- #1

Gavroy

- 235

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i am a little bit confused about the reactive power.

we defined it as

Q=UI sin(a)

and the apparent power as

S=UI

Now I have to solve this excercise:

Which percentage of heat loss in a power grid is caused by the reactive power, when the powerfactor is cos(a)=0,85?

Now I thought:

Q/S=sin(a) would be the solution

but instead of this it is said that one has to set:

Q= I² R sin²(a)

S= I² R

and therefore Q/S=sin²(a)

now my question is: when can I use Q=U I sin(a) and when do I have to use Q= I² R sin²(a)...can somebody explain this to me?