Power factor

[bobby]

Exams in 2 weeks and power factor is the only thing really that still bugs me can somone please enlighten me I think the standard forumla is simply divide but my awnsers are still wrong.

What is the power factor of a circuit with a capacitor connected in

series, given that resistance is 6ohms and the capactive reactance is 8ohms?

What is the power factor of a circuit with a 4ohms resistor and

10ohms inductor?

Appreciate any help.

 

[sarni31]

HI

ideally you need to know phasor diagrams, trig, pythagoras oh yeah and SOHCAHTOA

the values you have represent sohcahTan Opp Adj

so invTAN(for degrees) x 10/4

10/4 = 2.5

invTan 2.5 = 68 degs

... I think

 

[bobby]

Thanks for your help sarni but I now have the awnser to question 2 which is 0.8 Lagging, I just need to know how to work it out the programme gave me a hint saying "power factor forumla: R/Z" which gives me 0.4 which is one of the awnser to choose from but its wrong

Edit: just figured out that Z actually means impedance so if the question does not give me the impedance I have to work it out using the numbers they gave me and the impedance formula...divide that awnser by the resistance given to me and theres my awnser. BUT still stumped on the second question :| hopefully its their cock up.

 

[WayneL]

Hi Mate,

I'm doing the same as you - exam in two weeks

I think question two is worked out the same....

Find Z first

Z = √R

 

[bobby]

cheers for that wayne it makes sense and should be like that but according to this mock exam its actually 0.8 lagging I bashed in too many forumlas trying to get anything near that and it wern't happening. 0.37 wasnt even on the list of awnsers to pick from

 

[green-hornet]

http://www.allaboutcircuits.com/vol_2/chpt_11/3.html

 

[andy7751]

just checked waynes calculation and he is correct with the figures you have stated bobby