# Problem on loud speaker

1. Jun 24, 2006

### copresh1

i am a bit confused as to where to start on this problem, any hint will be appreciated.(problem attached)

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2. Jun 25, 2006

### Staff: Mentor

What is the equation for the force on a current-carrying wire in a magnetic field? What is the equation for the magnetic field generated by an air core coil? How can you use these two equations to help you solve this problem?

3. Jun 26, 2006

### copresh1

F=BLI
where B is flux density,L is lenght of conductor, and I is the current in the conductor.
magnetic field density=magnetic flux/c/s area
magnetic flux=mmf/reluctance
& reluctance=lenght of airgap/mew0 * c/s A

4. Jul 3, 2006

### copresh1

using f=BLI i found B=100T
next i used S=L/mew0*crosssectional area of air gap(L=10mm,c/sarea airgap=piD=3.14*0.001)
=2533029.59A/wb
next using H=IN/L(I=100mA,N=50,L=15mm this assumed)
=333.33A/m
but when i go to find relative permeabilty i am getting a huge value which i think is wrong.
apart from that my understanding is that i there are three elements involved here ie the coil,the magnet,the airgap(am i right?)but how do i find the mmf of the magnet bcoz i cannot find the reluctance of the magnet

5. Jul 5, 2006

### Staff: Mentor

How huge is the $$\mu_0$$ value that you are getting? The relative permeability of magnetic materials can be quite high, in the tens of thousands for some materials.

6. Jul 10, 2006

### copresh1

sorry for the delay in responding
the value i get for relative permeability is 238734.802
i will scan and attach the method that i have followed to solve the prob till now.