# Homework question!

1. Oct 3, 2007

### Nymn

Sorry if this is the wrong forum, it's a homework question I'm stuck on, though. :|

The question is that if a 60 Watt bulb has a hot resistance of 240 ohms when it's connected to a 120 VAC source, how much power is the bulb using?

Now I'm not exactly sure what equation to use, I thought it would be P = V squared divided by R? That just gives me 60 W, though. What equation do I use to get the power used?

2. Oct 3, 2007

### learningphysics

looks right to me. power being used is V^2/R = 60W.

Did you post the question exactly as it is written?

3. Oct 3, 2007

### Nymn

Yah, that's pretty much how it's written. Here's the exact question:

A 60 Watt incandescent bulb has a "hot" resistance of 240 ohms when connected to a 120 VACrms source. How much power is the light bulb using?

So...the answer is 60 Watts, then? Is it a trick question?

4. Oct 3, 2007

### learningphysics

Oh wait... in order to get average power, you need to use rms voltage, not peak voltage.

$$v_{rms} = \frac{v_{peak}}{\sqrt{2}}$$

then use average power = $$\frac{(v_{rms})^2}{r}$$

5. Oct 3, 2007

### learningphysics

Oh crap. sorry again... so they give you 120V rms not 120peak...

So then 60W is the answer. hmmm... strange question...

6. Oct 3, 2007

### Nymn

Maybe my teacher is checking to see if we're paying attention. :P