1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: What is the induced emf in one of the windings?

  1. Feb 10, 2013 #1
    1. A solenoid of length 3.64 cm and diameter 0.914 cm is wound with 170 turns per cm. If the current is decreasing at a rate of 36.3 A/s, what is the induced emf in one of the windings?


    this is what I did:
    Length of the solenoid l = 3.64cm
    = (3.64 cm)(10-2 m/ 1 cm)
    = 0.0364 m
    diameter d = 0.914 cm
    radius r = d/2
    = 0.914 cm /2
    = 0.457 cm
    = (0.457 cm)(10-2 m/ 1 cm)
    = 0.00457 m
    area A = πr2
    = (3.14)(0.00457 m)2
    = 6.55*10-5 m2
    the change of rate of current dI/dt = 36.3 A/s
    _______________________________________
    a)
    for one winding N = 1
    for one winding, n = N/1 cm
    = 1/10-2 m
    = 100 m
    per one meter of winding,
    n = 100
    the induced emf in one of the windings is
    ε = [μ0 n2A/l](dI/dt)
    = [(4π*10-7 T.m/A)(100)2(6.55*10-5 m2 )/(0.0364 m)](36.3 A/s)
    = 1.64*10-5 V <--------BUT THE ANSER IS WRONG...
    B. What is the induced emf in the entire solenoid?

    This is what I did:
    for entire solenoid, n = 170 turns/ 1 cm
    = 170 / 10-2 m
    = 17000 m
    per one meter, n = 17000
    the induced emf in the entire solenoid is
    ε = [μ0 n2A/l](dI/dt)
    = [(4π*10-7 T.m/A)(17000)2(6.55*10-5 m2 )/(0.0364 m)](36.3 A/s)
    but the answer was still wrong....
    could someone help me where did i do wrong, please \
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Feb 10, 2013 #2

    TSny

    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    Looks like you are using the formula B = μo(N/L)I for the field inside a solenoid. What do N and L stand for here?
     
  4. Feb 11, 2013 #3
    N=how many turns
    L=length of the wire?!
     
  5. Feb 11, 2013 #4

    TSny

    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    N is the total number of turns and L is the total length of the solenoid. So, N/L is the number of turns per unit length, which you found to be 17000 turns per m.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook