 ≈ 37 dBm (A). For reference, 5 W = 10·log10(5) ≈ 7 dBW (so B is wrong), 17 dBm = 50 mW, and 35 dBμ = 3.16 mW — none equal 5 W.

0.25 W = 250,000 μW. In dBμ (decibels relative to 1 μW): 10·log10(250000) ≈ 53.98 ≈ 54 dBμ (A). Note 0.25 W = 23.98 dBm (not 36 or 25 dBm) and 6 dBW = 4 W, so B/C/D are wrong. The exam uses dBμ here, not dBm.

0.4 kW = 400 W = 400,000,000 μW. In dBμ: 10·log10(4×10^8) = 80 + 6.02 ≈ 86 dBμ (A). In dBm it would be 10·log10(400,000) ≈ 56 dBm, so B/C/D (400, 6000, 34 dBm) are all wrong magnitudes.

All four statements correctly describe a power source: it converts energy into electricity (A); a battery is a chemical source (B); a DC supply converts AC/DC into regulated DC (C); and a transformer reshapes AC to form an AC source (D). Each is a valid characterization of a power source.

The electromotive force (电动势, EMF) of a source is defined as acting from the negative terminal toward the positive terminal inside the source (A). This is opposite to the external terminal voltage direction (B/D are wrong), and it is an intrinsic property, not dependent on load (C is wrong).

The terminal voltage of a source drives current through the external circuit from the positive terminal to the negative terminal (A). This is the opposite direction to the internal EMF (so D is wrong), and is not load-dependent in direction (C wrong).

Common external power sources for amateur equipment are switching power supplies, linear power supplies, and storage batteries (A, B, C). A standard cell (标准电池, D) is a precision voltage reference, not a practical power source for equipment.