I wan't to write the instantaneous value expression from the curves below.

The voltage is the bigger one and the other is the current and the scale for the voltage is 1 V and 10 mA for the current.

f = 1/(20E-3) = 50 Hz

For the voltage I get |U| = 2 V and α = 0° => u(t) = 2 Sin (2π50t) V

For the current I get |I| 10 mA and from the graphs it looks like the current is ~1,25 ms behind so I get α =(-1,25/20)*360 = -21,6° => i(t) = 10 Sin(2π50t - 21,6°) mA
The awnser for the current is wrong and it should be 10 Sin(2π50t - 30°) mA. What am I doing wrong when calculating the phase shift?



http://s32.postimg.org/yhmu67ryd/moment.png

It appears that you've incorrectly estimated the voltage-current lag. -30° implies α=-30=(lag/20)*360 = -5/3 ~= -1.6667 milliseconds. Surely you were given the equations for those two so you could find the derivatives of each, compute the exact peaks, and determine the lag from that?

Thanks!

I don't seem te get it now when I look at it again. I was not given the equations for thoes two and from the graphs it looks like the lag is about a half of a half which would be 5/4 = -1.25 ms.
Can you show me how you get -1,67 just from looking at the graphs?

From this grap I get: 50/4 and (-12/50)*360 =- 90 deg but if I look at the solution it uses -60 deg.


http://s33.postimg.org/ij47d6vgv/grafer222.png

Solved it... pi/3 = 60....