A complete circuit lets current flow — Science, 14–17 years
Electric devices work because charges can move through a complete path. This topic links batteries, wires, switches and lamps into one testable idea.
The path must be complete
A circuit is a complete route from one side of a cell, through components such as a lamp, and back to the other side. When the route is broken, charge cannot keep moving, so the lamp stays off. A switch simply opens or closes this route.
The problem of finding the fault
Early experimenters needed a way to explain why one lamp lit while another did not, and why a switch could control a whole device. The complete-path idea solves this problem: it tells us where to look first. Check every connection, not just the battery.
Tracing a simple circuit
Connect a cell's positive terminal to one lamp contact, then connect the lamp's other contact back to the negative terminal. Close the switch: the path is complete and the lamp lights. Open the switch: there is a gap, so the lamp goes out. The same test works for finding a loose wire.
A battery is not enough
A common mistake is to touch one wire to a battery and expect a lamp to light. That seems reasonable because the battery supplies the electrical push, but the push needs a return route. Without two connected sides, there is no continuous circuit and no steady current through the lamp.
Finding faults in real devices
This idea appears in torches, bicycle lights, doorbells and household wiring. When one stops working, a technician asks whether the path is broken, a contact is poor or a switch is open. Mains electricity is dangerous, so real repairs must be left to trained adults.
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