Open vs closed circuits: trace the entire path

Start at the source, not the switch

For a simple DC lamp circuit, trace from source positive through the switch and lamp back to source negative. When the switch contacts meet, that particular loop is closed and the source can drive current. When the contacts separate, that loop is open. An open external path does not remove the battery's terminal voltage. 'Closed' does not promise a safe current.

One open branch need not stop another; name the path you mean.

One resistor makes the distinction measurable

Imagine an isolated regulated 5 V DC supply, a switch, and a 1 kΩ resistor all in series. With an ideal closed switch and intact wires, the resistor sees 5 V and carries 5 V / 1,000 Ω = 5 mA. Open the switch and the steady-state resistor current becomes zero in the ideal model. You may now measure most of the supply voltage across the open switch.

Real open contacts and meters may leak tiny currents.

A closed path can be the wrong path

A wire accidentally connected directly across a source closes a loop that bypasses the intended load. This is a short circuit, not an especially effective way to power the load. An ideal zero-ohm short on an ideal voltage source has no finite current prediction in that ideal model; real sources, wires, protection, and contacts have limits and nonzero impedance. Real fault current is finite but can still cause heat, damage, or fire. Never test a source by shorting it.

A closed switch cannot rescue a disconnected return. Trace the breadboard rails too.

A switch is not the only possible break

A loose jumper, broken conductor, split breadboard rail, or badly seated resistor leg can open the intended loop while the switch appears closed. An open connection can be intermittent: touching the board may restore it briefly. A floating point may show voltage on a high-impedance meter yet supply no useful load current.

If a lamp stays dark, do not assume the lamp is defective. Compare its two connection nodes with the drawing, then inspect the complete supply and return routes with power removed.

Open does not always mean harmless

A capacitor connected across a supply may retain voltage after the switch is opened or the supply is disconnected. Charging current can stop while the capacitor still holds energy. Switching events can also involve brief transients even when a later DC calculation predicts zero current. Treat an open switch as a statement about one path, not proof that every component is discharged.

For beginners, stay with small, low-voltage parts and verify a capacitor's voltage before handling it. Never use this exercise on mains-powered or unknown stored-energy equipment.

Debug an apparent open safely

Turn off the low-voltage supply first. Draw the one intended load path, check each jumper against the actual breadboard strips, and inspect switch contacts and the return. On an unpowered, discharged circuit, a suitable meter's continuity mode can help locate a break, provided other parallel components cannot create misleading paths. Restore power only after the wiring is correct; measure voltage across the load in voltage mode rather than putting a current-mode meter across the supply.

1. Mark source, switch, load, and return as distinct points on a sketch.
2. With power off, check each intended connection and identify any parallel route.
3. With safe power restored, measure across the load and compare the result to the predicted circuit state.