# Inductance and Energy

Soaring Crane

## Homework Statement

After you measure the self-inductance of a coil, you unwind it and then rewind half the length of wire into a coil with the same diameter but half the number of turns. How does this change the self-inductance?

a. It is the same.
b. It is doubled.
c. It is quadrupled.
d. It is halved.
e. It is quartered.

See below.

## The Attempt at a Solution

L = (N* magnetic flux)/i = (N*B*A*cos theta)/i = (n*l*A*B*cos theta)/(i), where N = n*l (l = wire length)

L2 = [(l/2)*A*(n/2)*B]/i = (1/4)*[(L*A*n*B)/i] = (1/4)*L??

## Homework Statement

How much does the energy stored in an inductor change if the current through the inductor is doubled?

a. It remains the same.
b. It is doubled.
c. It is quadrupled.
d. It is halved.
e. It is quartered.

See below.

## The Attempt at a Solution

U = 0.5*L*I^2

U2 = 0.5*L*(2I)^2 = 4*0.5*L*I^2 = 4*U??

## Homework Statement

The magnetic flux through a loop is made to vary according to the relation magnetic flux = 6t^2 + 7t + 1, where the units are SI. The emf induced in the loop when t = 2 s is

a. 38 V
b. 39 V
c. 40 V
d. 31 V
e. 19 V

## Homework Equations

Emf_induced = -(d magnetic flux)/(dt)

## The Attempt at a Solution

Taking the derivative, |emf| = 12t + 7, where t = 2 s

|Emf| = 12*(2s) + 7 = 31 V??

Thanks.

## Answers and Replies

Mentz114
You don't seem to be having any problems with this. They all look correct.