Confusion - Diff Eq for a Circuit? What does my professor mean?

  • #1

Homework Statement



My professor wants us to derive a differential equation for the current in a circuit.

The circuit has a battery with voltage E, resistor with resistance (iR)??, and inductance L. It's in series.


The Attempt at a Solution



What REALLY confuses me are these things:

Why iR instead of R? Is the i supposed to represent current? Then why is it given for the resistance and not inductance? I don't think it could be a complex number.. it just wouldn't make sense.

Anyway, I assumed she meant R instead of iR. Then I set up a diff eq like this:

i(t)*R + L(d/dt)i(t) = 0
after a bit of math, I got
d(i(t))/i(t) = -(R/L)dt
and integrating some more math...
i(t) = Cexp(-(R/L)t) where C is some constant.
okay, that's fine. It seems to make sense to me

But then she wants us to solve it if i(0) = 0 and i(t=0) = 0.

First of all.. isn't she saying the exact same thing twice? i(0) = i(t=0) I'm guessing?
And second of all... according to my equation, i(0) = Cexp(0) = 0 implies C = 0, or that the whole expression is 0. How do I solve the differential equation if C = 0 exactly? Or does it only have a solution of 0?

Any clarification could help. Thanks!
 

Answers and Replies

  • #2
rock.freak667
Homework Helper
6,230
31
Shouldn't it be

E = i(t)R+ Ldi/dt
 
  • #3
ah, okay. that makes more sense. I should be able to figure something out - thanks!
 

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