# Calculating Vrms from a given function

Discussion in 'Homework Help' started by blalaw, Feb 11, 2014.

1. ### blalaw Thread Starter New Member

Oct 28, 2013
9
0
I need to calculate the effective RMS values given
a) g(t) = 2 + 3cos(100t) + 4cos(100t - 120)
b) h(t) = 2 + 3cos(100t) + 4cos(101t - 120)

I am pretty sure the first one is straight forward since 2 is a constant voltage then the value will just be two the second part would be 3/sqrt(2) and the last part would be 4/sqrt(2) in phase -120 degrees and just add them up if that is correct I believe.

The second part I am kind of lost because I am not sure what to do with the 101t since it is not the same as the 100t.

2. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,300
335
What result do you get for the first part after you add them up?

3. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Ask yourself if the method you are trying to use makes sense in the limiting cases.

For instance, what if you have

v(t) = 3V cos(ωt) + 4V cos(ωt + 180°)

Or what if you have

v(t) = 3V cos(ωt) + 4V cos(ωt + 0°)