# Ranking brightness of lightbulbs in a circuit

1. Mar 4, 2017

### ikihi

1. The problem statement, all variables and given/known data

In the circuit shown, all the light bulbs are the same. Why is the following statement true? :

A is the brightest. B and C are equally bright, but are dimmer than A.

https://pbs.twimg.com/media/C6GllsQVMAA4chz.jpg:large [Broken]

2. Relevant equations

P = I * V

3. The attempt at a solution

There is more of a voltage drop across B to C than A, so the power is less for both B and C than A since power is proportional to I and V? Since B and C are the same then they should be the same brightness because they each have the same resistance, in series?

Last edited by a moderator: May 8, 2017
2. Mar 4, 2017

### Staff: Mentor

No. lightbulb A is in parallel with B+C. So the voltage drop across B is half of that across A, and the drop across C is half that across A.

Try again?

3. Mar 4, 2017

### ikihi

The voltage drop of B+C equals the voltage drop of A. This is so because light bulb A equals the voltage drop across the equivalent of B+C since light bulbs in parallel have the same voltage. So B or C has half the voltage that A does; and because of that, the power for B and C is equal, but B or C is less power than A.

4. Mar 4, 2017

Yes, good.