Minimum resistance to prevent overheating in AC circuit

AI Thread Summary
The discussion centers on a calculation involving minimum resistance to prevent overheating in an AC circuit, where the user calculated resistance (R) as 8.2 ohms using RMS current. There is confusion regarding the answer key, which used the peak current (Io) instead of the RMS current (Irms), leading to a discrepancy. Participants confirm the user's calculation is correct and suggest that the error lies in the answer key, which can happen in educational materials. They recommend consulting the instructor or checking for errata from the book publisher for clarification. The conversation highlights the importance of using the correct current type in AC circuit calculations.
songoku
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Homework Statement
An alternating current supply is connected in series with a resistor R. The variation with time t (measured in seconds) of the current I (measured in amps) in the resistor is given by the expression I = 9.9 sin(380t). To prevent over-heating, the mean power dissipated in resistor R must not exceed 400 W. Calculate the minimum resistance of R
Relevant Equations
mean power = ##I_{rms}^2 \times R##
My calculation: ##400=\left(\frac{9.9}{\sqrt{2}}\right)^2 R## and I got R = 8.2 ohm

But the answer key used Io (9.9 A) instead of Irms

Why?

Thanks
 
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You are correct(8.2Ω). It's a mistake in the 'answer key' - it sometimes happens.
 
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It would only be speculation on my part as to "why" they did not use RMS current. Books make mistakes sometimes. Did you ask your instructor about it? Does the book publisher have a website for errata?
 
scottdave said:
It would only be speculation on my part as to "why" they did not use RMS current. Books make mistakes sometimes. Did you ask your instructor about it? Does the book publisher have a website for errata?
I haven't asked him, I just did the practice myself. The question is not from book, it is given by the teacher and on the last page there is working and answer to help the students.

Thank you very much for the help Steve4Physics and scottdave
 
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