Calculating Power in AC Circuits: Solving for Minimum and Maximum Power Values

AI Thread Summary
The discussion revolves around calculating average and instantaneous power in an AC circuit with a coil connected to a 240 V (rms) line and a resistance of 34 ohms. The average power is calculated as 847 W using the formula Pav = (0.5)V^2 / R. The user struggles with determining the minimum and maximum instantaneous power values, questioning the need for angular frequency and time. Responses clarify that if the load is purely resistive, the maximum and minimum power values can be derived from the sine squared function without needing specific time values. The key takeaway is that the maximum and minimum power values depend solely on the maximum and minimum values of the sine squared function.
physics noob
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hey guys, here's my problem...

coil connected to a 240 V (rms) ac line has a resistance of 34 ohms. what is the average power used

(.5)V^2 /R so Pav = 847 W

then it asks what are the min and max values of the instanteous power?

this is where I am stuck, i think i need to use the equation

P(t)= IVsin^2 (omega(T)) but how do i know what t is, and the angular freq? any help would be greatly appriciated,,,,also any knowledge on the topic would help, cause these ac circuits are kind of hard for me... thanks
 
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V0 = root2 * Vrms

got it
 
me again...im pretty sure i messed up every step on this problem... still need help
 
physics noob said:
me again...im pretty sure i messed up every step on this problem... still need help
The word "coil" usually implies some inductance, while "resistance" instead of "impedence" implies no inductance. If you have some inductance, then you need to rethink the problem. If the load is purely resistive, then the current and voltage are in phase and you can use your simple relationships between rms and peak voltage and current. You do not need the frequency or the specific times at which the instantaneous power is max and min; all you need is the maximum and minumum possible values of the squared sine function.
 
no, no inductance...you said use the square of the sine function, but doesn't that depend on angular frequency and time?,... can i just use

Vo = IoR ?
 
physics noob said:
no, no inductance...you said use the square of the sine function, but doesn't that depend on angular frequency and time?,... can i just use

Vo = IoR ?

The sine function does depend on frequency and time, but you are not asked to find when the minimum and maximum occur, just the power values at the minimum and maximum. What is the maximum (minimum) possible value of sine squared?
 
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