Vishal builds a circuit where a battery, bulb, and switch are all connected in one loop by wire

Why

Vishal's picture treats the circuit like a tap right next to a bucket, where being close to the thing you're controlling seems to matter. A single loop doesn't work by nearness, though — it works by whether the loop is broken or complete, anywhere along its length. A switch is just a gap you can open or close; open the gap ANYWHERE in the loop and current stops flowing through the whole loop at once, because a loop is only useful to current when every part of it connects. Close that same gap and current flows through every part again, however far the switch sits from the bulb. Picture a circle of people holding hands — if any one pair lets go, the message can't pass around the circle, no matter which pair it was. Test: move a switch to different points in the same loop and it turns the same bulb on and off identically each time. His sister is right: a switch works by breaking or completing the loop, and that works the same from any position in it.