How do you know if pf (power factor) is leading or lagging

[pokie_panda]

Homework Statement

power factor leading or lagging

v(t)=3.5cos(600t-120)
i(t)=2.3sin(600t+45)

Homework Equations

pf=cos(Vθ-Iθ)

The Attempt at a Solution

pf=cos(-75)
=.25881905

I get confused, How do we figure out whether it leads or lags.

 

[verty]

Read here: http://en.wikipedia.org/wiki/Leading_and_lagging_current

Then use the phase angles to decide which it is. (This should be correct, I apologize if it isn't.)

 

[NascentOxygen]

I get confused, How do we figure out whether it leads or lags.

You draw the phasor diagram.

The phasors rotate counter-clockwise. Consider a pair rotating together, and differing by an angle ≤ 90°. Whichever passes over an axis first is the leading phasor of that pair.

 

[rude man]

First, change the cos voltage to a sin function. Use sin(x) = cos(x - 90 deg).
Then, you get V = sin(wt + ψ₁) and i = sin(wt + ψ₂).

V leads i if ψ₁ > ψ₂ or ψ₁ - ψ₂ > 0.