# Homework Help: How much power does a light bulb connected to a 120 V outlet use if it draws .5 A of?

1. Jul 29, 2009

### graphicer89

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

A) How much power does a light bulb connected to a 120 V outlet use if it draws .5 A of current? Show your calculations

B) One kilowatt hour is a measure of energy which is equal to 3,600,000 J. How many hours would you have to leave on the light bulb from part A (60 W) in order to expend one kilowatt hour of energy (Remember that 1 W= 1 J/s)

C) If one kilowatt hour costs $.10 how long would you have to leave the light bulb from Part A) on in order to spend$1.00 on electricity? Show your calculations

2. Relevant equations

A) Current x voltage = energy

B) ????

3. The attempt at a solution

A) 120 V x .5 A = 60 W

B) I know that 60 w = 60 joules........so would it be 3,600,000 joules / 60 joules = 60,000 hours? I honest do not think that makes sense ..please help me out.....

C) If i got the right answer from B...then 60,000 hours x .10 = 6,000 hours?......please help me out i am so close to finishing physics...and i just need help with this.....

2. Jul 29, 2009

### queenofbabes

Re: How much power does a light bulb connected to a 120 V outlet use if it draws .5 A

60w = 60J per second.
So it's 60,000 seconds, for one kilowatt hour

for part C, how many kilowatt hours are you looking at?

3. Jul 29, 2009

### diazona

Re: How much power does a light bulb connected to a 120 V outlet use if it draws .5 A

You should have divided by 60 watts (not joules) there, and then you would have gotten 60000 seconds.

4. Jul 30, 2009

### Redbelly98

Staff Emeritus
Re: How much power does a light bulb connected to a 120 V outlet use if it draws .5 A

Not quite, that should be

Current x voltage = power

As the others hinted, power and energy are different, but related, concepts.

5. Jul 30, 2009

### graphicer89

Re: How much power does a light bulb connected to a 120 V outlet use if it draws .5 A

oh ok ......so A is right then........
B.....if its 60,000 seconds....divide that by 60 (60 seconds in minute)and i get 1,000 hours which makes a little bit more sense but still i dont know if im off or im on the answer......i feel that i have to keep dividing....so i think its something 16 hours????
C........hm well 60,000 seconds divided by .10 = 6,000 then divide again by 60 (seconds in 1 minute) gives me 100 so its 100 minutes or 1 1/2........same thing with this i really feel off.........please help me out.....this is basically easy math but i dont know why i cant capture the point....

6. Jul 30, 2009

### Staff: Mentor

Re: How much power does a light bulb connected to a 120 V outlet use if it draws .5 A

Yes, the power drawn by this bulb is 60 W.
60,000 seconds divided by 60 gives you 1000 minutes, not hours. Divide again by 60 to get hours.

First figure out how many kilowatt-hours you need to spend $1.00. And you already know how long it takes to burn one kilowatt-hour from part B. Multiply! 7. Jul 30, 2009 ### graphicer89 Re: How much power does a light bulb connected to a 120 V outlet use if it draws .5 A On C...so if it takes lets say 16 hours for one KW hour.....$.10 x 10= $1.00 so i multiply 16 hours x 10= 160 hours for it to reach$1.00.....is this right?

8. Jul 30, 2009

### Staff: Mentor

Re: How much power does a light bulb connected to a 120 V outlet use if it draws .5 A

Exactly. (But 16 hours is only roughly right for B. Figure that part out more carefully.)

9. Jul 30, 2009

### graphicer89

Re: How much power does a light bulb connected to a 120 V outlet use if it draws .5 A

Well i got 16.6666667 but basically rounded up = 17 hours....so then for C its 170 hours...am i on or off?

10. Jul 30, 2009

### Staff: Mentor

Re: How much power does a light bulb connected to a 120 V outlet use if it draws .5 A

I say you are smack on... bullseye!

11. Jul 30, 2009

### graphicer89

Re: How much power does a light bulb connected to a 120 V outlet use if it draws .5 A

Oh wow i actually smack into it....but seriously thanks for the help...