First, acha, draw the cell, bulb, and switch all in one simple loop with wires; the switch must be placed in that loop to break or complete it. An open switch stops the bulb because it breaks the path—like a lifted bridge—so the current from the cell cannot reach the filament at all. But I'll tell you, in my own house last week, the 'open' switch did not stop a faint glow in the old bulb due to some leakage in the damp wiring, which my spreadsheet-minded wife called a 'ghost current'. So sometimes, in reality, the switch is not a perfect barrier, ho na?
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