# Bulbs connected in series

1. Sep 16, 2013

### nil1996

1. The problem statement, all variables and given/known data
Two electric bulbs,each designed to operate with a power of 500 watt is connected in series on a 220 volt line.Now if they are connected to a line of 110v then the power generated by each bulb is??

2. Relevant equations

3. The attempt at a solution
When i solve it i get more variables that equations.
The equations that i have prepared are--
500=I2R1
500=I2R2
if i divide these equations i get R1=R2.But that is not possible.What is wrong in dividing these equations??
next--
220/(R1+R2)=I
I am not able to find any value here.

plz help

2. Sep 16, 2013

### Staff: Mentor

"Designed to operate with 500W" is always for some specific voltage. To me, it is unclear which voltage is meant here, I guess 220V or (if the bulbs are designed to be used in series) 110V. In both cases, you can calculate the resistance of the bulbs. That will stay the same* independent of the voltage applied.
At different voltages, the bulbs will use a different power - that's why the question asks to calculate the power.

*not exactly, but you can use this assumption

3. Sep 16, 2013

### CWatters

What mbf said.

It's not clear what voltage the bulbs are designed for. eg at what voltage they will consume 500W. You will get different answers if you assume they are designed for 220 or 110V. I would work the problem twice.

Perhaps start by assuming they are 220V rated and using P = V2/R to calculate R.

4. Sep 16, 2013

### nil1996

they are designed to consume 500watt power at 220volts but it is asked to calculate power consumed at 110 volts supply.

5. Sep 16, 2013

### nil1996

is the "V" here supply voltage or voltage across the resistors(which will be different).Also what is the "R" here(total resistance or individual).

6. Sep 16, 2013

### Staff: Mentor

Who is mbf?

@nil1996: V is the design voltage (220V) and P is the design power (500W). As both are for a single bulb, the resulting resistance value is for a single bulb as well.

7. Sep 16, 2013

### CWatters

Sorry I typed to fast.

8. Sep 16, 2013

### CWatters

It's still not 100% clear but I guess you mean each bulb consumes 500W when each bulb is connected to 220V.

In which case use the equation I posted to calculate the equivalent resistance..

P = V2/R
so
R = V2/P
= 2202/500
= 96.8 Ohms

Note that's for each bulb not for two in series or anything like that.

Now work out the power when connected as per the question.