Joined Feb 10, 2016
68
When i did the t=l/R i found the value of L and apply the formula V=l(Di/dt) then i have -200e^-1000t and when i did that by your method finding voltage across resistor and then the inductor still same answer -200e^-1000t
voltage across resistor is 200e^-1000t

Joined Feb 10, 2016
68
Good, except that doesn't matter whether the "other terminal" is grounded or not. Notice that which terminal is being referred to in the first place is not specified, nor is the direction of the current. You get to pick.

So, for the given voltage, what is the voltage across the resistor as a function of time?
Thank you for giving your valuable time you made my concepts stronger thank you sir.

#### oussama123443

Joined Nov 2, 2014
27
please note that the inductor reverses polarity when the source is disconnected and so the current will start flowing in the opposite direction and so VR=-VL( because VR+VL = 0) so use can use KCL or the decay equation to find the voltage. It also reaches steady state and should be no current after 5 Tau..remember inductor opposes CHANGE in current that's why the voltage is proportional to the change in current with respect to time.

#### WBahn

Joined Mar 31, 2012
26,398
please note that the inductor reverses polarity when the source is disconnected and so the current will start flowing in the opposite direction and so VR=-VL( because VR+VL = 0) so use can use KCL or the decay equation to find the voltage. It also reaches steady state and should be no current after 5 Tau..remember inductor opposes CHANGE in current that's why the voltage is proportional to the change in current with respect to time.
Read what you've just written and you will see that it is self-contradictory. If the inductor opposes a change in current, then how do you reconcile that with a claim that when the source is disconnected that the current somehow starts flowing in the opposite direction, which would constitute a really big change in the current.

Your claim is comparable to someone saying that, if it were a capacitor, that the charge on the capacitor would change polarity as soon as the source is disconnected.

#### oussama123443

Joined Nov 2, 2014
27
i meant to say inductor opposes change in current ( Faraday's Law) doesnt want the current to decrease when source is removed that's why is mathematically represented by an exponential decay equation so the current will decrease when E = 0V which means the voltage across inductor and resistor are of opposite polarity 0=VR+VL which means VR=-VL. the restore voltage is proportional to current so ti will behave as current and will decay exponentially. the inductor will decreases from the -E to 0V. does that answer your question ?

#### WBahn

Joined Mar 31, 2012
26,398
i meant to say inductor opposes change in current ( Faraday's Law) doesnt want the current to decrease when source is removed that's why is mathematically represented by an exponential decay equation so the current will decrease when E = 0V which means the voltage across inductor and resistor are of opposite polarity 0=VR+VL which means VR=-VL. the restore voltage is proportional to current so ti will behave as current and will decay exponentially. the inductor will decreases from the -E to 0V. does that answer your question ?
Please remember that this is Homework Help, so the goal is NOT to outright answer the TS's question, but rather to guide them toward answering it themselves.

#### oussama123443

Joined Nov 2, 2014
27
ok i just wanted to clarify the concept intuitively so he can answer it by hand and get a value mathematically.

#### WBahn

Joined Mar 31, 2012
26,398
Fair enough.

#### oussama123443

Joined Nov 2, 2014
27
cheers!

#### oussama123443

Joined Nov 2, 2014
27
i just wanted to correct something that " current will flow in the same direction as it was in the circuit in series with the source", it's just the polarity of the inductor that's going to change polarity.

#### WBahn

Joined Mar 31, 2012
26,398
What does "the polarity of the inductor" even mean?

#### oussama123443

Joined Nov 2, 2014
27
current doesn't change direction

#### JoeJester

Joined Apr 26, 2005
4,390
My professor does not actually teach anything
And yet your paying for the course of instruction ... why?

#### WBahn

Joined Mar 31, 2012
26,398
i just wanted to correct something that " current will flow in the same direction as it was in the circuit in series with the source", it's just the polarity of the inductor that's going to change polarity.
current doesn't change direction
So, again, what do you mean when you say that the polarity of the inductor is going to change polarity?

#### oussama123443

Joined Nov 2, 2014
27
And yet your paying for the course of instruction ... why?
<SNIP>

Moderators note: no swearing over here.

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#### WBahn

Joined Mar 31, 2012
26,398
<SNIP>

Moderators note: no swearing over here.
It was a perfectly valid question.

If I was posting to a car repair forum and the first thing I said was that my mechanic never fixes anything, wouldn't a reasonable question be, "Why do you do business with that mechanic?"

Your very first words were, "My professor does not actually teach anything...." So isn't a reasonable question, "Why do you do business with that professor (or, more generally, that school)?"

Or are you just trying to trick people into giving you the answer and/or provide some weak-tea excuse for why your fundamentals are so poor?

#### oussama123443

Joined Nov 2, 2014
27
i'm not the person that posted the question to begin with. I'm just like you trying to help this kid that posted the question and his "professor ": being sarcastic. i studied this in my previous courses and i know the topics thats why i tried to help

#### JoeJester

Joined Apr 26, 2005
4,390
@oussama123443

The TS hasn't responded for days. The TS was seen seven days before my question.

Teachers not teaching is a common statement from some of thread starters in the homework section.

I didn't see your response until after the moderators snipped your response, so I have no idea of what you said.

It seems to me that the excuse "teachers aren't teaching" is becoming more common and "research" is a foreign activity for some that post here. I've seen good people get frustrated when dealing with some inquiries. Sometimes it's dragging out point by point when the rules remind them to provide everything they've done, including schematics and such.

So yeah, I question why they are spending their money for nothing. Do I actually believe teachers don't teach? I'm sure there is a percentage of them where that statement rings true. Of course if it's not your money your spending, you might not care.

Equality of access doesn't mean equality of success. The students can cause an inverse relationship.

The only place success comes before work is in the dictionary.
--- May V. Smith

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#### WBahn

Joined Mar 31, 2012
26,398
i'm not the person that posted the question to begin with. I'm just like you trying to help this kid that posted the question and his "professor ": being sarcastic. i studied this in my previous courses and i know the topics thats why i tried to help
Ah, my oversight and my apologies -- still, the foul language has no place here.

#### WBahn

Joined Mar 31, 2012
26,398
@oussama123443
I didn't see your response until after the moderators snipped your response, so I have no idea of what you said.
He told you to go do a certain obscene thing to yourself.