# Homework Help: Energy stored in coil with given relationship between current and flux

1. Aug 12, 2012

### Trip1

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

The relationship between the current in an N-turn coil and the flux created by it is given as:

$$\Phi = \frac{ai}{b+ci}$$

Determine the energy stored in the coil when the current varies from 0 to I.

2. Relevant equations

$$W = N \int{i d\Phi}$$

3. The attempt at a solution

Started by differentiating the given relationship with respect to i, using the quotient rule

$$\frac{d\Phi}{di} = \frac{a(b+ci) - cai}{(b+c{i}^2)} \\ \\ \Rightarrow d\Phi = \frac{a(b+ci)}{{(b+ci}^2)}di - \frac{aci}{(b+ci)^2}di$$

I then proceed to substitute this expression for $$d\Phi$$ into the equation for W above, and setup two integrals (one for each term), integrating with respect to i from 0 to I.

Problem is, the integrations are very complex to do by hand, and they aren't in a general form to lookup in a table. This leads me to believe i've made a mistake somewhere, but i can't seem to find it.

Any help would be greatly appreciated, thanks.

2. Aug 12, 2012

### TSny

You might try integration by parts.