# Need help differentating

1. May 2, 2014

### Thepiman

1. The problem statement, all variables and given/known data

i= v/r (1-e^-Rt/L)

How would i go about differentiating this formula to get di/dt? Would I use the product rule or another rule?

2. Relevant equations

i= v/r (1-e^-Rt/L)

3. The attempt at a solution

di/dt= ?

2. May 2, 2014

### 6c 6f 76 65

Open the parenthesis, then differentiate

3. May 2, 2014

### Staff: Mentor

You have many options. You can use the product rule or distribute the $v/r$ inside the parenthesis and derive the sum. If you make no mistake, the answer will be the same.

Go ahead, start solving and tell us what you get.

4. May 2, 2014

### Thepiman

I got di/dt= R/L x e^-Rt/L

Using the chain rule.

5. May 2, 2014

### Staff: Mentor

What happened to $v/r$?

6. May 2, 2014

### Thepiman

Does it not cancel out?

7. May 2, 2014

### Staff: Mentor

Are r and R different variables? I suspect from what you wrote that they aren't. If you mean them to be the same, then be consistent by not mixing upper and lower case letters. That is, don't use r and R interchangeably.

You have i = (V/R)(1 - e^(-Rt/L)) = (V/R) - (V/R)e^(-Rt/L)
Now differentiate with respect to t. The answer you got above is incorrect.