A 120-V fish-tank heater is rated at 130 W. Calculate a) the current through the heater when it is operating, and b) its resistance.

a) 1.1 A b) 110 \Omega Note: in part b), even though the formula and final answer are correct, I forgot to write that the 120 V is squared, even though during my calculations it is in fact squared.

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Giancoli's Physics: Principles with Applications, 7th Edition

Electric Currents

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18-2 and 18-3: Electric Current, Resistance, Ohm's Law

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Problem 35

A 120-V fish-tank heater is rated at 130 W. Calculate

the current through the heater when it is operating, and
its resistance.

$1.1 \textrm{ A}$
$110 \Omega$

Note: in part b), even though the formula and final answer are correct, I forgot to write that the

120 \textrm{ V}

is squared, even though during my calculations it is in fact squared.

Giancoli 7th Edition, Chapter 18, Problem 35 solution video poster

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VIDEO TRANSCRIPT

This is Giancoli Answers with Mr. Dychko. The power output of this fish tank heater is the current through it, times the voltage across it. And we can solve for the current by dividing both sides by V, and on one side we get I isolated, and on the other side we get power divided by voltage. So the current is 130 watts divided by 120 volts, which is 1.1 amps. And then in Part B, we could say that power is voltage squared divided by resistance. And we can solve for R, by multiplying both sides by R over P. And on the left side, we have R, and on the right side we have voltage squared divided by power. So it's a 120 volts squared, divided by 130 watts, which is 110 ohms of resistance.

COMMENTS

By cj_vana on Mon, 4/3/2017 - 8:41 AM

For part b, should the 120V be squared?

By Mr. Dychko on Mon, 4/17/2017 - 1:35 PM

Hi cj_vana, thanks for pointing this out. It turn out that I did, in fact, square the $120 \textrm{ V}$ when doing the calculation, even though I forgot to write that the number is squared in the working. I'll make a note about this for other students.

All the best,
Mr. Dychko

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