Take the derivative of an equation

atty

New member
Joined
Jul 17, 2021
Messages
6
Hi,

I'm trying to learn electronics and math in my spare time, and now I'm stuck somewhere
with electronics because my math isn't quite that good yet.

1626506890425.png


My question is how do they get to the 3. equation, I think you have to take the derivative of the 2. equation and then add it into the 1. equation ?

Can anyone help?

Thank you in advance.
 

Subhotosh Khan

Super Moderator
Staff member
Joined
Jun 18, 2007
Messages
25,024
Hi,

I'm trying to learn electronics and math in my spare time, and now I'm stuck somewhere
with electronics because my math isn't quite that good yet.

View attachment 28281


My question is how do they get to the 3. equation, I think you have to take the derivative of the 2. equation and then add it into the 1. equation ?

Can anyone help?

Thank you in advance.
Please try the "action" as you have suggested (described) and show us the result.

If you have trouble, please tell us exactly where did you get stuck - we will unstuck you.
 

HallsofIvy

Elite Member
Joined
Jan 27, 2012
Messages
7,620
Hi,

I'm trying to learn electronics and math in my spare time, and now I'm stuck somewhere
with electronics because my math isn't quite that good yet.

View attachment 28281


You are given that \(\displaystyle L\frac{di(t)a}{dt}+ R_1i(t)+ v(t)= 0\) and that \(\displaystyle i(t)= C\frac{dv(t)}{dt}+ \frac{v(t)}{R_2}\) and are told to replace i(t) in the first equation by its form in the second equation. That will give you \(\displaystyle L\frac{d(C\frac{dv(t)}{dt}+ \frac{v(t)}{R_2})}{dt}+ R_1i(t)+ v(t)= 0\). Yes, you now have to differentiate that expression- and since that expression already has a derivative the result will have a second derivative. That will give \(\displaystyle LC\frac{d^2v(t)}{dt^2}+\frac{1}{R_2}\frac{dv(t)}{dt}+ R_1C\frac{dv(t)}{dt}+ \frac{R_1}{R_2}v(t)+ v(t)= 0\).
 

atty

New member
Joined
Jul 17, 2021
Messages
6
Please try the "action" as you have suggested (described) and show us the result.

If you have trouble, please tell us exactly where did you get stuck - we will unstuck you.

I would like to calculate the derivative of the 2nd equation and then add this result with the 1st equation to get to the 3rd equation, but I don't know how to get started.
 

atty

New member
Joined
Jul 17, 2021
Messages
6
Okay, but could you show me how to calculate the derivative of i(t)?
 

Eugenio

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Jul 4, 2021
Messages
35
Differentiating equation 2, we get:
[math]\frac{\mathrm{d} i}{\mathrm{d} t}=c\frac{d^2v}{dt^2}+\frac{1}{R_2}\frac{dv}{dt}[/math]
Replacing in 1, leads to:[math]L\left [C\frac{d^2v}{dt^2}+\frac{1}{R_2}\frac{dv}{dt}) \right ]+R_1\left [C\frac{dv}{dt}+\frac{1}{R_2}v(t)) \right ]+v(t)=0[/math]
 

atty

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Joined
Jul 17, 2021
Messages
6
Differentiating equation 2, we get:
[math]\frac{\mathrm{d} i}{\mathrm{d} t}=c\frac{d^2v}{dt^2}+\frac{1}{R_2}\frac{dv}{dt}[/math]
Replacing in 1, leads to:[math]L\left [C\frac{d^2v}{dt^2}+\frac{1}{R_2}\frac{dv}{dt}) \right ]+R_1\left [C\frac{dv}{dt}+\frac{1}{R_2}v(t)) \right ]+v(t)=0[/math]

Thanks for helping.
 
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