Lightbulbs in Parallel

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.


New Member
Aug 8, 2012
Reaction score

Members don't see this ad.
Hey, so I'm not sure what I'm doing wrong in answering the following question:

Three identical light bulbs are connected to a battery. What will happen if the middle bulb burns out?

The answer I got was the light bulbs will now burn less brightly, and here follows my logic:

We know from Kirchhoff's First Law, the sum of the voltage drop across resistors must equal the voltage of the battery. As the light bulbs are the only source of resistance across the circuit, we know the voltage drop remains constant and must be the total voltage of the battery (I.e. the drop in voltage crossing these resistors must be the voltage of the battery). In an example, let us say we are providing V voltage.

In parallel, the resistance of these is as follows:

For 2 lightbulbs it is (10)(10)/20 = 5 Ohms total.

For 3 lightbulbs it is (5)(10)/(15) = 3.33Ohms total for the whole set of lightbulbs in parallel.

Now, as V is constant, and the resistance is changing based on how many lightbulbs we set up in parallel, I SHOULD decrease as the resistance increases for subtracting a lightbulb, as it must give the same value of V.

Why is it the case the TPR Physical sciences states it remains the same. This involves modulating the voltage coming from the battery, but the battery doesn't know that downstream it has less resistance. It should still pump out the same amount of voltage, regardless, surely?

Any help on this would be appreciated.