Circuit help

[bobby2017]

Hi guys can someone please help?

 

[bobby2017]

Again guys I have absolutely no idea what i'm looking at, could you guys point me in the right direction or if you are feeling really generous just do it for me haha !!
Thanks in advance once again

 

[marconi]

Bobby2017: Indeed there are many generous folk in the EF but they tend to baulk at doing other's homework. This is the second electrical science problem you have had no inkling about how to proceed. Does your tutor know of your difficulty? You said you are reading mechanical engineering; nevertheless these sorts of problems you ought to be able to tackle to be a rounded engineer.

Please let us know why you (or the whole class) have been set problems you cannot answer.

 

[Baddegg]

What courses are you doing exactly @bobby2017?.....and what is your background that got you to this point?.....I’m not judging as I couldn’t answer the questions myself having never tried/looked before, but I find it a little bemusing that someone has found themselves in the situation you’re in and not have even an inkling of where to start, as before not judging or being a critic that’s not me I admire anyone that gets up and tries to better emselves but this thread is making me curious.....

 

[DPG]

I don't see how these two questions have come up on a mechanical engineering course. Are you sure you are in the right classroom?

 

[Julie.]

I don't see how these two questions have come up on a mechanical engineering course. Are you sure you are in the right classroom?

I would have thought it's a first degree course, electrical students do a lot of mechanical stuff in the first year and conversely mechanical engineering students do a similar amount of electrical stuff.

These are the sorts of questions from basic circuit theory

To the op:

You usually get recommended a book or two per subject, I would suggest you do invest in one for this subject if you are struggling, from my experience you don't get much help from the lecturers as they expect you to be able to learn it

Back in the '70s the on-campus bookshop would sell 2nd hand books from the previous year's students, or the su would run a similar scheme - once you move on you sell the books back.

Don't know if that's the case now.

 

[littlespark]

If the tutor is expecting you to know these answers, then maybe HE is in the wrong classroom.

 

[bobby2017]

Bobby2017: Indeed there are many generous folk in the EF but they tend to baulk at doing other's homework. This is the second electrical science problem you have had no inkling about how to proceed. Does your tutor know of your difficulty? You said you are reading mechanical engineering; nevertheless these sorts of problems you ought to be able to tackle to be a rounded engineer.

Please let us know why you (or the whole class) have been set problems you cannot answer.

Hi its not just me in my class that is struggling, its pretty much the whole class that is struggling, we have had a "stand in" teacher who isn't really the best of help to be honest. we have raised this as a class to our course leader, but his response was that they are struggling to recruit a good permanent electronics teacher. So for the mean time, we can either make do with what we have or go without a teacher (both are just as bad as each other by the way). So its a lose lose situation. On the brighter side for us, with no disrespect meant for anyone, is that we only have this 1 assignment for electronics and this is the final question on the assignment. I think i have managed the rest of the assignment with a lot of research but these circuits just go right over my head.
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Hi its not just me in my class that is struggling, its pretty much the whole class that is struggling, we have had a "stand in" teacher who isn't really the best of help to be honest. we have raised this as a class to our course leader, but his response was that they are struggling to recruit a good permanent electronics teacher. So for the mean time, we can either make do with what we have or go without a teacher (both are just as bad as each other by the way). So its a lose lose situation. On the brighter side for us, with no disrespect meant for anyone, is that we only have this 1 assignment for electronics and this is the final question on the assignment. I think i have managed the rest of the assignment with a lot of research but these circuits just go right over my head.

 

[marconi]

Do you know how to do complex number algebra eg:

(a + bj) divided by (c+dj)?

 

[bobby2017]

Do you know how to do complex number algebra eg:

(a + bj) divided by (c+dj)?

Yes i can do algebra and solve equations. Its just knowing what equations to use to for each question that i struggle with

 

[marconi]

Do you know how to work out the total resistance of two resistors in series or in parallel? If you do then you can do the same for impedances - see a little further down this post.

Also do you know:

1. that the current(I) through an inductor lags the voltage (V) across it? So one can say the voltage V = jX x I where j is square root of -1 or a phase shift of 90 degrees and X is the reactance of the inductor at the frequency of the voltage V?

2. For a capacitor, V = -jX x I where this time -j means a phase shift of -90 degrees - the current leads the voltage this time.

3. that the current through a resistor is in-phase with the voltage across it so V = R x I? So no j required.

4. the reactance X of an inductor is 2 x pi x f x L where f is frequency and L is the inductance of the inductor.

5. For a capacitor X = 1/(2pi x f x C) where C is the capacitance of the capacitor.

6. That in alternating current circuits we deal in impedance which is an amalgam of resistance and the two types of reactance viz inductive and capacitive.

