# Homework Help: At what rate must the current be to changed to produce 40V emf

1. Dec 8, 2005

### mr_coffee

A 20 H inductor carries a steady current of 4.0 A. At what rate must the current be changed to produce a 40 V emf in the inductor?

I odn't get this one because it gives you all the information!!
The formula is:
EMF = -L di/dt
they want u to find di/dt, but they already give you it has a steady current of 4.0A, so isn't di/dt just i, which is 4.0A?
Then the EMF would be 40, and they say L = 20H, so i'm stuck on what intermediate step i'm missing! thanks

Last edited: Dec 8, 2005
2. Dec 8, 2005

### Integral

Staff Emeritus
For a steady current, at any value,

$$\frac {dI} {dt} = 0$$

3. Dec 8, 2005

### mr_coffee

So basically, I can disregard that they told me it had a steady current of 4.0A right? and then just use
di/dt = EMF/-L to find the di/dt?

4. Dec 8, 2005

### mr_coffee

thanks that was the trick!! i did that the last time, but forgot the minus sign!

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook