In the circuit shown how will the glow of bulbs A and C be affected when the key k is switched on?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Brightness of A increases but that of C decreases
The brightness of a bulb depends on the current through it. Bulb A is in series with the rest of the circuit and carries the total current. Bulb C is in one branch of a parallel arrangement, and bulb B, with the key, forms the other branch.
Key open: only C is in the circuit, in series with A.
Key closed: B is connected in parallel with C, so the resistance of that part falls.
Bulb A. The total resistance of the circuit decreases, so the total current from the battery increases. A carries this current, so A becomes brighter.
Bulb C. The potential difference across the parallel part is the battery emf minus the drop across A. The drop across A has increased, so this potential difference decreases. The current in C falls, so C becomes dimmer.