# Homework Help: Which is the constant of differentiation?

1. Oct 20, 2015

### Calpalned

When the textbook differentiated with respect to time, I see that the middle term is R(dI/dt). Why can't it be I(dR/dt)? When I differentiate, how do I know which letter to differentiate?
3. The attempt at a solution
2. Relevant equations
1. The problem statement, all variables and given/known data

Last edited by a moderator: Oct 20, 2015
2. Oct 20, 2015

### LCKurtz

L, R, and C are constants. You are differentiating with respect to time and the constants don't vary.

3. Oct 20, 2015

### SammyS

Staff Emeritus
Why did you ignore the template? You even went through the trouble to change the font color.

R is a constant just like L and C. You had no objection to treating them as constants.

4. Oct 20, 2015

### Calpalned

I see... If I look at the original equation (1.2) has $\frac{dI}{dt}$ and that tells me that I is not a consant?

5. Oct 20, 2015

### SammyS

Staff Emeritus
Actually, it helps to understand something of the physics behind this problem.

V is voltage or emf. I is current.

6. Oct 20, 2015

### Calpalned

The example was on differential equations in general, so the textbook didn't give me background information on the physics.

7. Oct 21, 2015

### Staff: Mentor

You could use the product rule, differentiating the product RI w.r.t. time. What result would you get?