帮我解决这道题，并生成这道题的讲解视频。题目和第（3）问的解答步骤已经上传至图片，你可以参考。---Here is the extracted content from the image: --- **Question Stem:** (3) 要对Umax和Umin的成因理解都非常到位才行。很难，但非常好；是2024年中考压轴改编。 (Translation: To fully understand the causes of Umax and Umin, it is very important. It's difficult, but very good; it's an adaptation of a key problem from the 2024 mid-term exam.) **Chart/Diagram Description:** * **Type:** Circuit Diagram * **Main Elements:** * **Voltage Source:** A DC voltage source providing 18V. * **Resistors:** * R0: A resistor connected in series with the voltage source. * R1: A resistor with a value of 10Ω, connected in series with R0. * R2: A resistor with a value of 25Ω, connected in parallel with R1. * **Voltmeter:** Labelled 'V', with a measurement range of 0-15V. It is connected in parallel across the combination of R1 and R2. * **Current Path:** A main current path flows from the positive terminal of the 18V source, through point A, then through R0, then splits into R1 and R2 in parallel, and returns to the negative terminal. * **Current Annotation:** An arrow indicates a current of 0.6A flowing in the main circuit path (presumably through R0). **Analysis/Solution Content:** **分析：** (Analysis:) Lmax, Lmin 分别对应这2种因: (Lmax, Lmin correspond to these 2 reasons:) **Case 1:** R2↑ R1↓ A↑ A = 0.6A / R2 = 0.5Ω Ro+R1 / Rmax = 3V / 15V = 1/5 不可能 (Impossible) Umin = 0V **Case 2:** R2↑ A↓ V = 15V / R2 = 25Ω - = (18V - 3U) / (Ro + 10Ω) 3V / 25Ω (1) Ro + 10Ω = (18V - 3U) / 0.6A (2) Ro + R1 + R2min = 18V / 0.6A (3) U = 0.6A x R2min **或** (OR) 18V / (35Ω + Ro) * x * 25Ω = 3U 0.6A * (20SΩ - Ro) = U **联立①②③得：** (Combine ①②③ to get:) Ro^2 + 15Ro - 450 = 0 Ro = 15Ω --- Here is the complete and accurate extraction of content from the image: --- **Question Stem:** 7. 如图甲所示, 电源电压为 18V, 电压表量程 0~15V, 电流表量程 0~3A, R1 的 I-U 图像 如图乙所示。滑动变阻器 R2 标有“25Ω 2A”。闭合开关 S, 移动滑动变阻器的滑片 P 到某位置时, 电流表的示数为 1A。 **Sub-questions:** (1) 求 R1 的阻值。 (2) 求此时滑动变阻器 R2 的电功率。 (3) 在电路中 M 点再串联接入一个定值电阻 R0, 电流表更换为 0~0.6A 量程, 移动滑动变阻器的滑片 P, 在保证电路安全的情况下, 电压表示数的最大值与最小值之比为 3:1, 求定值电阻 R0 的阻值。 **Supplementary Instruction for Answering:** (解答时, 要求有必要的文字说明、公式和计算步骤等, 只写最后结果不得分) *(Translation: When answering, provide necessary textual explanations, formulas, and calculation steps; merely writing the final result will not earn points.)* **Chart/Diagram Description:** **Diagram 甲 (Jia - Circuit Diagram):** * **Type:** Circuit Diagram. * **Main Elements:** * A power source (battery symbol) is shown at the top left. * A switch (S) is connected in series with the power source. * An ammeter (A) is connected in series in the circuit, labeled with 'M' at its left terminal. * A fixed resistor R1 is connected in series after the ammeter. * A sliding rheostat R2 is connected in series after R1. It has a movable slider labeled 'P'. The rheostat is drawn with a resistor symbol and an arrow passing through it, indicating its variable nature. * A voltmeter (V) is connected in parallel across the variable part of the sliding rheostat R2. Specifically, it connects from the slider 'P' to the right-hand terminal of R2. The voltmeter has a label '15' next to it, indicating its range is 0-15V. * The overall circuit forms a series connection of the ammeter, R1, and R2 with the power source. **Diagram 乙 (Yi - I-U Graph for R1):** * **Type:** Line Chart (I-U graph). * **Main Elements:** * **X-axis:** Labeled "U/V", representing voltage in Volts. The scale ranges from 0 to 10. * **Y-axis:** Labeled "I/A", representing current in Amperes. The scale ranges from 0 to 1.0. * **Data Series:** A straight line originates from the origin (0,0) and extends to the point where U = 10V and I = 1.0A. This linear relationship indicates Ohm's Law for resistor R1. * **Label:** "乙" (Yi) is written below the X-axis. **Handwritten Solution/Calculation:** **(1)** R1 = U / I R1 = 10V / 1A = 10Ω **(2)** U1 = R1 * I = 1A * 10Ω = 10V U2 = U - U1 = 18V - 10V = 8V P2 = U2 * I = 8V * 1A = 8W **(3)** Umax = 0.6A * 10Ω = 6V Umin = 18 - 6 - 0.6R0 = 12 - 0.6R0 [The following steps imply a ratio of Umax (from voltmeter range, 15V) to Umin as 3:1, leading to 15 / Umin = 3, so Umin = 5V] 12 - 0.6R0 = 5 0.6R0 = 7 R0 = 35 / 3 ---