# Confusion

Discussion in 'Homework Help' started by k31453, May 21, 2013.

1. ### k31453 Thread Starter Member

May 7, 2013
54
0
Hi i got this question ?

have to find Vr and VL

so can i use this formula :

VR = IR
VL = I * XL???

confuse ?? need help ??

Last edited: May 22, 2013
2. ### WBahn Moderator

Mar 31, 2012
24,490
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Go back to basics.

V = L di/dt

3. ### k31453 Thread Starter Member

May 7, 2013
54
0
ahhh make sense a bit !!

so answer is 0.002 * (20/1) = 0.04 for VL

Last edited: May 22, 2013
4. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,901
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Check the time scale.

5. ### k31453 Thread Starter Member

May 7, 2013
54
0
so 40V ? is that right?

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,901
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Yes 40V is the right answer. So from 0s to 1ms you have 40V across the coil. But what next?

7. ### WBahn Moderator

Mar 31, 2012
24,490
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And here is a shining example of why I am always screaming about units!

The answer is NOT 0.002 * (20/1) = 0.04, it is

0.002H * 20A / 1ms = 0.04 HA/ms

You don't have to remember what the base units are for henries or farads, just use the formulas:

V = L di/dt => V = HA/s => H = Vs/A
Q = CV =? F = C/V = Cs/Vs = As/V

0.04 HA/ms = 0.04 (HA/ms) (1000/m) (Vs)/(HA) = 40V

Or even more simply:

2mH * 20A / 1ms = 40 (HA/s)(Vs)/(HA) = 40V

The majority of mistakes that you will make will mess up the units -- just like happened here. If you track your units, you will catch most of these mistakes when you make them. If not, you just blindly accept a wrong answer and next thing you know you have just applied 40V to something that you were expecting would see 40mV and some piece of flight-critical hardware has just gone bye-bye and the planeload of passengers is now at the bottom of a smoking crater.

May 7, 2013
54
0

am i right?

9. ### screen1988 Member

Mar 7, 2013
310
4
No, both are wrong. You need to check it again.
VR = I* R and because R is constant VR and IR has the same shape.
UL = Ldi/dt and you need to consider i in each time periods.

10. ### k31453 Thread Starter Member

May 7, 2013
54
0
yeah i know voltage in resistor doesn't go above 40 and -40 but i dont get it with the inductor ?

11. ### WBahn Moderator

Mar 31, 2012
24,490
7,664
What is a voltage of 40?

What is a height of 3?

What is a weight of 10?

What is a volume of 6?

What is a speed of 13?

Do any of these make any sense to you?

Quit being sloppy about the units. That, or get the hell out of engineering. What would you say should happen to a doctor that didn't bother to check his units and ended up prescribing someone a thousand times more of a medicine than they should have and the person died as a result? Oh, but you say, if a doctor makes a mistake they could kill some one. Well, doctors are generally limited to killing people one at a time. Engineers that can't be bothered to use simple and effective error detecting procedures in their work kill people in job lots.

The voltage in the resistor does not exceed +40V or -40V.

Units matter.

You've already found that, between t=0s and t=1ms, that the voltage across (not "in", you also need to start using the proper terminology to minimize miscommunication) the inductor was 40V. So why does your graph show it changing in this time period?

What is the voltage across an inductor if the current isn't changing?

Is the current changing in the inductor in the time periods 1ms<t<2ms or 4ms<t<5ms?

What is the voltage across the inductor during these time periods?

Is this consistent with your chart?

You need to not only start tracking and checking your units, but you need to start always asking if your answer makes sense. Do those to things, relentlessly, and you will see your grades go up considerably and you will let far fewer mistakes get past you on the job and you will catch many of the mistakes that others did let get past them and you will be much more valuable to your employer and/or customers and you will make a lot more money and you will kill a lot fewer people.

May 7, 2013
54
0
is it right?

13. ### WBahn Moderator

Mar 31, 2012
24,490
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40 WHAT????

Quatloos?

Gorillas?

UNITS! UNITS! UNITS!

Other than the units issue and the fact that there is no indication (except by context) of what this is a graph of, you've got it.

14. ### k31453 Thread Starter Member

May 7, 2013
54
0
i know it is Volt bro i didnt put in there !!!

15. ### JoeJester AAC Fanatic!

Apr 26, 2005
4,073
1,774
I find it unusual you have two threads dealing with this one problem.

WBahn's price for helping you is you identifying the units. A cheap price. The reader could assume it was volts, or they could assume it was 40 mV, or 40 GV. The only thing for sure is it's 40 something. Attention to detail provides clarity to the reader.

16. ### WBahn Moderator

Mar 31, 2012
24,490
7,664
I don't care whether you know it or not. I had hoped that the fact that it's already been pointed out to you that you are making mistakes and not catching them because you are being sloppy with units would make you aware of their value. But, apparently, that was wishful thinking on my part. So please consider switching to a field other than engineering where your disregard for even the simplest error detecting practices is less likely to get someone killed someday.

17. ### WBahn Moderator

Mar 31, 2012
24,490
7,664
That was my first thought, but the other thread is actually different problem. The only thing they have in common is that it is a series RLC circuit. The other problem has different component values and an AC voltage source instead of a piecewise linear current source.

Thanks for the backup.

I know I am more than a bit of a Units Nazi, but I am completely unapologetic about it. I have very sound and motivating reasons for having the attitude that I do.

18. ### JoeJester AAC Fanatic!

Apr 26, 2005
4,073
1,774
You may think your a unit Nazi .... but your not. Effective communications occurs when the recipient understands the message.

19. ### atferrari AAC Fanatic!

Jan 6, 2004
3,321
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Do not worry.

At the Naval Academy, I was taught by strict professors, Physics, Thermodynamics and Navigation to: recognize first all your knowns (with actual units!) and then your uknowns.

The whole to be properly stated at the right side of the paper sheet, doing all your calculations trailing units along it. Use pencil and never a pen (not admited in nautical charts).

Solved one of my problems before starting: being a little disorganized no more!

BTW, even today, my notebooks in the last 20++ years are all writen in pencil and could reconstruct a report for each survey just from the data in there. My thanks to them!