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Lesson 11 · Two laws together
One board, both laws, and a shortcut
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One board, both laws, and a shortcut

Real analysis never uses one law at a time. A single board hands you a series string to walk with the loop rule and a junction to balance with the node rule, and the skill being examined is picking the right one for the line you are on. The test is simple: if the question is in volts and there is a closed path, walk the loop. If it is in amps and there is a junction, balance the node.

Between them sits a shortcut worth memorising, because it saves a step every time. Two resistors in series across a supply share the volts in proportion to their resistance, which is the voltage divider: Vout=VinR2R1+R2V_{out} = V_{in} \dfrac{R_2}{R_1 + R_2} — V-out equals V-in, times R-two over R-one plus R-two. VinV_{in} is the supply across the pair, in volts; VoutV_{out} is the voltage at the tap between them, also in volts; and the subscript convention is fixed and worth stating once: R1R_1 is the upper leg, from the supply down to the tap, and R2R_2 is the lower leg, from the tap down to ground — both in ohms, or both in kilohms, since only their ratio matters.

The divider is not a new law. It is KVL with the resistances substituted in: the same current flows through both, so each takes volts in proportion to its own resistance. Which also tells you when the shortcut stops being true — the moment anything DRAWS current from the tap, the two legs no longer carry the same current, and the divider quietly lies to you. A divider feeding a real load is a divider you have to solve properly.

Two rails to hold on to. The tap voltage is always less than the supply, always: no arrangement of passive resistors invents volts. And the bigger resistor always takes the bigger share, which is the one-second check that catches a swapped R1R_1 and R2R_2 before the arithmetic does.

V₁V₂V₃Vs

Vs=V1+V2+V3V_{s} = V_{1} + V_{2} + V_{3}

  • VsV_{s}= Source voltage
  • V1V_{1}= Drop across element 1
  • V2V_{2}= Drop across element 2
  • V3V_{3}= Drop across element 3

each variable a voltage

Kirchhoff's Voltage Law (Three-Element Loop) solver →
IinI₁I₂I₃

Iin=I1+I2+I3I_{in} = I_{1} + I_{2} + I_{3}

  • IinI_{in}= Current into the node
  • I1I_{1}= Branch 1 current
  • I2I_{2}= Branch 2 current
  • I3I_{3}= Branch 3 current

each variable a electric current

Kirchhoff's Current Law (Node with Three Branches) solver →
R₁R₂VinVout

Vout=VinR2R1+R2V_{out} = V_{in} \frac{R_{2}}{R_{1} + R_{2}}

  • VoutV_{out}= Output voltage (voltage)
  • VinV_{in}= Input voltage (voltage)
  • R1R_{1}= Upper resistance (resistance)
  • R2R_{2}= Lower resistance (resistance)
Voltage Divider solver →