Question

The circuit shown in Fig. contains a battery, a rheostat and two identical lamps. What will happen to the brightness of the lamps if the resistance of the rheostat is increased?

• A. Lamp P Less bright, Lamp Q Brighter
• B. Lamp P Less brighter, Lamp Q Less brighter
• C. Lamp P Brighter, Lamp Q Less brighter
• D. Lamp P No change, Lamp Q Brighter

Answer 1

The correct option is A Lamp P Less bright, Lamp Q Brighter

Initially, when the rheostat is set at a very low value of resistance or negligible resistance, it will act as a short-circuit for the $lamp$ $Q$, thus the current will flow through the rheostat section. It is the case of only $lamp$ $P$ in series with the battery. $Lamp$ $P$ glows brightly.

When the resistance of the rheostat is increased slightly some current starts flowing through the $lamp$ $Q$. Thus it starts glowing very dim.

When the resistance of the rheostat is increased considerably the current in the $lamp$ $Q$ and rheostat section moves through the $lampQ$ owing to increased resistance in the rheostat path. It is the case of a series connection involving $lamp$ $P$ , $lamp$ $Q$ and the battery. This new voltage drop alongside the $lamp$ $Q$ will reduce the voltage drop along $lamp$ $P$ . Thus the $lamp$ $P$ gets less bright and the $lamp$ $Q$ brightens.