7. So a capacitor C in series with a resistor R would have an impedance
Z = R - j/(2pi x f x C).

8. An inductor L in series with a resistor R and capacitor C would have an impedance of
Z = R + j(2pi x F x L) - j/(2pi x f x C)

9. In general, Z = R + j(Xind - Xcap) and if the applied voltage V across an impedance Z is V = Z x I. Z is a complex number. I is the current through the impedance.

10. If one works out the current I by dividing V/Z = a +bj then the amplitude of the current is the modulus of a + bj and the phase of the current with respect to the voltage is the argument of a + bj.

Have a go using these cues to tackle the problem you have been set.

 

[bobby2017]

Do you know how to work out the total resistance of two resistors in series or in parallel? If you do then you can do the same for impedances - see a little further down this post.

Also do you know:

1. that the current(I) through an inductor lags the voltage (V) across it? So one can say the voltage V = jX x I where j is square root of -1 or a phase shift of 90 degrees and X is the reactance of the inductor at the frequency of the voltage V?

2. For a capacitor, V = -jX x I where this time -j means a phase shift of -90 degrees - the current leads the voltage this time.

3. that the current through a resistor is in-phase with the voltage across it so V = R x I? So no j required.

4. the reactance X of an inductor is 2 x pi x f x L where f is frequency and L is the inductance of the inductor.

5. For a capacitor X = 1/(2pi x f x C) where C is the capacitance of the capacitor.

6. That in alternating current circuits we deal in impedance which is an amalgam of resistance and the two types of reactance viz inductive and capacitive.

7. So a capacitor C in series with a resistor R would have an impedance
Z = R - j/(2pi x f x C).

8. An inductor L in series with a resistor R and capacitor C would have an impedance of
Z = R + j(2pi x F x L) - j/(2pi x f x C)

9. In general, Z = R + j(Xind - Xcap) and if the applied voltage V across an impedance Z is V = Z x I. Z is a complex number. I is the current through the impedance.

10. If one works out the current I by dividing V/Z = a +bj then the amplitude of the current is the modulus of a + bj and the phase of the current with respect to the voltage is the argument of a + bj.

Have a go using these cues to tackle the problem you have been set.

Thank you so much for that information, you have been more help to me than my teacher has been for the past 6 weeks in a single post !!
I'll have a go and post my results on here
Thanks again

 

[Julie.]

The real clue is in the question, it asks for a series of answers, if you answer each one in order, they lead on to the next.

So I just asks for the current in one branch, so from ohms law I = V /Z, you know V, and you know Z as it's R+jX, X being 2 pi f L

This will be a complex number, so you can convert it to polar to get magnitude and phase angle.

Part II asks for the current in the other branch, so repeat the above remembering that capacitors lead and the calculation for Z is slightly different to an inductor.

Part III asks for the total current, so add the two branches, either via cartesian form as you have calculated above, or even in their polar form noting the sign /direction of the complex part.

Part IV is the impedance, so you could calculate it via standard series/parallel method (using the complex values), or as this has lead you down a simpler path Z=V/I - since you have V and the calculation of total current from part III - this would be an easy way to get the complex impedance

And so on...

I don't know how far you are down your course, but in any ac system calculations the use of complex numbers is pretty standard in anything but the simplest problem.

What level of course are you doing?

This is important as the more basic courses won't use complex numbers, and the more advanced would solve using laplace transforms - it depends on where this is going, simple circuit theory (DC, ac and simple DC transient) wouldn't go past the answers above, if you're moving on to control systems or more advanced step functions then laplace is the way to go.

There are many different ways to solve the question posted - it's probably the way we have said, but it does depend on the level and type of course.

 

[HappyHippyDad]

Is any of what ive done here correct?
I really dont understand what ive done but i guess ive done something? View attachment 55230

"I really don't understand what I've done, but I guess I've done sonething?".
That has got to be the quote of the year
Good for you for giving it a go Bobby.

 

[bobby2017]

"I really don't understand what I've done, but I guess I've done sonething?".
That has got to be the quote of the year
Good for you for giving it a go Bobby.

And we are only in January ? thanks happyhippydad
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This is what i have managed to do so far but i have a feeling i'm way off what is expected of me.
Can somebody tell me what J is? I know it was mentioned above that its the square root of -1 but when i type this into a calculator it says error

 

[Lucien Nunes]

First thing...

You've used a mixture of values for frequency, 240Hz in one case, 50Hz in another. The SPICE parameters shown by the voltage source in the schematic show 50Hz but the question specifically states that the source is a variable frequency supply. I wonder what their intention was; If it is to provide a general solution for any frequency, then an f term will appear in every answer.