• Support PF! Buy your school textbooks, materials and every day products Here!

Contracting Loop in a Magnetic Field (emf and current)

  • Thread starter GDGirl
  • Start date
  • #1
50
0

Homework Statement


An elastic circular loop in the plane of the paper lies in a 0.71 T magnetic field pointing into the paper. If the loop's diameter changes from 19.3 cm to 7.2 cm in 0.54 s,

a. what is the magnitude of the average induced emf?
HELP: Use Faraday's Law and evaluate the rate of change of magnetic flux through the loop.

b. If the loop's resistance is 2.6 Ω, what is the average induced current I during the 0.54 s?


Homework Equations


[tex]\Delta[/tex][tex]\Phi[/tex]/[tex]\Delta[/tex]t=emf
[tex]\Delta[/tex][tex]\Phi[/tex]=magnetic flux = BAcos[tex]\theta[/tex]

The Attempt at a Solution


So I used the above equation to find the emf
[tex]\pi[/tex](.0605)2(.71)/.54 = .0151
Where the value for r is the amount that the radius changes in the time given.
This is wrong, and I'm not quite sure what else to try...
 

Answers and Replies

  • #2
nrqed
Science Advisor
Homework Helper
Gold Member
3,573
192

Homework Statement


An elastic circular loop in the plane of the paper lies in a 0.71 T magnetic field pointing into the paper. If the loop's diameter changes from 19.3 cm to 7.2 cm in 0.54 s,

a. what is the magnitude of the average induced emf?
HELP: Use Faraday's Law and evaluate the rate of change of magnetic flux through the loop.

b. If the loop's resistance is 2.6 Ω, what is the average induced current I during the 0.54 s?


Homework Equations


[tex]\Delta[/tex][tex]\Phi[/tex]/[tex]\Delta[/tex]t=emf
[tex]\Delta[/tex][tex]\Phi[/tex]=magnetic flux = BAcos[tex]\theta[/tex]

The Attempt at a Solution


So I used the above equation to find the emf
[tex]\pi[/tex](.0605)2(.71)/.54 = .0151
Where the value for r is the amount that the radius changes in the time given.
This is wrong, and I'm not quite sure what else to try...

For the change of area, you must use : [itex] \pi R_i^2 - \pi R_f^2 [/itex]
where Ri and Rf are the initial and final radii.
I see what you did (you took the difference of diameter and divided this by 2 to get a radius ) but that does not give the correct change of area.
 
  • #3
50
0
Oh, that makes sense. Thanks!
 

Related Threads for: Contracting Loop in a Magnetic Field (emf and current)

  • Last Post
Replies
7
Views
752
  • Last Post
Replies
6
Views
10K
  • Last Post
Replies
1
Views
8K
  • Last Post
Replies
1
Views
1K
Replies
10
Views
2K
Replies
3
Views
1K
  • Last Post
Replies
0
Views
2K
Replies
20
Views
3K
Top