Series
The left board holds a 6 volt (V) battery and two bulbs in one loop, one after the other. This is a series circuit. There is only one path, so the current, the flow of charge, must pass through every part in turn.
Resistance is how hard a part resists the current, measured in ohms (Ω). Bulb A has 100 Ω and bulb B has 200 Ω. In series the current must get through both, so the total is 100 + 200 = 300 Ω.
Ohm's law gives the current: 6 V ÷ 300 Ω = 0.02 amps, or 20 milliamps (mA). With one path, the same 20 mA flows through every part. A meter anywhere in the loop reads the same, because charge cannot pile up or leak out.
Voltage is the push. Bulb A uses 20 mA × 100 Ω = 2 V of the battery's 6 V; bulb B uses 20 mA × 200 Ω = 4 V. The bigger resistance takes the bigger share. This is a voltage divider.
Kirchhoff's voltage law says: go once round any closed loop and the voltage gained equals the voltage used. The battery gives 6 V; the bulbs use 2 V + 4 V = 6 V. Nothing is left over and nothing is missing.
Parallel
On the right board, the two bulbs sit side by side. Each bulb connects straight across the battery, so each one gets the full 6 V. This is a parallel circuit: every branch, each separate path, has the same voltage across it.
Follow the current to the junction, a point where wires meet. There it splits: each 100 Ω bulb takes 6 V ÷ 100 Ω = 60 mA. The branches join again, so 120 mA flows back to the battery. Faster dashes mean more current.
Kirchhoff's current law says: the current flowing into a junction equals the current flowing out. Here 120 mA flows in and 60 mA + 60 mA flows out. Charge is never made or lost at a junction; it only picks a path.
Two 100 Ω bulbs in parallel pass 120 mA, just as one 50 Ω resistor would: 6 V ÷ 120 mA = 50 Ω. Each branch is another path, so the total is less than the smallest branch. Rule: 1/R = 1/R1 + 1/R2.
In real life
Now take bulb B out of each board. In series, the only path now has a gap, so the current stops and bulb A goes dark too. In parallel, bulb A keeps its own path and its full 6 V, so it stays lit.
House sockets are wired in parallel. Each one gets the full mains voltage, about 230 V in Europe or 120 V in North America, and switching one appliance off leaves the rest running. Wired in series, one gap would stop everything.
In detail
;; step 1/11 · series Series loop: battery + -> bulb A -> bulb B -> battery - One path only: no junctions, no branches. Battery: 6 V = four 1.5 V cells (ideal, no internal resistance).
The boards are a teaching model: an ideal 6 V battery, wires with no resistance, and each bulb treated as a fixed resistor. A real bulb's filament has a higher resistance when it is hot than when it is cold. Series and parallel rules after OpenStax University Physics Volume 2, section 10.2; Kirchhoff's junction and loop rules after section 10.3. Current is drawn in the conventional direction, out of the battery's + side and back into its - side; the electrons in the wires drift the other way. In the figure the dashes move at a speed set by the current. Mains: 230 V nominal in IEC 60038 (used across Europe); about 120 V at US household outlets (HyperPhysics). House wiring adds fuses and breakers for safety, which this lesson leaves out.