Finding induced emf of solenoid

AI Thread Summary
The discussion revolves around calculating the induced electromotive force (emf) in a 15-turn coil wound around an 800-turn solenoid as the current increases from 0 to 3.1 A over 0.28 seconds. Participants initially struggle with the calculations, particularly in applying the correct number of turns for the solenoid when determining the magnetic field (B). The correct formula for emf involves using the magnetic field generated by the solenoid, which is based on its 800 turns, rather than the 15 turns of the coil. After clarifying the use of the right parameters, one participant successfully computes the correct induced emf. The discussion highlights the importance of correctly identifying the variables in electromagnetic calculations.
number14
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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!
 
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Hi number14,

number14 said:

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
 
number14 said:
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.
 
number14 said:
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!
 

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