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

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

 PhysOrg.com science news on PhysOrg.com >> Heat-related deaths in Manhattan projected to rise>> Dire outlook despite global warming 'pause': study>> Sea level influenced tropical climate during the last ice age
 Mentor Blog Entries: 9 For a steady current, at any value, $$\frac {dI} {dt} = 0$$
 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?

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

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