# Homework Help: Watts (HW)

1. Feb 5, 2008

### bNo

Hello, I'm having problems with this question. I'm not exactly sure how to determine how much watts in a day, or in a second the lamp takes.

1. The problem statement, all variables and given/known data
Theres a lamp that absorbs 60 W when connected to a 120-V source.
Q: Find the cost of operating the lamp for 24 hrs when electricity cost 6.8cents/kWh.

2. Relevant equations
current = power/voltage

3. The attempt at a solution
1)I took a guess and said 60W equals in one second.
60W(60sec x 60min x 24hrs) = 5,184,000
2)Then, 24hrs into kW = 24000 kW in a day.
3)(5,184,000/24000) x 6.8 cents = 1468.8 cents.

I'm pretty sure I'm missing a formula for time. Thanks for any help.

2. Feb 5, 2008

### Staff: Mentor

No, a watt is a unit of power, and a kWh is a unit of energy (power multiplied by time). You are charged for energy by the electric utility company, so they charge you \$/kWh.

The rest is just being careful with units and cancelling units.

Like in that equation, you were not being careful about units. See if this makes more sense, and keep it going to get to your answer:

$$E = Power * Time = 60W * \frac{1kW}{1000W}* 24h * \frac{60 min}{h}$$

3. Feb 5, 2008

### Staff: Mentor

EDIT -- Remember to cancel units when you have the same unit over itself, like

$$\frac{W}{W} = 1$$

4. Feb 5, 2008

### bNo

Thank you for the help. That was the equation I was missing.

Your right I should have put in the units for the result. I need to learn how to do that javascript. It would look a lot nicer than, 60W(60sec/min x 60min/hr x 24hrs)

5. Feb 6, 2008

### Staff: Mentor

There's a sticky post at the top of the PF tutorials forum about using LaTex:

https://www.physicsforums.com/forumdisplay.php?f=151

You can also click on the QUOTE button on a post with LaTex in it (like mine above) to see the underlying tex that was used to make the LaTex image. There are also some buttons in the Advanced Reply window that help you to create LaTex equations.