The Bulb That Dimmed
Siti builds a circuit with one cell, one bulb, and wires. The bulb lights brightly. She then adds a second identical bulb in the circuit. In arrangement P, the two bulbs are connected one after the other in a single loop. In arrangement Q, each bulb is on its own branch connected across the same cell. Siti notices that the bulbs in P are dimmer than the single bulb was, while the bulbs in Q are as bright as the single bulb.
1. Explain why the bulbs in arrangement P are dimmer than the single bulb, using the idea of resistance.
Read the worked answer
In arrangement P the bulbs are in series, so the current must pass through both bulbs one after the other. Each bulb adds resistance, so the total resistance of the circuit is greater than with one bulb. With the same cell, a greater total resistance means a smaller current, so each bulb is dimmer.
The cell is not 'used up' faster in a way that explains the dimness; the key is that the same current passes through both bulbs and that current is smaller than before.
2. Explain why the bulbs in arrangement Q stay as bright as the single bulb, and state one disadvantage of arrangement Q.
Read the worked answer
In arrangement Q each bulb is on its own branch connected directly across the cell, so each branch gets the full cell voltage. The current through each bulb is the same as when it was alone, so each bulb is as bright as before.
A disadvantage is that the cell must supply current to both branches at once, so it runs down faster. Another disadvantage is that Q needs more wiring. Also, if one bulb in P fails, the whole loop stops working, while in Q the other bulb can still light.