电流-电压
读数
发生了什么
电流等于电压除以总电阻。串联时电阻相加，每多接一个电阻，整个回路的电流就变小。并联时每个电阻都有自己的通路，总电阻变小，电池推动的电流更大。移动的小点按实际电流的快慢移动；图表显示 I-V 直线，斜率为 1/R。
- Ohm's lawI = V / R
- Resistors in seriesR = R₁ + R₂
- Resistors in parallel1/R = 1/R₁ + 1/R₂
考试大纲
- GCSE Physics AQA 4.2.2, Edexcel, OCRseries and parallel circuits, Ohm's law and power
- A-Level Physicsinternal resistance and terminal voltage
挑战
先预测：R₁ = R₂ 时，在相同电压下，并联电流是串联电流的两倍、四倍，还是一半？先算出来，再切换开关验证。
是串联电流的四倍：串联时 I = V/2R₁，并联时 I = 2V/R₁，比值为 4 倍。
系统地学
GCSE Combined and Separate Science 中讲这个主题的课：先看分步讲解，再做考试风格的题目，每次数字都不同。 课程为英文。
常见问题
How do you add resistors in series and in parallel?
In series they add, R = R₁ + R₂. In parallel the reciprocals add, 1/R = 1/R₁ + 1/R₂, so the total is smaller than the smallest resistor.
Why do parallel circuits draw more current?
Each branch is another path for charge, so the total resistance drops. With the same battery voltage, I = V/R goes up.
What is internal resistance?
A real battery has some resistance inside it. When current flows, part of the EMF is used up inside, so the terminal voltage is V = ε − Ir.
What resistor do I add in parallel to get a given total?
Rearrange 1/R = 1/R₁ + 1/R₂ to R₂ = R₁R/(R₁ − R). To bring a 20 Ω resistor down to 12 Ω, add R₂ = 20 × 12 / (20 − 12) = 30 Ω in parallel. The target must be below 20 Ω, because parallel always lowers the total. Pick Parallel R₂ for a total under Solve for to see each step.