Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Phasor problem help

  1. Mar 2, 2006 #1
    for i = 0.4A cos (wt + 60) , how do we get i(rt) =0.35A cos(wt + 30) and then i(ct) = 0.2A cos(wt+ 120) ??
  2. jcsd
  3. Mar 2, 2006 #2
    v = 30V (cos 30 + j sin30) is given
  4. Mar 2, 2006 #3
    the circuit consists of capacitor and resistor
  5. Mar 2, 2006 #4


    User Avatar
    Science Advisor
    Homework Helper

    A good way to present a problem like this is to describe the circuit as best as you can. With your third post, it appears you have power supply V and a capacitor and resistor connected in some fashion. How is your resistor and capacitor connected?

    You gave in your first post that the current thru your resistor is at a different angle [itex]\theta [/itex] compared with your capacitor. Do you notice anything special about the difference between the two angles?
  6. Mar 2, 2006 #5
    no,there is just a z and the current and voltage is given,not even a circuit.
    the current for capacitor and resistor is the answer and i don't understand how they got the answer
  7. Mar 3, 2006 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    There are several possible configurations between input and output, based on two components (which component, resistor or capacitor, is first) and configurations series vs parallel vs T.

    Is this correct - v = 30V (cos 30 + j sin30) ? Is this at time, t=0?

    or should it be 30V (cos (wt+30) + j sin (wt+30)) = 30V e j (wt+30)?

    Then look at the voltage v, and see if it is phase with the current in the resistor.

    Welcome to the world of complex analysis.

    See also - http://en.wikipedia.org/wiki/Phasor_(electronics)

    http://en.wikibooks.org/wiki/Electronics:Phasors [Broken]
    Last edited by a moderator: May 2, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook