Calculate the resistance, then find the current at the new voltage

AI Thread Summary
To calculate the cost of running a 78.9 W light bulb at a reduced voltage of 59.2 V, first determine the resistance using the formula R = V^2 / P, which gives a resistance of approximately 146.5 ohms. Next, find the current at the new voltage using I = V / R, resulting in a current of about 0.404 A. The power consumed at 59.2 V can then be calculated as P = VI, yielding approximately 24.1 W. Finally, to find the monthly cost, use the formula for energy cost, which results in approximately 35.29 cents for 30 days. The initial calculation was incorrect due to not adjusting the power consumption for the lower voltage.
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A 78.9 W, 107.3 V light bulb is plugged into a 59.2 V outlet. If energy costs 4.9 cents/kW*h, how much does it cost per month (30 days) to leave the light bulb turned on? Answer in units of cents.

This is what I did:

(78.9W/107.3V)(59.2V)(24hours)(30days)(4.9cents/kW*h)(1kW/1000W)

I got 153.58 cents. I know the answer is incorrect, but where did I go wrong?
 
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P=VI
V=IR

Calculate the resistance, then find the current at the new voltage. Then use conversion factors to get your price.
 
Thanks, that helped!
 

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