If a 75 W lightbulb is 15% efficient, how many joules of light energy does the bulb produce every minute?

5400 W-hr

Explanation:

Given:

Power used by energy efficient bulb = 15 W

Power used by incandescent bulb = 60 W

Total time of bulb used = 4 hours/day × 30 days = 120 hours

Now,

Energy used by the individual bulb by using them for 120 hours

we know,

Energy = Power × Time

thus,

Energy consumed by energy efficient bulb = 15 W × 120 hour = 1800 W-hr

Energy consumed by  incandescent bulb = 60 W × 120 hour = 7200 W-hr

hence,  the energy saved will be = 7200 - 1800 = 5400 W-hr

Explanation:

12000j is work done

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