Question

The figure (Figure 1) shows voltage and current graphs for a resistor.

Draw the resistor’s voltage and current phasors at t=15ms.
Draw the vectors with their tails at the origin. The orientation of your vectors will be graded. The exact length of your vectors will not be graded.

Answer 1

f=25 hz and r=20 ohms. not sure how to do the graph

Answer 2

@ganeshie8 , do you have any idea?

Answer 3

Can you identify the phase angle; the difference in phase between the voltage and current?

Answer 4

no.

Answer 5

at t= 0 the phasor is vertical, because this is a cosine.

you are asked to rotate the phasor to t = 15ms

you have been told freq -- and you can also see period from the drawing....
.04s

so you can figure out how far through a 2π cycle the phasor is at t = 15ms which gives you the vectors angle.

Answer 6

how do i do that??

Answer 7

it takes 40ms for a fully cycle. do you see that from the wave diagram?

Answer 8

yeah, i was able to find the resistance and hz without problem, but i can't figure out how to do angle.

Answer 9

it wasn't given to us. we solved for it

Answer 10

well at t= 0, the angle is pi/2, right? it's a cosine wave.

agree?

Answer 11

yeah, some of the equations are cos wt and some are sin wt. so i'm confused

Answer 12

wouldn't get tied up in stuff like that, the vector at t= 0 has to point up in the vertical direction because V(t) = Vmax at t= 0. thus you are measuring from that point.

the amount you have to measure? well the period is .04s = 40ms. so the vector rotates a full 2π in 40ms. the question is, how much does it rotate in 15ms. that is what you are drawing the phasor for.

Answer 13

2pi*.015/.04=2.35, but how is that an angle?

Answer 14

hint:
we can write both your function, voltage and current as below:
\[\Large \begin{gathered}
V\left( t \right) = 10\cos \left( {\frac{{2\pi }}{{0.04}}t} \right) \hfill \\
I\left( t \right) = 0.50\cos \left( {\frac{{2\pi }}{{0.04}}t} \right) \hfill \\
\end{gathered} \]

Answer 15

functions*

Answer 16

yes, but how does that give you an angle?

Answer 17

@saiken2009
keep the π in the angle and remember that 2π = 360, yes?

Answer 18

convert to degrees if you are using a protractor

Answer 19

3pi/4

Answer 20

is that for current or voltage? we need two vectors.

Answer 21

maybe these formulas can be useful:
\[\Large \begin{gathered}
V\left( t \right) = 10\cos \left( {\frac{{2\pi }}{{0.04}}t} \right) = 10\sin \left( {\frac{{2\pi }}{{0.04}}t + \frac{\pi }{2}} \right) \hfill \\
I\left( t \right) = 0.50\cos \left( {\frac{{2\pi }}{{0.04}}t} \right) = 0.50\sin \left( {\frac{{2\pi }}{{0.04}}t + \frac{\pi }{2}} \right) \hfill \\
\end{gathered} \]

Answer 22

since we have to work with sinusoidal functions

Answer 23

ok, so one of you is using sin, the other cosine. i'm getting more confused..............

Answer 24

more explanation:
here is the relationship between rotating vectors and sinusoidal functions: