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Lesson 2 of 6 · 15 minutes · Free

Series & Parallel Circuits

Distinguish shared current in series from shared voltage in parallel.

Try the experiment ↓Practice challenge ↓

See the idea

Series resistors share one current path. Their voltage drops add to the supply voltage, and their resistances add. A larger resistor takes a larger share of the voltage drop.

Parallel resistors connect across the same two nodes. Each branch has the full supply voltage. Branch currents add at the supply, and total resistance is lower than either branch resistance.

A change that adds resistance in series reduces source current; adding a parallel load increases source current. This distinction is central to understanding why a power supply can become overloaded when more loads are connected.

Series: Req = R1 + R2 Parallel: Req = R1R2 / (R1 + R2)

Series: same current. Parallel: same voltage. Check the source current in both cases.

Work through an example

  1. For 12 V, R1 = 1,000 Ω and R2 = 2,000 Ω in series: Req = 3,000 Ω, so I = 4 mA.
  2. The voltage drops are 4 V and 8 V. Their sum is 12 V.
  3. In parallel, currents are 12 mA and 6 mA; source current is 18 mA and Req ≈ 666.7 Ω.

Watch for this

Do not add parallel resistances directly. Also, a real source’s voltage may sag as load increases; this model assumes it does not.

Predict, then test

  1. Set R1 to 1,000 Ω and R2 to 2,000 Ω, then change the topology.
  2. In parallel mode, increase R2 while leaving R1 unchanged. Its branch current falls; the R1 branch stays the same with this ideal source.

Your virtual bench

Change a setting. See the circuit respond.

Swipe the diagram sideways to explore the connections.

±12 VR1 1000 ΩR22000 ΩReturn reference: 0 V
Source current4 mA
Supply power0.048 W
R1 voltage4 V
Load / R2 voltage8 V

Branch currents: R1 4 mA · R2 4 mA. Equivalent resistance: 3,000 Ω.

Model: Ideal low-voltage DC source and linear resistors; zero wire resistance except the selected connection fault. Values are calculated, not equipment measurements. Browser simulation only—do not connect live equipment.

Check your understanding

Two equal resistors R are connected in parallel. Their equivalent resistance is:

Put it to work

With a 12 V source and two 1,000 Ω resistors, compare source current in series and parallel.

Show the worked solution

Series: 12/2,000 = 6 mA. Parallel: 12 mA in each branch, or 24 mA total. The topology changes source current by a factor of four.

Try the experiment and answer the check, then record your completion.

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