“动态电路”是指由于电路中的局部电阻变化而引起总电路及各支路的电压、电流及电功率等变化的一类电路。在动态电路中U与I、△U与△I的比值问题是一种非常重要的题型。这类问题比较复杂,难度系数高,所涉及的物理量较多,解决这类问题的关键是抓住动态电路中变化量与不变量的联系。 “Dynamic Circuit ” refers to a class of circuits that cause changes in voltage, current, and electric power of the total circuit and each branch due to local resistance changes in the circuit. In the dynamic circuit U and I, △ U and △ I ratio problem is a very important question type. This kind of problem is more complex, the difficulty coefficient is high, and the physical quantity involved is more. The key to solve this kind of problem is to seize the relationship between the variation and the invariant in the dynamic circuit.