# Finding induced emf of solenoid

## Homework Statement

A 800-turn solenoid, 18 cm long, has a diameter of 2.9 cm. A 15-turn coil is wound tightly around the center of the solenoid.
If the current in the solenoid increases uniformly from 0 to 3.1 A in 0.28 s, what will be the induced emf in the short coil during this time?

## Homework Equations

Emf=delta_flux/delta_t

## The Attempt at a Solution

I keep getting the wrong answer. Originally I just plugged in 800 and 15 for the n's, but then I used turns per meter. Both were wrong. I think I may be using the wrong equation. Any help would be appreciated!

alphysicist
Homework Helper
Hi number14,

## Homework Statement

A 800-turn solenoid, 18 cm long, has a diameter of 2.9 cm. A 15-turn coil is wound tightly around the center of the solenoid.
If the current in the solenoid increases uniformly from 0 to 3.1 A in 0.28 s, what will be the induced emf in the short coil during this time?

## Homework Equations

Emf=delta_flux/delta_t

## The Attempt at a Solution

I keep getting the wrong answer. Originally I just plugged in 800 and 15 for the n's, but then I used turns per meter. Both were wrong. I think I may be using the wrong equation. Any help would be appreciated!

What did you do to get your answers? (what equations did you use, and what numbers did you plug into them)

I used B=(mu_0 *N*I)/l delta_flux = B*pi*r^2 and
Emf=-N * delta_flux/delta_t

N=15, I=3.1, l=.18, delta_t=.28 , r=.0145

and I got: 3.6 * 10^-5

alphysicist
Homework Helper
I used B=(mu_0 *N*I)/l delta_flux = B*pi*r^2 and
Emf=-N * delta_flux/delta_t

N=15, I=3.1, l=.18, delta_t=.28 , r=.0145

and I got: 3.6 * 10^-5

I'm not getting that answer from using those numbers. What are you getting for just the B part by itself?

Last edited:
B=3.3*10^-4

Wait, this time i got 1.2*10^-5 . I'm using a calculator that is very finicky.

alphysicist
Homework Helper
B=3.3*10^-4

I think here you are using N=15; but the B is the magnetic field from the solenoid, which has N=800.

thanks so much for the help, i got the right answer finally!!!