# Homework Help: Differential equations: Circuit RL

1. Oct 3, 2015

### masterchiefo

1. The problem statement, all variables and given/known data
A simple electrical circuit consists of a voltage source E(t) = t*e-t volts, a
resistor R = 1 and an inductor L = 1/10 H connected in series. It is assumed that I(0) = -10/81
a) The differential equation that governs the current I (t) in this circuit .
b) Find the time manually t1 where the current is maximum . Explain your answer.

2. Relevant equations

3. The attempt at a solution
a)
i=dq/dt
VR+VL =E
Ri+L*di/dt=E

I(t) = E(t)/1 *(1-e-R*t/L)
I(t) = t*e-t/1 *(1-e-1*t/(1/10))

b) Not sure how I have to use this info I(0) = -10/81
do I do d/dt(I(t)) to find the max I?
I was also thinking of doing the limit of t to infinite

2. Oct 3, 2015

### vela

Staff Emeritus
Up to here is fine.

Now you need to solve the differential equation correctly. Do you know how to solve first-order differential equations?

3. Oct 4, 2015

### rude man

where did you get these last two equations???
As vela said, you need to solve the ODE. Unfortunately, it's a bit messy. When you finally get i(t) you do indeed do di/dt = 0 etc. to find imax and the corresponding t1.

Not sure what the question meant by "find the time manually".

4. Oct 4, 2015

### masterchiefo

it means by hand, as we can do it pretty quick on calculator.
also the equations are from wikipedia and my note book are the same for i(t)

5. Oct 4, 2015

### rude man

Where in wikipedia?

6. Oct 4, 2015

7. Oct 4, 2015

### masterchiefo

also what do I do with i(o)=-10/-81? I use it to find the constant C in i(t)?

8. Oct 4, 2015

### rude man

Lisez bien le suivant:
"On appelle échelon de tension le passage brutal de la tension appliquée à l'ensemble {R + L} d'une valeur nulle à une valeur non nulle : on suppose qu'à t < 0, la tension du générateur est nulle et qu'à partir de t = 0 elle est égale à une constante E."

But in your case, E is not a constant, what we call a step function, commonly written here as E U(t). In your case, E = t exp(-t) which is a much more complicated voltage. Which is why you need to solve the differential equation to get i(t), then do di/dt = 0 etc. as you were previously advised to do.

9. Oct 4, 2015

### rude man

exactly right. You will find that i(0) = -10/81 was chosen for great convenience for you, not arbitrarily.