Circuit builder

Series — one loop, one current. Every component carries the same current; the battery voltage splits across them in proportion to resistance (V = IR for each). Remove every component and the loop is open — nothing lights up, matching the real "one broken bulb kills the string" behaviour of old series fairy lights.

Parallel — every branch gets the full terminal voltage directly, and draws its own current independently (I = V/R). Removing a branch just means one fewer path; the others keep working exactly as before — the real reason household wiring is parallel, not series.

The capacitor is the odd one out: once a DC circuit reaches steady state a fully charged capacitor blocks current completely. In series, that stops current everywhere in the loop and the whole EMF appears across the capacitor (Q = CV). In parallel, only that branch goes to zero current — it just charges up to the terminal voltage while every other branch keeps flowing normally.

Internal resistance — a real cell isn't a perfect voltage source; it has some internal resistance r, so the voltage at its terminals drops below the EMF once current flows: V = E − Ir. Set r above zero and watch "Terminal voltage" fall away from "Battery EMF" as you add more components.

Exactly two idealisations, both disclosed: the bulb is a fixed 9Ω resistor (a real filament's resistance actually rises with temperature — modelling that needs a separate thermal simulation, not a shortcut in the circuit maths), and the wires plus the battery's EMF are ideal (no resistance of their own; every ohm you see comes from a component you placed or the internal-resistance slider). Every equation applied to whatever you build — Ohm's law, the series and parallel combination rules, Kirchhoff's laws, and the capacitor's DC steady-state behaviour — is solved exactly, with no fudge factors.