Homework Help: Power and Resistor problem

1. Jan 25, 2009

johnson.3131

A blow-dryer and a vacuum cleaner each operate with a voltage of 120 V. The current rating of the blow-dryer is 13 A, and that of the vacuum cleaner is 3.0 A.
(a) Determine the power consumed by the blow-dryer.
1 W

(b) Determine the power consumed by the vacuum cleaner.
2 W

(c) Determine the ratio of the energy used by the blow-dryer in 21 minutes to the energy used by the vacuum cleaner in one-half hour.
Ebd / Evc = 3

Relevant Equations R=E^2/P
I=E/R

I mostly get confusedon what E is and what R is in the problem

2. Jan 26, 2009

Gib Z

Hello johnson!

I'm afraid I just don't see how you get yours answers for Part a) and b). Here, we could use the equation $$P = IV$$, so for eg part a), I get approximately 1.6 kW, not 1 W. I think there may be a misunderstanding here.

For part c), Power is the rate of Energy per time, ie $$P = E/t$$. Rearrang the equation for E (Energy). We can work out how much energy each item takes by then replacing P with the value we got earlier, and t with the time we are told (remember to convert to SI units). Then we can divide appropriately to get the ratio.

We don't need R anywhere.

3. Jan 26, 2009

johnson.3131

thanks soo much...i just figured out that I messed up the equations like crazy...i have the right answers now.