Obtain the solution of the following differential equations. L \frac{di}{dt}+Ri=E, where

oliviayychengwh

oliviayychengwh

Answered question

2021-12-27

Obtain the solution of the following differential equations.
Ldidt+Ri=E, where L,R, and E are constants when t=0,i=0

Answer & Explanation

jean2098

jean2098

Beginner2021-12-28Added 38 answers

We have:
Ldidt+Ri=E where L,R,E are constant when t=0,i=0 (1)
Rewriting the equation (1) we get:
didt+RLi=EL (2)
Observe that this is a linear equation in i is of the form dydx+P(x)y=Q(x). (3)
We know that the integrating factor for the differential equation (2) is: eP(x)dx
Now using this in equation (1) we get:
The integrating factor of the equation (2) is:
eRLdt=eRLdt=eRtL
Now multiplying the equation (2) with the integrating factor eRxL we get:
eRtL(didt+RLi)=eRtL(EL)
eRtLdidt+eRLi=eRtL(EL)
d(eRtLi)dt=eRtL(EL)
d(ieRtL)=ELeRtLdt
d(ieRtL)=ELeRtLdt
(ieRtL=EL×LReRtL+C
where C is an arbitrary constant.
So ieRtL=EReRt
Karen Robbins

Karen Robbins

Beginner2021-12-29Added 49 answers

dydx+P(x)y=Q(x)(1)
The solution by integrating factor will be
I(x)=eP(x)dx(2)
I(x)Q(x)=I(x)S(x)dx(3)
E=E0
Ldidr+Ri=E
didt+RLi=E0L
Using equation number 1
P(t)=RL(Constant)
Q(t)=E0L(Constant)
Now, using equation number 2 we get I
P(t)dt=RLdt=RtL
I=eP(t)dt
I=eRtL
Applying equation number 3 we get
I(t)×i(t)=Q(t)I(t)dt
i(t)eRtL=E0LeRtLdt
i(t)eRtL=E0¬{L}׬{L}ReRtL+C=E0L+C
Which can be arranged to the required to be proven
t=0i=0
0=E0Re0+CC=E0R
karton

karton

Expert2022-01-10Added 613 answers

dIdt=ERILLdIdtERI=1.LRln(ERI)=t+Cln(ERI)=RtL+C1.ERI=C2eRLt.I=ERC3eRLt
If we assume we’re modeling an RL circuit, at t=0 the current should be zero, because reasons (inductors are modeled as open circuits at time 0).
So C3=ER. Therefore I=ER(1eRLt)
So we’re looking for when eRLt=0.5. That's when t=RLln0.5=RLln2.

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