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Question: 

Estimate the resistance of the $120$-$\textrm{V}_\textrm{rms}$ circuits in your house as seen by the power company, when

  1. everything electrical is unplugged, and
  2. two 75-W lightbulbs are on.

Source: Giancoli, Douglas C., Physics: Principles with Applications, 7th Edition, 2014.

Quick Answer: 
  1. Resistance appears infinite.
  2. $96 \Omega$

Giancoli 7th Edition, Chapter 18, Problem 49

(0:53)

Chapter 18, Problem 49 is solved.

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Transcript for this Giancoli solution

This is Giancoli Answers with Mr. Dychko. he power output of a device is the rms current that it draws, multiplied by the rms voltage across it. And we can solve this for V rms by dividing both sides by the rms current. And that gives V rms is power divided by rms current. But we don't know what this is, but we know that peak current instead. So we'll have to work with that. So, we know the peak -- or -- the rms current is peak current divided by root two. And so, we can substitute this in place of I rms. So, when we divide by rms current, it's the same as multiplying by the reciprocal of the rms current. So we're gonna multiply by the reciprocal of this here, so which means flip it over. So we have, V rms is gonna be P times root two over I naught, peak current. And, that's gonna be 1500 watts, times root two, divided by 6.4 amps, which is about 330 volts rms.