# I need help figuring watts used

Discussion in 'Homework Help' started by Lightning bug, Apr 1, 2009.

1. ### Lightning bug Thread Starter New Member

Apr 1, 2009
3
0
Hello
I just took my first electric class and there are two questions I just cant get. #1. If you have a 100 watt 110 volt light bulb being feed with 120 volts how many watts are being used. And the second one is hard to explain but they showed a Y phase power diagram and wanted to know which legs you would tie together for high voltage. Im lost.

2. ### PRS Well-Known Member

Aug 24, 2008
989
35
The term 110 volts harkens back to the days when power companies weren't as adept at regulating their voltage supply. Nowadays the supply is usually close to 120 volts if not right on. The NEC, which specifies design criteria for electrical systems in the USA, defines the actual voltage used in calculations as 120V. Why a company may still stamp their product with 110 volts these days is beyond me. It's probably a business decision, certainly not an engineering thing. At any rate a bulb stamped 100 Watts will use 100 watts.

As for your 3-phase problem, you need to make a drawing or at least specify the problem as your homework puts it.

3. ### veritas Active Member

Feb 7, 2008
167
0
Power is calculated as P = IV or P = V^2/R. If you assume R will be the same at both voltages (in reality, it's actually dependant on the temperature of the filament), then you can find P2 = (V2^2)/(V1^2) * P1

4. ### subtech Senior Member

Nov 21, 2006
123
5
One of the most important parts of working with things electrical is to break down problems into the basic parts.
Your light bulb problem needs just that.
Use the ratings of the lamp to calculate its resistance.
Once you know that, you can apply the 120 Volts and calculate to solve your problem.
I agree with PRS concerning your three phase problem.

5. ### Darren Holdstock Active Member

Feb 10, 2009
262
11
veritas:
Quite right - you've rather cruelly been set a question with insufficient data for a correct answer. You need the V-I characteristics of the bulb, as it's not linear.

6. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,403
1,227
The NEC may specific the calculations, but the FAA (Federal Airway Administration) specifies the tower light bulbs at x watts at 117V. So the FAA requirement is directly related to the question. As to why the Manufacturer would specify other than 120V ... the FAA is one reason.

This person is starting out, and everyone should know what the instructor is illustrating with the question.

7. ### Lightning bug Thread Starter New Member

Apr 1, 2009
3
0
Thanks Joe Im as new to this stuff as a person can get and my head is spinning already. From what I have read from everyones help I assume this is one of two things and I would like everyones opinion on it please. # 1 is it a trick question and like PRS said a 100 wattt bulb is going to use 100 watts. And if its not a trick question would this be correct. If I look at ohms law P I
V R then I need either amps,volts,or resistance to find watts. So should I start at the bulb and find the resistance by dividing the wattage of the bulb by the voltage of the bulb to get .909 ohms then multiply .909 x 120 volt to get 109 watts being used ?

8. ### wr8y Active Member

Sep 16, 2008
232
1
I think you want to follow the idea of post 3, and assume the filiment temperature does not vary with voltage. It's not reality, but I thinkt you instructor is simply trying to get you to think about what's happening.

Fact is, the temperature variation from 110v to 120v isn't THAT big of a deal.

If you can, show your work so he/she can see that you tried, and what you based your answer on!

You start with a 110 volt bulb consuming 100 watts. So find the resistance:

R = E^2/W ..... 110^2 / 100 = 121 ohms

and watts equal the square of the voltage divided by the resistance you just found, but plug in the higher voltage:

W = E^2 / R

so,

120^2 / 121 = 119 Watts

Again, the resistance really would have changed a bit - but I think your instructor assumes you will ignore that, anyway. You might, if you have room, not only show your work, but also indicate that the temperature of the filiment would change, throwing off your numbers and that you could not know how much it would change!!!

I don't see what else you are expected to do with this...

Last edited: Apr 3, 2009
9. ### Lightning bug Thread Starter New Member

Apr 1, 2009
3
0
That makes sence to me, although I messed up on figuring the ohms which I have straightened out now. Also guys I found the answer on the Wye low voltage wiring problem. When I posted this I forgot part of the problem but he wanted to know what wires you tie together to wire a motor up with wye low voltage Then which ones went on the line feeds L1 L2 and L3. Thanks Again