Why does lamp N get brighter when lamp M breaks?

[ravsterphysics]

Homework Equations

Power = VI

The Attempt at a Solution

[/B]
Since lamp M breaks, lamp L gets more of the current so its brightness increases and lamp N's brightness stays the same because the current at point 2 is the same if the other lamp breaks or not, so N's brightness should not be affected.

So my answer is A but the correct answer is D?? What have I done wrong?

 

[ehild]

View attachment 110973

Homework Equations

Power = VI

The Attempt at a Solution

[/B]
Since lamp M breaks, lamp L gets more of the current so its brightness increases and lamp N's brightness stays the same because the current at point 2 is the same if the other lamp breaks or not, so N's brightness should not be affected.

So my answer is A but the correct answer is D?? What have I done wrong?

The current through N will also change, as the resultant resistance changes when lamp M breaks.

 

[ravsterphysics]

The current through N will also change, as the resultant resistance changes when lamp M breaks.

I get it now.

since the parallel combo is no more, resistance has increased across the circuit so less current for both, so N decreases but L increases since it has more current in resistance than in parallel. Thanks.

 

[ehild]

It is correct, but can you derive what are the powers on the lamps in both cases, to prove your statement? The EMF of the battery is E and each lamp has resistance r.