You buy a 75-W lightbulb in Europe, where electricity is delivered to home at 240 V

You buy a 75-W lightbulb in Europe, where electricity is delivered to home at 240 V. If you use the lightbulb in the United States at120 V (assume its resistance does not change). how bright will it be relative to 75-W 120-V Bulbs? [ Hint: assume roughly that brightness is proportional to power consumed]

Solutions

Expert Solution

resistance of bulb is found by:

\(\mathrm{P}=\mathrm{V}^{2} / \mathrm{R} \quad 75=240^{2} / \mathrm{R} \quad \mathrm{R}=768 \mathrm{ohms}\)

Now find the power it uses when plugged into \(120 \mathrm{~V}\) :

\(\mathrm{P}=\mathrm{V}^{2} / \mathrm{R}=120^{2} / 768=18.75 \mathrm{Watts}\)

So compared to a 75 watt bulb is it is only:

18.75 / 75 = 1/4 as bright

Dr. OWL answered 3 years ago

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