Hello everyone!

I am brand new to all about circuits and an EE student. I'm taking "electric circuits" and need help.


we are learning about inductors and I need some guidance (my professor isn't the best). On my homework we are given peicewise equations for current and need to calculate power. I under stand the math but I'm confusted about the equation. It has a B1 and B2 that are constants. I cant find it anywhere in the book. What do they stand for and how do I figure them out? Thank You in advance

 

My crystal ball is out of order, so what is the equation?

 

i=[B1cos(200t) + B2sin(200t)]e^-50t for t>=0
i=75mA t<=0
voltage across is 4.25 V at t=0

 

Sorry I don't really understand that equation.
Do you have a diagram of the circuit?

 

no that is literally all they gave us. the solutions manual has i(0) = B1 = 75mA and than after come calc they solve for B2. i get how they get the B2 because once you take the derivative and simplify you get an linear equation times the orginal "cos(200t)". I just have no idea where the B1 and B2 come from.

 

my professor said we might see something with the B1 and B2 but next explained what it was

 

think i figured it out. according to the RLC circuit wiki page they are "constants determined by boundary conditions" which would be my starting i and ending i. Thank you for trying!

 

Your instructor started out explaining inductors......with that?

 

they have been "introduced" in the EE intro class but not really much math. He explained what they are (kinda)......but then gave us the voltage and current equations using L and derivative/integral. But as I as said "isn't the best" haha. Glad the internet exists and I'm capable of teaching myself things through solutions haha.

 

i=[B1cos(200t) + B2sin(200t)]e^-50t for t>=0
i=75mA t<=0
voltage across is 4.25 V at t=0

You aren't given the inductance?

You aren't given whether this is a series or parallel RLC circuit?

Is this the current through the inductor?

What do you know about the current at t = 0- and t = 0+?

Does that let you solve for one of the constants?

What do you know about the voltage across the inductor?

IF you had the inductance, would that let you solve for the other constant?

 

the inductance is 500mH. There is no circuit. this is a basic level class (1270). everything is across/through just the inductor. No series or parallel RLC because there is no actual RLC. Yes current through the inductor. T<=0, i=75mA, T>=0 i= that equation. I'm able to figure out B1 = 75mA. But i want to know how I'm supposed to know that B1 = the current at t=0. The voltage across the inductor is 4.25 at t=0. The actual question is "Calculate the power at the terminals of the inductor at t=25 ms. I have no issues with the calculating power.

 

So apparently that WASN'T literally all that they gave you. They also gave you the inductance, which is a critical piece of information.

How you are supposed to know that B1 is the current through the inductor at t=0 is by understanding the answer to earlier question I asked. Namely:

What do you know about the current (through an inductor) at t = 0- and t = 0+?

You are given the current through the inductor at all time. What is the relationship between the current through an inductor and the voltage across it? Can you use this relationship and the expression for the current through the inductor to get the voltage across the inductor? What does that expression evaluate to at t=0+? Do you know the voltage across the inductor at t=0+? Does that allow you to solve for the second constant?

 

wow now I feel stupid haha. I guess I just needed to slow down and look at what had. I was thinking B1 and B2 were some magically EE magic.

Thank You so much!!!

 

Sometimes all it takes is the right question asked at the right time to make a light click on. Unfortunately, knowing what the right question is often times does seem to require magic.

 

haha so true! by the way I liked that so I printed it and hung it next to Johnny Cash (I have a dry erase board that's background is old Johnny) haha

 

i=[B1cos(200t) + B2sin(200t)]e^-50t for t>=0
i=75mA t<=0
voltage across is 4.25 V at t=0

Which textbook is this problem from?