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One change, explained

Why does the lamp light when the switch closes?

Change one word and watch an interrupted circuit become a complete path.

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Start with the question

The source and lamp are both present. Why is there no current while the switch is open?

A steady current needs a continuous closed path. An open switch interrupts that path; closing it completes the circuit and places the rated voltage across the lamp.

Change just this

Change switch S from open to closed to complete the circuit.

scene switch_lamp

source V 6V #blue
switch S open #gray
lamp L 6V 3W #gold

V <-> S <-> L <-> V
scene switch_lamp

source V 6V #blue
switch S closed #gray
lamp L 6V 3W #gold

V <-> S <-> L <-> V

Compare before and after

Why does the lamp light when the switch closes?

01Switch open: lamp off

switch lamp Static circuit diagram + + − − V V 6V 6V S open S open L 6V / 3W L 6V / 3W ideal wires · lamps use rated hot resistance ideal wires · lamps use rated hot resistance
Diagram description

Static physics diagram: switch_lamp.

02Switch closed: lamp on

switch lamp Static circuit diagram + + − − V V 6V 6V S closed S closed L 6V / 3W L 6V / 3W ideal wires · lamps use rated hot resistance ideal wires · lamps use rated hot resistance
Diagram description

Static physics diagram: switch_lamp.

The physics behind it

The source and lamp are both present. Why is there no current while the switch is open?

This model uses a 6 V source and a 6 V / 3 W lamp. At its ideal rated operating point, the lamp current is about 0.5 A. Change open to closed to compare both states.

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