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

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SUMMARY

The discussion focuses on calculating the cost of running a 78.9 W light bulb at a reduced voltage of 59.2 V. The user initially attempted to compute the monthly cost using the formula (Power/Voltage)(New Voltage)(Time)(Cost per kWh), resulting in an incorrect answer of 153.58 cents. The correct approach involves first calculating the resistance of the bulb using Ohm's Law (V=IR), then determining the current at the new voltage before applying the cost conversion factors to find the accurate monthly expense.

PREREQUISITES
  • Understanding of Ohm's Law (V=IR)
  • Basic knowledge of electrical power calculations (P=VI)
  • Familiarity with unit conversions (kW to W, hours to days)
  • Ability to calculate energy costs based on usage
NEXT STEPS
  • Calculate resistance using the formula R = V^2 / P for the light bulb.
  • Determine the current at 59.2 V using the formula I = V/R.
  • Research energy cost calculations for different voltage scenarios.
  • Explore the impact of voltage on power consumption in electrical devices.
USEFUL FOR

Electrical engineers, physics students, and anyone involved in energy cost analysis or electrical device performance optimization.

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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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