# Having trouble on two problems- magnetism/inductance

1. Dec 11, 2005

### Grandmas

Not to be lazy or anything, but I was having a lot of trouble on these two physics problems for hw and don't know where to start for either.

I think for 1 I use Faraday's Law and for 2 I usee F=BILsin Theta... not sure

1. A square, single-turn coil 0.23 m on a side is placed with its plane perpendicular to a constant magnetic field. An emf of 16 mV is induced in the winding when the area of the coil decreases at a rate of 0.12 m2/s. What is the magnitude of the magnetic field?
2. A singly charged positive ion has a mass of 2.30 10-26 kg. After being accelerated through a potential difference of 260 V, the ion enters a magnetic field of 0.530 T, in a direction perpendicular to the field. Calculate the radius of the path of the ion in the field.

If possible post how to do it with answer, or explain where to start and i'll try and figure it out from there. Would be much help, thanks!

Last edited: Dec 11, 2005
2. Dec 12, 2005

### mezarashi

For 1, use the relationship (Faraday's Law):

$$V = -\frac{d\phi}{dt}, where$$ $$\phi = BA$$
Hint: You know that B is constant, but the change is in A.

For 2, you can use the Lorentz force equation in conjunction with the equation for circular motion (i.e):

$$F = qv \times B = m\frac{v^2}{r}$$