Final answer:
When the battery is plugged in and the battery switch is off, the positive bus is still powered. The bulb is on when the switch is open due to a small amount of current flowing through the filament. When the switch is closed, a complete path is created for the current to flow, causing the bulb to light up.
Step-by-step explanation:
When the battery is plugged in and the battery switch is off, the bus that is still powered is the positive bus. This is because the positive terminal of the battery is connected directly to the positive bus, providing a continuous supply of power. To understand why the bulb is on when the switch is open, we need to consider the circuit.
When the switch is open, there is no complete path for charges to flow from the positive terminal of the battery to the negative terminal. However, the bulb is still on because there is a small amount of current flowing through the filament due to the internal resistance of the battery. When the switch is closed, a complete path is created for charges to flow, which includes the bulb. The current flows from the positive terminal of the battery, through the switch, then through the bulb, and back to the negative terminal of the battery. As a result, the bulb lights up.