Solve Unresolved Past Paper Question: Physics 11/11

  • Thread starter FunkyDwarf
  • Start date
  • Tags
    Paper
In summary, Daniel was trying to find the power factor for a parallel LCR circuit, but he was having trouble because he made a mistake in the equation. He fixed the problem by remembering that -iwC is the correct symbol for the power factor.
  • #1
FunkyDwarf
489
0
Past paper question - please help, still unresolved! (11/11)

Hey guys

Im studyin for my physics end of year exam (only first year so its prob easy peasy for you guys) but unfortunately i have no answer key as yet so i don't know if what I am doing is right. I get a plausable answer but i just wanted to be sure incase i never get the answers from lecturers (im badgering em heaps don't worry)

Ok this might take a while to set out in latex thing but ill give it a go

The question is about power factor of a parallel LCR ciruit but in a nutshell i need to find C such that the imaginary part of the complex impedance vanishes (ie angle is zero)

Were given that
[tex]L= 5 \\
R = 4 [/tex] and were doing this at mains freq of [tex] \omega = 50Hz[/tex]

Unfortunately i can't upload the image right now but all you need to know is that this parallel circuit has 2 arms, one just has the capacitor the other the resistor and inductor in series.

So, working out impedances:
[tex] Z = \frac{1}{(R+\omega Li)} - \omega Ci[/tex] Now I've always used minus i for the capacitor part because of the angle difference in the phasor, i assume i do that here.

Next i try to get the imaginary parts off the bottom.
[tex] \frac{1}{(R+ \omega Li)} * \frac{(R- \omega Li)}{(R-\omega Li)} = \frac{R- \omega Li}{((R^2)+(\omega L)^2)}[/tex]

Then putting it back in and making it one big happy fraction
[tex]\frac{(R-\omega Li - (\omega Ci)(R^2 +(\omega L)^2)}{(R^2 +(\omega L)^2)}

= \frac{(R-\omega Li - \omega CR^2 i - \omega ^3 L^2Ci)}{(R^2 +(\omega L)^2)}
[/tex]
Now just taking the imaginary bit and equating to zero (getting rid of denominator for obvious reasons)
[tex]
(-\omega L-\omega CR^2-(\omega )^3 L^2C = 0
[/tex] Then just shoving in those numbers and ignoring the minus sign (which has me rather worried) i get C = 7.9 x 10^-5 farad.

Hope this makes sense and I am right!

Cheers for help (please don't flame my stupidity if i made a stupid mistake, first time with latex too)
-G
 
Last edited:
Physics news on Phys.org
  • #2
The [tex] tag closes with [ / tex] (without the spaces of course)

Daniel.
 
  • #3
yah just fixing that now cheers :D tryin to get omega to work too :S

hmm some of the changes arent saving. anyway
 
Last edited:
  • #4
It's \omega and the same for the other symbols and functions.

Daniel.
 
  • #5
yah its not saving the changesEDIT... ah there we go
 
  • #6
Ok i found out what's wrong, its the fact that i used -iwC instead of +iwC, which screws it all up. However i always thought that due to phasors and phase differences etc your supposed to use the minus. Anyone?
 
Last edited:

Related to Solve Unresolved Past Paper Question: Physics 11/11

1. What is the purpose of solving unresolved past paper questions in Physics?

Solving unresolved past paper questions in Physics helps students to practice and apply their knowledge of the subject, identify any areas of weakness, and improve their problem-solving skills. It also gives them a better understanding of the types of questions that may appear on their exams.

2. How do I approach solving unresolved past paper questions in Physics?

The most effective approach is to start by carefully reading and understanding the question, then breaking it down into smaller parts and applying relevant concepts and formulas. It is also important to check your work and make sure you have answered the question correctly.

3. Is it necessary to solve unresolved past paper questions in Physics in a specific order?

No, there is no specific order that needs to be followed. It is recommended to start with easier questions and gradually move on to more challenging ones. You can also focus on specific topics or areas that you need more practice in.

4. How many unresolved past paper questions in Physics should I solve?

It is not about the quantity, but the quality of your practice. It is better to thoroughly understand and solve a smaller number of questions than to rush through a large number of questions without fully grasping the concepts. It is recommended to solve a variety of questions from different topics.

5. Can solving unresolved past paper questions in Physics guarantee a good grade on my exam?

While solving unresolved past paper questions can greatly improve your understanding and preparation for the exam, it is not a guarantee of a good grade. It is important to also review class notes, textbooks, and other study materials to comprehensively prepare for the exam.

Similar threads

  • Advanced Physics Homework Help
Replies
9
Views
626
Replies
2
Views
1K
  • Advanced Physics Homework Help
Replies
5
Views
1K
  • Advanced Physics Homework Help
Replies
19
Views
3K
  • Introductory Physics Homework Help
Replies
4
Views
306
  • Engineering and Comp Sci Homework Help
Replies
19
Views
2K
  • Introductory Physics Homework Help
Replies
8
Views
337
  • Advanced Physics Homework Help
Replies
3
Views
5K
  • Introductory Physics Homework Help
Replies
3
Views
832
  • Introductory Physics Homework Help
Replies
19
Views
915
Back
Top