# Homework Help: Period of spring -> magnetic field

1. Sep 25, 2009

### Jackyo

http://img142.imageshack.us/img142/7485/physicsb.jpg [Broken]

Hi,

sorry that I only mention a first strategy for solution for the following problem, but I can not think of a better one.

For the question you will need to look at the image in the appendix:
1. You will see a frame, consisting of the bars F,G,H and I.
2. Between the bar HI is a capacitor C;
between the bar GH is a spring with spring constant k.
3. The density in the bar is everywhere the same.
4. Assume that all bars and the spring are conductive
5. Do not include friction in the calculation

6. orthographic to the frame is a magnetic field B

What is the time of one periods of the spring?

Obviously there is a tension V=- d(B*A)/dt, because of the changing of the frame-area A.
Because the resistance of the frame is ~0 no current will be detected.
I could now create the function x -> V(x), which attaches every elongation x the current tension V(x).

But what is the next step for finding the solution? What can I calculate with the capacity C?

Jenny

Last edited by a moderator: May 4, 2017
2. Oct 1, 2009

### Jackyo

No idea?
I'm not sure why you are calling this quantity a Tension--- tension has units of force (Newtons in SI units), while the emf/induced electric potential $V=-B\frac{dA}{dt}$ (I assume that $B$ is uniform and constant?) has units of potential (Newtons per Coulomb in SI)---you need to multiply $V$ by something in order to find the tension